Updated on 2024/03/29

写真a

 
KITA, Tomohiro
 
Affiliation
Faculty of Science and Engineering, School of Advanced Science and Engineering
Job title
Professor
Degree
工学博士(材料) ( Japan Advanced Institute of Science and Technology Hokuriku )

Committee Memberships

  • 2014.12
    -
    Now

    電子情報通信学会シリコンフォトニクス時限研究専門委員会  専門委員

  • 2014.12
    -
    Now

    電子情報通信学会シリコンフォトニクス時限研究専門委員会  専門委員

  • 2016.06
    -
    2019.05

    電子情報通信学会  和文論文誌C編集委員

  • 2016.06
    -
    2019.05

    電子情報通信学会  和文論文誌C編集委員

Professional Memberships

  •  
     
     

    日本物理学会

  •  
     
     

    The Institute of Electronics, Information and Communication Engineers

  •  
     
     

    応用物理学会

  •  
     
     

    Optical society, America

Research Areas

  • Electron device and electronic equipment   半導体工学 / Optical engineering and photon science

Research Interests

  • 非線形光学効果

  • 半導体レーザ

  • 光エレクトロニクス

  • シリコンフォトニクス

  • 半導体工学

Awards

  • 石田實記念財団研究奨励賞

    2014.11   石田寛記念財団   大容量光通信用シリコンフォトニック波長可変レーザに関する研究

  • 第23回応用物理学会 講演奨励賞

    2007.10   応用物理学会  

Media Coverage

  • Quantum dots and silicon photonics combine in broadband tunable laser

    Newspaper, magazine

    Laser focus world  

    2015.11

  • Tunable laser diode taps NIR for data transmission

    Newspaper, magazine

    Photonics Spectra  

    2015.08

  • 波長可変、広範に 東北大 情通機構 小型レーザー開発

    Newspaper, magazine

    2015.05

  • 東北大とNICT ヘテロジニアス波長可変レーザの開発に成功 超小型・広帯域光デ

    Newspaper, magazine

    電波タイムズ  

    2015.05

  • 光ファイバー波長数2倍 東北大など レーザー部品開発

    Newspaper, magazine

    日経産業新聞  

    2015.05

 

Papers

  • Demonstration of 2D beam steering using large-scale passive optical phased array enabled by multimode waveguides with reduced phase error

    Yamato Misugi, Hideaki Okayama, Tomohiro Kita

    Applied Physics Express   15 ( 10 )  2022.10

     View Summary

    Optical phased arrays (OPAs) have received considerable attention as solid-state beam scanners. However, conventional OPAs that actively control the phase difference between arrays are characterized by excessive power consumption for high-precision beam emission. In this study, we fabricated an OPA comprising Bragg grating and arrayed waveguide grating (AWG). Multi-mode waveguide is used in AWG to reduce the effect of manufacturing error. This device realizes wide and high-resolution two-dimensional beam steering only by sweeping wavelength. FWHM of the emitted beam is 0.534° × 2.27°, and the steering range is 43.9° × 13.5° with 1/64 of the power consumption of conventional OPA.

    DOI

    Scopus

    7
    Citation
    (Scopus)
  • High-extinction-ratio Si optical modulator loaded with integrated polarizer

    Hiroki Kojima, Junichi Fujikata, Tomohiro Kita

    Japanese Journal of Applied Physics   61 ( SK )  2022.08

     View Summary

    We propose a carrier-depletion type Si optical modulator for transverse electric (TE) polarization loaded with an integrated polarizer. The integrated TE-pass polarizer with a bent waveguide has a compact and simple structure and demonstrate a polarization extinction ratio of 41.3 dB and insertion loss of 0.67 dB. This Si optical modulator loaded with the integrated polarizer achieved a maximum extinction ratio of 54.5 dB. It is expected to have a wide range of applications in digital coherent optical communication and optical sensing fields.

    DOI

    Scopus

  • Microoptics

    Toshio Watanabe, Tomonari Sato, Taro Arakawa, Kiichi Hamamoto, Satoshi Iwamoto, Tomohiro Kita, Muneharu Kuwata, Shinichi Saito, Shinji Yamashita

    Japanese Journal of Applied Physics   61 ( SK )  2022.08

    DOI

    Scopus

  • Ultrafast Silicon MZI Optical Switch with Periodic Electrodes and Integrated Heat Sink

    Tomohiro Kita, Manuel Mendez-Astudillo

    Journal of Lightwave Technology   39 ( 15 ) 5054 - 5060  2021.08

     View Summary

    The compact, high-speed, low-power consumption Mach-Zehnder interferometer (MZI)-type optical switch using thermo-optical effect is a key device in future optical integrated circuits. In this study, we achieved ultra-high-speed switching operation of 0.4 μs and low electrical power operation of 23 mW with a small device footprint by loading an asymmetric doped MZI structure and an integrated heat sink. This optical switch can be fabricated in the standard manufacturing process of silicon photonic devices, has high practicality and will be a useful tool in various optical integrated circuits.

    DOI

    Scopus

    13
    Citation
    (Scopus)
  • 1.55-µm Si-Photonics-Based Heterogeneous Tunable Laser Integrated with Highly Stacked QD-RSOA

    A. Matsumoto, W. Masuda, K. Akahane, T. Umezawa, N. Yamamoto, T. Kita

    2021 Conference on Lasers and Electro-Optics, CLEO 2021 - Proceedings    2021.05

     View Summary

    We demonstrated a Si-photonics-based heterogeneous tunable laser in the 1.55-µm-band with quantum dot reflective semiconductor optical amplifier. Relatively low threshold current could be achieved owing to high gain characteristic.

  • C-band Tunable Dual-Wavelength Laser with a Quantum Dot SOA

    Wataru Masuda, Kissho Iwanaga, Atsushi Matsumoto, Tomohiro Kita

    Conference Digest - IEEE International Semiconductor Laser Conference    2021

     View Summary

    A C-band tunable dual-wavelength laser for a RoF light source is demonstrated. The difference frequency was tunable in the range of 22.6 GHz to 192.0 GHz. The generated dual-wavelength laser amplified by EDFA could be converted to a corresponding radio frequency through a high-speed photodetector.

    DOI

    Scopus

  • Narrow linewidth hybrid wavelength-tunable laser with optical negative feedback circuit

    Tatsuki Komatsubara, Toshiaki Okachi, Nobuhide Yokota, Hiroshi Yasaka, Tomohiro Kita

    IEEE International Conference on Group IV Photonics GFP   2021-December  2021

     View Summary

    A hybrid laser with an optical negative feedback circuit integrated on a silicon photonics chip was investigated. Our numerical analysis verified that the laser can function as a wavelength-tunable laser with a spectral linewidth of less than 10 kHz. The result was supported by experiments.

    DOI

    Scopus

  • Thermo-optic phase shifter with sub-microsecond switching time and low power consumption

    Masaki Okamoto, Manuel Mendez-Astudillo, Tomohiro Kita

    Japanese Journal of Applied Physics   59 ( SO )  2020.08

     View Summary

    We propose a thermo-optic phase shifter utilizing multimode interference in a silicon waveguide. The electrodes were attached to the section where the multimode interference had the minimum optical electric field around the sidewalls. We designed the multimode waveguide width to reduce optical propagation loss in the C-L wavelength band (1.53-1.625 μm). We succeeded in developing a low optical loss thermo-optic phase shifter with less than 0.1 dB in the wavelength range of 1.5 to 1.6 μm. Moreover, wavelength dependence was reduced by adjusting the number of electrodes. An optical switch with the designed thermo-optic phase shifter was fabricated and used to estimate the optical switching time. Switching time of several microseconds was obtained by normal pulse operation, and ultrafast optical switching of about 500 ns was realized by overdrive control.

    DOI

    Scopus

    3
    Citation
    (Scopus)
  • Si光変調器の高消光比

    藤方 潤一, 北 智洋

    電子情報通信学会 技術研究報告   vol. 120, no. 243, OPE2020-70   125 - 128  2020

  • Wavelength Tunable filter with Curved Directional Coupler

    Yoshiaki Ito, Manuel Mendez-Astudillo, Tomohiro Kita

    MOC 2019 - 24th Microoptics Conference     106 - 107  2019.11

     View Summary

    We investigate a structure to reduce the wavelength-dependence of a ring resonator's transmission characteristics by replacing the conventional directional coupler of the racetrack resonator with a curved directional coupler. We show its potential as a filter for broadband wavelength tunable single mode lasers

    DOI

    Scopus

    3
    Citation
    (Scopus)
  • High Speed and Low Power Consumption, Thermo-optic Phase Shifter

    Masaki Okamoto, Manuel Mendez-Astudillo, Tomohiro Kita

    MOC 2019 - 24th Microoptics Conference     144 - 145  2019.11

     View Summary

    We propose a thermo-optic phase shifter which uses multimode interference. Electrodes are attached to the section where the multimode interference has minimum optical power in the sidewalls. We use the Silicon large thermo-optic constant to create an integrated optical device to control the state of light in several microseconds, for both heating and cooling times.

    DOI

    Scopus

  • Athermal Silicon Ring Resonators with TiO<inf>2</inf> Hybrid-Polymer Claddings

    Tomohiro Kita, Manuel Mendez-Astudillo, Mieko Masaka, Freddy Susanto Tan, Okihiro Sugihara

    MOC 2019 - 24th Microoptics Conference     134 - 135  2019.11

     View Summary

    Thermo-optic coefficients of organic-inorganic hybrid polymers with rutile-TiO2 nanoparticles were carefully estimated using SOI ring resonators. Athermal operation of the SOI ring resonators with TiO2 hybrid-polymer claddings was achieved. The hybrid-polymer cladding can be applied to diverse silicon waveguide structures to control the temperature dependence of silicon photonic devices.

    DOI

    Scopus

  • Fabry-Perot Cavity Using Two Row Photonic Crystal in a Multimode Waveguide

    Manuel Mendez-Astudillo, Hideaki Okayama, Tomohiro Kita

    2019 Conference on Lasers and Electro-Optics, CLEO 2019 - Proceedings   Part F129-CLEO_SI 2019  2019.05

     View Summary

    We experimentally present a Fabry-Perot cavity that uses two-row photonic crystals in a multimode waveguide as the reflecting elements in an add-drop configuration to achieve fine FSR tuning and maximum footprint efficiency.

    DOI

    Scopus

    2
    Citation
    (Scopus)
  • Camera sensor platform for high speed video data transmission using a wideband electro-optic polymer modulator

    Xiaoyang Cheng, Feng Qiu, Andrew M. Spring, Masaru Sasaki, Tsubasa Kashino, Masaaki Ozawa, Hideyuki Nawata, Tomohiro Kita, Okihiro Sugihara, Shiyoshi Yokoyama

    Optics Express   27 ( 3 ) 1877 - 1883  2019.02

     View Summary

    In this work, 1 GHz video data was collected by a CMOS camera and successfully transmitted by the electro-optic (EO) modulator driven by an external modulation module integrated onto the same chip. For this application, the EO modulator component included a polymer waveguide modulator, which performed a 20 GHz bandwidth, clear eye diagram opening with a Q factor of 10.3 at 32 Gbit/s and a drive voltage of 1.5 V pp . By utilizing a thermally stable EO polymer, the wide-band polymer modular can yield a photonic integrated camera sensor system which is a reliable processing platform for real-time data processing.

    DOI PubMed

    Scopus

    1
    Citation
    (Scopus)
  • Compact thermo-optic MZI switch in silicon-on-insulator using direct carrier injection

    Manuel Mendez-Astudillo, Masaki Okamoto, Yoshiaki Ito, Tomohiro Kita

    Optics Express   27 ( 2 ) 899 - 906  2019.01

     View Summary

    In this paper we present a compact direct current injection thermo-optic switch based on a Mach-Zehnder Interferometer configuration that is suitable for autonomous vehicle applications as it has a low heating resistance value of 97 Ω, a rapid 2.16 μs switching time constant, and a Pπ of 28 mW. The device relies on multimode interference to achieve low optical insertion losses of less than 1.1 dB per device, while allowing direct current injection to heat the waveguide and achieve fast operation speeds. Furthermore, the total resistive value can be tailored as the heating elements are placed in parallel.

    DOI PubMed

    Scopus

    33
    Citation
    (Scopus)
  • Rectangular resonators on silicon-on-insulator with large sensing area for object detection

    Manuel Mendez-Astudillo, Hideaki Okayama, Hirochika Nakajima, Tomohiro Kita

    Japanese Journal of Applied Physics   58 ( SJ )  2019

     View Summary

    In this paper we experimentally demonstrate the possibility of detecting an object using a rectangular resonator with a surface area of 200 μm2. The optical biosensor is fabricated on the standard silicon-on-insulator platform, and the object is sensed with the evanescent field that extends from the top of the device. It is possible to detect the presence of the object by measuring the shift of the resonant wavelengths in the transmission spectrum. We compare the performance of the device when it is operated in TE and TM polarization, obtaining superior performance with TM polarization, whereby it achieves a shift of 1.5 nm when a large element is placed on its surface.

    DOI

    Scopus

    1
    Citation
    (Scopus)
  • Compact narrow-linewidth optical negative feedback laser with Si optical filter

    Aoyama Konosuke, Kobayashi Shuhei, Wada Masashi, Yokota Nobuhide, Kita Tomohiro, Yasaka Hiroshi

    APPLIED PHYSICS EXPRESS   11 ( 11 )  2018.11  [Refereed]

     View Summary

    A compact narrow-linewidth semiconductor laser is demonstrated by hybridly integrating an optical filter constructed from Si waveguides with a single-mode semiconductor laser, whose spectral linewidth is narrowed through optical negative feedback. We design the optimum structure of a reflective ring filter for an optical negative feedback laser, and a narrow spectral linewidth of 160 kHz is realized by the hybridly integrated laser source whose effective device length is less than 1.5 mm.

    DOI

    Scopus

    6
    Citation
    (Scopus)
  • Phase-Shifted Multimode Bragg Gratings in Silicon-on-Insulator for Sensing Applications

    Manuel Mendez-Astudillo, Hideaki Okayama, Tomohiro Kita, Hirochika Nakajima

    IEEE International Conference on Group IV Photonics GFP   2018-August   49 - 50  2018.10

     View Summary

    In this paper, we experimentally characterize the sensitivity of different types of multimode Bragg gratings in Silicon-On-Insulator and demonstrate a high sensitivity of 120 nm per refractive index unit using Bragg gratings for TM polarized light.

    DOI

    Scopus

    1
    Citation
    (Scopus)
  • Two-Wavelength Tunable Laser Diode using a Quantum Dot SOA and a Silicon Photonic External Cavity

    Tomohiro Kita, Atsushi Matsumoto, Naokatsu Yamamoto, Manuel Mendez-Astudillo, Hirohito Yamada

    2018 IEEE International Semiconductor Laser Conference (ISLC)   2018-September   23 - 24  2018.09

     View Summary

    We propose a tunable two-wavelength heterogeneous quantum dot laser diode. The tunable two-wavelength laser consists of a quantum dot semiconductor optical amplifier and an external cavity fabricated from silicon photonics technology as the two-wavelength tunable filter. We successfully demonstrated two-wavelength lasing oscillation by tuning the difference frequency from approximately 34 GHz to 400 GHz.

    DOI

  • Tunable Dual-Wavelength Heterogeneous Quantum Dot Laser Diode with a Silicon External Cavity

    Tomohiro Kita, Atsushi Matsumoto, Naokatsu Yamamoto, Hirohito Yamada

    Journal of Lightwave Technology   36 ( 2 ) 219 - 224  2018.01  [Refereed]  [Invited]

     View Summary

    We propose a tunable dual-wavelength heterogeneous quantum dot laser diode. The tunable dual-wavelength laser consists of a quantum dot semiconductor optical amplifier as the optical gain medium and an external cavity fabricated from silicon photonics technology as the wavelength tunable filter. We successfully demonstrated dual-wavelength lasing oscillation by tuning the difference frequency from approximately 34 to 400 GHz.

    DOI

    Scopus

    13
    Citation
    (Scopus)
  • Narrow Spectral Linewidth Silicon Photonic Wavelength Tunable Laser Diode for Digital Coherent Communication System

    Tomohiro Kita, Rui Tang, Hirohito Yamada

    IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS   22 ( 6 ) 23 - 34  2016.11  [Refereed]  [Invited]

     View Summary

    We demonstrated wavelength tunable laser diodes with Si-wire waveguide ring resonators as an external optical cavity. The footprint of the optical cavity including the semiconductor optical amplifier is as small as 0.8 mm x 0.6 mm which is less 1/10 of those made of silicon oxinitride material. Wavelength tuning range of approximately 55 nm was demonstrated, which covers entire L-band of the optical communication wavelength system. Furthermore, a spectral linewidth narrower than 100 kHz was obtained by optimizing the design of the external optical cavity. The optical nonlinearity of silicon is described and the influence on laser oscillation is also discussed.

    DOI

    Scopus

    44
    Citation
    (Scopus)
  • Heterogeneous quantum dot/silicon photonics-based wavelength-tunable laser diode with a 44 nm wavelength-tuning range

    Tomohiro Kita, Naokatsu Yamamoto, Atsushi Matsumoto, Tetsuya Kawanishi, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   55 ( 4 ) 04EH11-1 - 04EH11-4  2016.04  [Refereed]

     View Summary

    A heterogeneous wavelength-tunable laser diode combining quantum dot and silicon photonics technologies is proposed. A compact wavelength-tunable filter with two ring resonators was carefully designed and fabricated using silicon photonics technology. The tunable laser combining the wavelength-tunable filter and an optical amplifier, which includes InAs quantum dots, achieved a 44.0 nm wavelength-tuning range at around 1250 nm. The broadband optical gain of the quantum dot optical amplifier was effectively used by the optimized wavelength-tunable filter. This heterogeneous wavelength-tunable laser diode could become a breakthrough technology for high-capacity data transmission systems. (C) 2016 The Japan Society of Applied Physics

    DOI

    Scopus

    18
    Citation
    (Scopus)
  • Instability of Silicon Photonic Wavelength Tunable Laser Diodes due to the Nonlinear Optical Effect of Silicon

    Tomohiro Kita, Hirohito Yamada

    2016 INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE (ISLC)    2016  [Refereed]

     View Summary

    The silicon photonic wavelength tunable laser diode is a good candidate for the integrated light source of the digital coherent optical communication system. However, optical nonlinear effects easily occur due to the strong light confinement of Si photonic wire waveguides. The impact on laser operation caused by nonlinearity was discussed.

  • Quantum dots and silicon photonics combine in broadband tunable laser

    Tomohiro Kita, Naokatsu Yamamoto

    LASER FOCUS WORLD   51 ( 11 ) 45 - 48  2015.11  [Refereed]

     View Summary

    A new wavelength-tunable laser diode combines quantum-dot (QD) technology and silicon photonics with large optical gains around the 1310 nm telecom window and is amenable to integration of other passive and active components towards a truly integrated photonic platform.

  • Ultra-compact wavelength-tunable quantum-dot laser with silicon-photonics double ring filter

    Tomohiro Kita, Naokatsu Yamamoto, Tetsuya Kawanishi, Hirohito Yamada

    APPLIED PHYSICS EXPRESS   8 ( 6 ) 062701 - 062704  2015.06  [Refereed]

     View Summary

    Compact, wavelength-tunable light sources are desired for the enhancement of information communication technology and bio-imaging applications. We propose a compact, wavelength-tunable laser diode with a wide wavelength-tunable range around 1230 nm consisting of a quantum-dot optical amplifier and a silicon photonic tunable filter. High-quality InAs quantum dots grown with the sandwiched sub-nano separator technique were used as the optical gain medium. The wavelength-tunable filter was constructed with ring resonators fabricated using silicon photonics. The single-mode laser oscillation was demonstrated with a 28.5-nm wavelength-tunable range. (C) 2015 The Japan Society of Applied Physics

    DOI

    Scopus

    24
    Citation
    (Scopus)
  • Optimum waveguide-core size for reducing device property distribution of Si-wire waveguide devices

    Munetoshi Soma, Tomohiro Kita, Yuichiro Tanushi, Munehiro Toyama, Miyoshi Seki, Nobuyuki Yokoyama, Minoru Ohtsuka, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   54 ( 4 ) 04DG03  2015.04  [Refereed]

     View Summary

    We investigated the waveguide-core size distribution of ring resonators fabricated on a 300mm silicon-on-insulator (SOI) wafer using a CMOS-compatible process featuring ArF immersion lithography. These ring resonators were constructed in a Si-wire waveguide with a standard core size of 400nm width and 220nm height. The group refractive indices of the waveguide were derived from the transmission spectra of the ring resonators. From the deviation of these group refractive indices, the waveguide-core width distribution was estimated to be 5 nm, and the waveguide-core height distribution was estimated to be 1 nm. Moreover, the device property distribution of various Si-wire waveguide depended on the estimated fabrication error was calculated. The waveguide core with the smallest device property distribution had a 540nm width and a 160nm height, and this waveguide has a device property distribution of 2/3 value compared with the standard core size. (C) 2015 The Japan Society of Applied Physics

    DOI

    Scopus

    7
    Citation
    (Scopus)
  • Narrow-spectral-linewidth silicon photonic wavelength-tunable laser with highly asymmetric Mach-Zehnder interferometer

    Rui Tang, Tomohiro Kita, Hirohito Yamada

    OPTICS LETTERS   40 ( 7 ) 1504 - 1507  2015.04  [Refereed]

     View Summary

    We propose a narrow-spectral-linewidth silicon photonic wavelength-tunable laser with a novel external wavelength-tunable filter, which consists of two silicon ring resonators with different circumferences and a highly asymmetric Mach-Zehnder interferometer (MZI), the two optical paths of which have significantly different lengths. Calculations and experimental results indicated that the gain difference between longitudinal modes was increased by the highly asymmetric MZI. Consequently, a narrow spectral linewidth of 12 kHz and a stable single-mode oscillation were obtained. (C) 2015 Optical Society of America

    DOI

    Scopus

    23
    Citation
    (Scopus)
  • Silicon Photonic Hybrid Ring-Filter External Cavity Wavelength Tunable Lasers

    Naoki Kobayashi, Kenji Sato, Masahiko Namiwaka, Keisuke Yamamoto, Shinya Watanabe, Tomohiro Kita, Hirohito Yamada, Hiroyuki Yamazaki

    JOURNAL OF LIGHTWAVE TECHNOLOGY   33 ( 6 ) 1241 - 1246  2015.03  [Refereed]

     View Summary

    Si-photonic hybrid ring external cavity wavelength tunable lasers by passive alignment techniques with more than 100-mW fiber-coupled power and linewidth narrower than 15 kHz along the whole C-band are demonstrated. These attractive performances are achieved due to very low loss Si-wire waveguides, of which loss is lower than 0.5 dB/cm. Obtained results show excellent features of Si-photonics toward commercial products.

    DOI

    Scopus

    152
    Citation
    (Scopus)
  • Compact silicon photonic wavelength-tunable laser diode with ultra-wide wavelength tuning range

    Tomohiro Kita, Rui Tang, Hirohito Yamada

    APPLIED PHYSICS LETTERS   106 ( 11 ) 111104  2015.03  [Refereed]

     View Summary

    We present a wavelength-tunable laser diode with a 99-nm-wide wavelength tuning range. It has a compact wavelength-tunable filter with high wavelength selectivity fabricated using silicon photonics technology. The silicon photonic wavelength-tunable filter with wide wavelength tuning range was realized using two ring resonators and an asymmetric Mach-Zehnder interferometer. The wavelength-tunable laser diode fabricated by butt-joining a silicon photonic filter and semiconductor optical amplifier shows stable single-mode operation over a wide wavelength range. (C) 2015 AIP Publishing LLC.

    DOI

    Scopus

    33
    Citation
    (Scopus)
  • Silicon Photonic Wavelength-Tunable Laser Diode With Asymmetric Mach-Zehnder Interferometer

    Tomohiro Kita, Keita Nemoto, Hirohito Yamada

    IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS   20 ( 4 ) 8201806_1 - 8201806_6  2014.07  [Refereed]

     View Summary

    We fabricated wavelength-tunable laser diodes using a Si photonic wavelength filer that consists of ring resonators and an asymmetric Mach-Zehnder interferometer. The footprint of the optical cavity including the semiconductor optical amplifier is small, 2.6 mm x 0.5 mm, which is about 1/9 of those for tunable laser diodes made of silicon oxynitride. The wavelength could be tuned over approximately 62 nm, which covers the entire L-band of the optical communication wavelength range. The maximum output power reaches 42.2mW. Furthermore, a spectral line width narrower than 100 kHz was obtained. Such tunable laser diodes with narrow spectral line widths are suitable as light sources integrated into other digital coherent devices.

    DOI

    Scopus

    36
    Citation
    (Scopus)
  • Long external cavity Si photonic wavelength tunable laser diode

    Tomohiro Kita, Keita Nemoto, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   53 ( 4 ) 04EG04_1 - 04EG04_4  2014.04  [Refereed]

     View Summary

    We fabricated wavelength-tunable laser diodes with external cavity consist of Si photonic wire waveguide ring resonators. About 51.5 nm wavelength tuning operation, which covers the entire L-band of the optical communication wavelength range, was obtained. The cavity length dependence of the spectral line width was verified to obtain narrower spectral linewidth. The observed spectral linewidth of 3.78 mm long cavity and 8.19 mm long cavity are 131.0 and 64.8 kHz, respectively. The advantage of long external cavity to design the narrow spectral linewidth Si photonic wavelength tunable laser diode was confirmed. (C) 2014 The Japan Society of Applied Physics

    DOI

    Scopus

    22
    Citation
    (Scopus)
  • Wide-band Wavelength Tunable Laser Diode with Si Photonic Filter

    Tomohiro Kita, Rui Tang, Hirohito Yamada

    2014 24TH IEEE INTERNATIONAL SEMICONDUCTOR LASER CONFERENCE (ISLC 2014)     149 - 150  2014  [Refereed]

     View Summary

    We investigated the wide-band wavelength tuning operation to increase the possibility of silicon photonic wavelength tunable laser diodes which is composed semiconductor optical amplifier and silicon photonic waveguide filter. The wide-band wavelength tuning operation of 94.1 nm was demonstrated.

    DOI

  • High Output Power and Narrow Linewidth Silicon Photonic Hybrid Ring-Filter External Cavity Wavelength Tunable Lasers

    K. Sato, N. Kobayashi, M. Namiwaka, K. Yamamoto, T. Kita, H. Yamada, H. Yamazaki

    2014 EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION (ECOC)   33   1241 - 1244  2014  [Refereed]  [Invited]

     View Summary

    Si-photonic Hybrid Ring-filter External Cavity (SHREC) wavelength tunable lasers by passive alignment techniques with over 100-mW fiber-coupled power and linewidth narrower than 15 kHz along the whole C-band are demonstrated. Obtained results show excellent features of Si-photonics towards commercial products.

    DOI

    Scopus

    152
    Citation
    (Scopus)
  • Highly-stabilized operation of Si photonics wavelength tunable laser diode

    Tomohiro Kita, Keita Nemoto, Hirohito Yamada

    Extended abstract of Solid State Device and Materials    2013.09  [Refereed]

  • Nonlinear optical photonic crystal waveguide with TiO2 material

    Koji Uchijima, Tomohiro Kita, Hirohito Yamada

    OXIDE-BASED MATERIALS AND DEVICES IV   8626  2013

     View Summary

    Titanium dioxide (TiO2) has several attractive properties such as high linear and nonlinear refractive indices and the wide bandgap for applying the nonlinear optical devices. We have been studied channel waveguides with TiO2 as the submicron size core to verify their feasibility as the nonlinear optical devices. In this study, we analyzed optical propagation in the TiO2 photonic crystal waveguide and demonstrated the enhanced optical nonlinearity due to slow light effect.

    DOI

    Scopus

    1
    Citation
    (Scopus)
  • Anomalous localization modes in Bragg-grating based on high index-difference waveguide

    Tomohiro Kita, Koji Uchijima, Hirohito Yamada

    SILICON PHOTONICS VIII   8629  2013

     View Summary

    We observed photonic band-gap disappearance on Bragg-grating wavelength filters with Si photonic-wire waveguides, and studied the physical mechanism with theoretical and numerical analyses. This is a unique phenomenon observed in channel waveguides with very high index-contrast between the waveguide core and cladding materials. The photonic band-gap disappearance was observed in structures where two different optical field distributions of standing wave degenerate.

    DOI

    Scopus

  • Narrow-Spectral-Linewidth Wavelength-Tunable Laser Diode with Si Wire Waveguide Ring Resonators

    Keita Nemoto, Tomohiro Kita, Hirohito Yamada

    APPLIED PHYSICS EXPRESS   5 ( 8 ) 082701-1 - 082701-3  2012.08  [Refereed]

     View Summary

    We fabricated wavelength-tunable laser diodes with Si wire waveguide ring resonators as an external optical cavity. The footprint of the optical cavity including the semiconductor optical amplifier is as small as 2.73 x 0.89 mm(2), which is about 1/5 of those made of silicon oxinitride material. About 44-nm-wavelength tuning operation, which covers the entire L-band of the optical communication wavelength range, was obtained. Furthermore, spectral linewidth narrower than 100 kHz was obtained by optimizing the design of the external optical cavity. The tunable laser diodes with narrow spectral linewidth are suitable as light sources for digital coherent optical transmission systems. (C) 2012 The Japan Society of Applied Physics

    DOI

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    50
    Citation
    (Scopus)
  • Analysis of Phase Matching Conditions for Generating Second Harmonic in ZnO Channel Waveguides

    Yuta Taira, Tomohiro Kita, Edgar Yoshio Morales Teraoka, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   51 ( 4 ) 04DG04-1 - 04DG04-4  2012.04  [Refereed]

     View Summary

    We describe a phase-matching scheme for generating the second harmonic in ZnO channel waveguides using a modal phase-matching (MPM) method. We found that MPM can be achieved between the TM00 mode of the fundamental wave and the TM02 mode of the second-harmonic wave by using structural dispersion of the waveguide mode. Furthermore, we calculated the normalized conversion efficiency by taking the overlap integral of each waveguide mode and the effective area of the fundamental wave into account, and obtained a sufficiently high conversion efficiency. These results show that ZnO channel waveguides are very attractive for generating second-harmonic devices. (C) 2012 The Japan Society of Applied Physics

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  • Theoretical analysis of vertical coupling optical I/O interface with reflectors

    Tohoku-Section Joint Convention Record of Institutes of Electrical and Information Engineers, Japan   2012   16 - 16  2012

    DOI CiNii

  • TiO2 photonic crystal waveguide for nonlinear optical applications

    Tohoku-Section Joint Convention Record of Institutes of Electrical and Information Engineers, Japan   2012   18 - 18  2012

    DOI CiNii

  • Analysis of the Nonlinear Optical Parameter of ZnO Channel Waveguides

    Edgar Yoshio Morales Teraoka, Tomohiro Kita, Daniel H. Broaddus, Atsushi Tsukazaki, Masashi Kawasaki, Alexander L. Gaeta, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   50 ( 4 ) 04DG01  2011.04  [Refereed]

     View Summary

    In this paper, we analyze the nonlinear optical parameter gamma estimated from experimental results of spectral broadening in ZnO channel waveguides. The effective area was calculated for waveguides of various dimensions and using the resulting data, the nonlinear optical parameter gamma was obtained analytically. We compare these values with those estimated from measurements, and found good agreement between them. We extended the analysis to calculate the optimal size for maximum nonlinearity and found that the fabricated waveguides were very close to the optimal size. (C) 2011 The Japan Society of Applied Physics

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    2
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  • Optical Crossing and Integration Using Hybrid Si-Wire/Silica Waveguides

    Younosuke Wakayama, Tomohiro Kita, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   50 ( 4 ) 04DG20  2011.04  [Refereed]

     View Summary

    We studied the feasibility of an integrated optical circuit consisting of Si-wire waveguides and silica-based waveguides. In order to realize such a hybrid waveguide optical circuit, we investigated a waveguide junction structure based on adiabatic mode conversion with tapered Si and a waveguide crossing structure of heterogeneous waveguides by numerical analysis and measurements. We estimated the optical coupling loss between the waveguides to be 0.39 dB from the measurements. We also estimated the insertion loss of the waveguide crossing to be less than 0.7 dB, and the crosstalk between the waveguides to be less than -35 dB by numerical analysis. (C) 2011 The Japan Society of Applied Physics

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    7
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  • Variation of optical properties by the crystalline phase transition of polycrystalline silicon

    Hidenori Iwata, Tomohiro Kita, Hirohito Yamada

    SILICON PHOTONICS VI   7943   50  2011

     View Summary

    We studied a characteristics trimming technique in Si photonic-wire waveguide devices. In order to trimming device properties, we utilized refractive index change of amorphous silicon when it crystallizes by annealing. We fabricated MZI devices with amorphous silicon waveguides, and demonstrated the trimming of the transmission spectra by thermal annealing and also laser annealing after finished the device fabrication process. We observed 5.2 % of the refractive index change owing to crystallization by annealing in a nitrogen atmosphere and 5.8% of change by laser crystallization.

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  • Wavelength Tunable Laser Diodes with Si-Wire Waveguide Ring Resonator Wavelength Filters

    Ken Suzuki, Tomohiro Kita, Hirohito Yamada

    SILICON PHOTONICS VI   7943   51  2011

     View Summary

    We fabricated wavelength tunable laser diodes consisting of ring resonator wavelength filters with Si-wire waveguides. More than 45 nm wavelength tuning operation was obtained by about 100 mW heating power. We also measured spectral linewidth of the tunable laser, and observed about 200 kHz of spectral linewidth.

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    13
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  • Nonlinear optical waveguides with rutile TiO(2)

    Koichi Abe, Edgar Yoshio Morales Teraoka, Tomohiro Kita, Hirohito Yamada

    OXIDE-BASED MATERIALS AND DEVICES II   7940 ( 49 )  2011

     View Summary

    We investigated a possibility of making rutile TiO(2) channel waveguides for nonlinear optical applications. Single mode conditions, the group velocity dispersion and the nonlinear-optical parameters for the rutile TiO(2) channel waveguides were calculated using the finite-element method. We also fabricate channel waveguides with rutile TiO(2) using reactive ion etching. The propagation loss measured by the cut-back method was around 6 dB/mm. According to our simulation, around 70 nm of spectral broadening owing to the self-phase modulation will be expected by a pumping optical pulse with 360 W peak power.

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    6
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  • Fabrication of ZnO channel waveguides for nonlinear optical applications

    Edgar Yoshio Morales Teraoka, Tomohiro Kita, Atsushi Tsukazaki, Masashi Kawasaki, Hirohito Yamada

    OXIDE-BASED MATERIALS AND DEVICES II   7940 ( 48 )  2011

     View Summary

    We present a fabrication procedure for ZnO channel waveguides intended for nonlinear optical applications. Ar ion milling was used to etch the single crystal thin film samples, and the effects of bias power, chamber pressure and Ar flow rate were investigated, finding optimal parameters for waveguide fabrication. The effect of sidewall roughness was estimated by comparing the results of cut-back measurements and an analytical model. We show an easy and effective method for the fabrication of ZnO channel waveguides.

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    1
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  • Self-phase modulation at visible wavelengths in nonlinear ZnO channel waveguides

    Edgar Yoshio Morales Teraoka, Daniel H. Broaddus, Tomohiro Kita, Atsushi Tsukazaki, Masashi Kawasaki, Alexander L. Gaeta, Hirohito Yamada

    APPLIED PHYSICS LETTERS   97 ( 7 ) 71105 - 71107  2010.08  [Refereed]

     View Summary

    We report the observation of nonlinear optical effects in the visible spectrum using ZnO channel waveguides. We demonstrate sixfold spectral broadening and a 2 pi phase shift due to self-phase modulation in ZnO channel waveguides using femtosecond optical pulses with a center wavelength of 840 nm. We measured a value for the nonlinear parameter gamma of 13.9 +/- 3.0 W-1 m(-1), which is more than 1300 times that of a highly nonlinear fiber. The calculated intensity-dependent refractive index is found to be consistent with previously reported values of bulk single-crystal ZnO. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3480422]

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    23
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  • Experimental and numerical analysis study of 1-D photonic crystal in Si photonic-wire waveguides

    Tomohiro Kita, Hirohito Yamada

    SILICON PHOTONICS V   7606  2010

     View Summary

    We have systematically studied Bragg Grating Filters in Si photonic-wire waveguides. Highly uniform samples have been fabricated in a specialized foundry service for Si photonics. The reflected wavelength has been controlled over an 180 nm range. By changing the design parameters we controlled the photonic band-gap to be between 6 to 16 nm. Thermal tuning of the reflected wavelength was also measured, resulting in a wavelength shift of more than 3 nm.

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  • ZnO Channel Waveguides for Nonlinear Optical Applications

    Edgar Yoshio Morales Teraoka, Tomohiro Kita, Atsushi Tsukazaki, Masashi Kawasaki, Yasuo Ohtera, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   49 ( 4 ) 04DG15-1 - 04DG15-4  2010  [Refereed]

     View Summary

    ZnO channel waveguides were fabricated for the first time, for the purpose of being used in nonlinear optical applications. Wide-bandgap ZnO was chosen for the core material to avoid two-photon absorption at wavelengths above 800nm which are used for biomedical applications. We calculated the single-mode conditions and group-velocity dispersion of the propagating modes for the waveguides. We also fabricated channel waveguides using single-crystal ZnO thin film grown on sapphire substrate. Measured propagation losses with a cut-back method were approximately 3 to 6 dB/mm for single-mode waveguides. (C) 2010 The Japan Society of Applied Physics

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    9
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  • Non-linear optical ZnO waveguides

    モラレス 芳男, 北 智洋, 塚崎 敦, 川崎 雅司, 大寺 康夫, 山田 博仁

    IEICE Technical Report   OPE2008 ( 143 ) 35 - 40  2008.12

  • Fabrication of a few-electron In0.56Ga0.44As vertical quantum dot with an Al2O3 gate insulator

    T. Kita, D. Chiba, Y. Ohno, H. Ohno

    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES   40 ( 6 ) 1930 - 1932  2008.04  [Refereed]

     View Summary

    For the study of spin-dependent transport in narrow-gap semiconductor nanostructures, we fabricated a few-electron quantum dot based on an In0.56Ga0.44As resonant tunneling diode structure with an Al2O3 gate insulator formed by atomic layer deposition and an air-bridge drain electrode. This gated quantum dot device allows us to control the number of electrons from zero to a few dozen without leakage problem. We describe the processing techniques and the characteristics of the low-temperature transport. (C) 2007 Elsevier B.V. All rights reserved.

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  • Fine structure in magnetospectrum of vertical quantum dot

    Oleksiy B. Agafonov, Tomohiro Kita, Hideo Ohno, Rolf J. Haug

    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES   40 ( 5 ) 1630 - 1632  2008.03  [Refereed]

     View Summary

    The electronic transport properties of a gated vertical quantum dot fabricated of an asymmetrical InGaAs/AlGaAs double-barrier resonant tunneling heterostructure are studied experimentally. At a temperature of 15 mK, a series of small current peaks are observed far below the voltage of a main resonance peak. The voltage position of these peaks appeared to be strongly dependent on the presence of magnetic field oriented perpendicular to the plane of the barriers. The occurrence of the peaks is attributed to tunneling mechanisms involving inter- Landau- level resonant tunneling, longitudinal-optical (LO)-phonon-assisted tunneling and to electrostatic effects such as Coulomb blockade (C) 2007 Elsevier B.V. All rights reserved.

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  • Effect of vertical electric fields on exciton fine structure of GaAs natural quantum dots

    S. Marcet, T. Kita, K. Ohtani, H. Ohno

    PHYSICA E   40   2069  2008  [Refereed]

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    3
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  • A few-electron vertical In0.56Ga0.44As quantum dot with an insulating gate

    T. Kita, D. Chiba, Y. Ohno, H. Ohno

    APPLIED PHYSICS LETTERS   91 ( 23 ) 232101  2007.12  [Refereed]

     View Summary

    Using an Al2O3 gate insulator by atomic layer deposition and air-bridge drain electrode, we fabricated a quantum dot with few electrons based on an In0.56Ga0.44As resonant tunneling diode structure. Artificial atomic properties manifested themselves in magnetotransport, enabling the determination of effective electron g factors. Results show that the insulating gate structure used here is effective for realizing quantum dots made of narrow-gap semiconductors for studying spin-related phenomena. (c) 2007 American Institute of Physics.

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    2
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  • Combinatorial synthesis and high throughput evaluation of thermoelectric power factor in Mg-Si-Ge ternary compounds

    M. Watanabe, T. Kita, T. Fukumura, A. Ohtomo, K. Ueno, M. Kawasaki

    APPLIED SURFACE SCIENCE   254 ( 3 ) 777 - 780  2007.11  [Refereed]

     View Summary

    Discrete phase libraries of thermoelectric compounds, Mg(x)Si(y)Ge(1-y), were fabricated by a combinatorial pulsed laser deposition followed by annealing as a thin film form on an integrated ceramic substrate. In the substrate are embedded four probe electrical contacts to each sample, lead wires and pads to be accessed by needle probes. Resistivity and Seebeck coefficient were evaluated electrically, while temperature difference was locally given to each sample by a local heater also embedded in the substrate. The sample temperature (300-673 K) was controlled by a heating stage and temperature difference at the two contact points for each sample was evaluated by an infrared camera. The dependences of polarity and absolute values of Seebeck coefficient on the composition agree well with the data in literature. (C) 2007 Elsevier B.V. All rights reserved.

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  • Quantum Hall effect in polar oxide heterostructures

    A. Tsukazaki, A. Ohtomo, T. Kita, Y. Ohno, H. Ohno, M. Kawasaki

    SCIENCE   315 ( 5817 ) 1388 - 1391  2007.03  [Refereed]

     View Summary

    We observed Shubnikov-de Haas oscillation and the quantum Hall effect in a high-mobility two-dimensional electron gas in polar ZnO/Mg(x)Zn(1-x)O heterostructures grown by laser molecular beam epitaxy. The electron density could be controlled in a range of 0.7 x 10(12) to 3.7 x 10(12) per square centimeter by tuning the magnesium content in the barriers and the growth polarity. From the temperature dependence of the oscillation amplitude, the effective mass of the two-dimensional electrons was derived as 0.32 +/- 0.03 times the free electron mass. Demonstration of the quantum Hall effect in an oxide heterostructure presents the possibility of combining quantum Hall physics with the versatile functionality of metal oxides in complex heterostructures.

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    533
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  • (In,Ga)As gated-vertical quantum dot with an Al2O3 insulator

    T. Kita, D. Chiba, Y. Ohno, H. Ohno

    APPLIED PHYSICS LETTERS   90 ( 6 ) 062102  2007.02  [Refereed]

     View Summary

    The authors fabricated a gated-vertical (In,Ga)As quantum dot with an Al2O3 gate insulator deposited using atomic layer deposition and investigated its electrical transport properties at low temperatures. The gate voltage dependence of the dI/dV-V characteristics shows clear Coulomb diamonds at 1.1 K. The metal-insulator gate structure allowed the authors to control the number of electrons in the quantum dot from 0 to a large number estimated to be about 130. (c) 2007 American Institute of Physics.

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    7
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  • Bias voltage dependence of the electron spin injection studied in a three-terminal device based on a (Ga,Mn)As/n(+)-GaAs Esaki diode

    M. Kohda, T. Kita, Y. Ohno, F. Matsukura, H. Ohno

    APPLIED PHYSICS LETTERS   89 ( 1 ) 012103 - 012105  2006.07  [Refereed]

     View Summary

    We investigated injection of spin polarized electrons in a (Ga,Mn)As/n(+)-GaAs Esaki diode (ED) by using a three-terminal device integrating a (Ga,Mn)As ED and a light emitting diode (LED). Electroluminescence polarization (P-EL) from the LED was measured under the Faraday configuration as a function of bias voltages applied independently to the Esaki diode and to the LED. The maximum P-EL of 32.4% was observed when the valence electrons near the Fermi energy of (Ga,Mn)As are ballistically injected into the LED. (c) 2006 American Institute of Physics.

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    38
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  • Pulse-width and magnetic-field dependences of current-induced magnetization switching in a (Ga,Mn)As magnetic tunnel junction

    D Chiba, T Kita, F Matsukura, H Ohno

    JOURNAL OF APPLIED PHYSICS   99 ( 8 ) 08G514  2006.04  [Refereed]

     View Summary

    We have investigated the pulse-width and magnetic-field dependences of current-induced magnetization switching in a (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction. Critical current to induce magnetization switching shows a linear dependence on pulse width from 10 to 1000 mu s. The magnetic-field dependence appears to indicate that the observed current-induced magnetization switching proceeds through metastable magnetization structures. (C) 2006 American Institute of Physics.

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    7
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  • Spin injection with three terminal device based on (Ga,Mn)As/n(+)-GaAs tunnel junction

    T. Kita, M. Kohda, Y. Ohno, F. Matsukura, H. Ohno

    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 12   3 ( 12 ) 4164 - 4167  2006  [Refereed]

     View Summary

    We have fabricated a three terminal device integrating a (Ga,Mn)As Esaki diode and a light emitting diode (LED) to investigate the bias-voltage dependence of the injection of spin polarized electrons. The electroluminescence polarization (P-EL) from the LED was measured under Faraday configuration as a function of bias voltages applied independently to the Esaki diode and to the LED. The polarization shows strong dependence on the bias applied to the Esaki diode when the LED bias is fixed. The maximum P-EL of 32.4% was observed when the valence electrons near the Fermi energy of (Ga,Mn)As are injected into the LED. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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    5
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  • Spin-polarized transport in adiabatic quantum point contact with strong Rashba spin-orbit interaction

    T Kita, T Kakegawa, A Akabori, S Yamada

    SOLID STATE COMMUNICATIONS   136 ( 8 ) 479 - 483  2005.11  [Refereed]

     View Summary

    We first report 0.5(2e(2)/h) conductance quantization in adiabatic quantum point contacts (QPCs) fabricated at high In-content InGaAs/InAlAs single heterojunctions under no magnetic field. This quantization seems difficult to understand, since the spin one-dimensional (I D) subbands in the QPCs are generally degenerated when B = 0. However, this observation is reproducible in various QPC samples with different dimensions but not likely so definite as the conductance quantization in usual QPCs. It is noted that this particular heterojunction 2DEG is found to have high electron mobility of &lt; 5 X 10(5) cm(2) Ns as well as very large Rashba spin-orbit (SO) coupling constant of &lt; 35 X 10(-12) eVm. So that, the QPCs realized here can be regarded as a kind of Tomonaga-Luttinger wire with an enhanced Rashba interaction. In such a case, a mode coupling between the Rashba splitting ID subbands gives rise to a spin-polarized transport in each +/- k direction. This theory could be the one plausible candidate to explain the 0.5(2e(2)/h) conductance quantization observed here in the adiabatic QPC. This finding would be developed to novel spin-filters or spin-directional coupler devices based on nonmagnetic semiconductors. (c) 2005 Elsevier Ltd. All rights reserved.

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    1
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  • Current-driven magnetization reversal in a ferromagnetic semiconductor (Ga,Mn)As/GaAs/(Ga,Mn)As tunnel junction

    D Chiba, Y Sato, T Kita, F Matsukura, H Ohno

    PHYSICAL REVIEW LETTERS   93 ( 21 ) 216602 - 216605  2004.11  [Refereed]

     View Summary

    Current-driven magnetization reversal in a ferromagnetic semiconductor based (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K. Magnetoresistance measurements combined with current pulse application on a rectangular 1.5x0.3 mum(2) device revealed that magnetization switching occurs at low critical current densities of 1.1-2.2x10(5) A/cm(2) despite the presence of spin-orbit interaction in the p-type semiconductor system. Possible mechanisms responsible for the effect are discussed.

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    147
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  • Nonmagnetic impurity effects in MgB2

    K Watanabe, T Kita

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   73 ( 8 ) 2239 - 2244  2004.08  [Refereed]

     View Summary

    We study nonmagnetic impurity effects in MgB2 using the quasiclassical equations of superconductivity for a weak-coupling two-band model. Parameters in the model are fixed so as to reproduce experiments on MgB2 as closely as possible. The quasiparticle density of states and the specific heat are calculated for various values of the interband impurity scattering. The density of states changes gradually from a two-gap structure into the conventional single-gap structure as the interband scattering increases. It is found that the excitation threshold is not a monotonic function of the interband scattering. Calculated results for the specific heat are in good agreements with experiments on samples after irradiation.

    DOI

  • Spin-polarized transport in Rashba quantum point contacts

    T Kita, T Kakegawa, M Akabori, S Yamada

    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES   22 ( 1-3 ) 464 - 467  2004.04  [Refereed]

     View Summary

    Two types of quantum point contacts (QPCs), side-gate quantum wire and split-gate QPCs, were fabricated at high In-content InGaAs/InAlAs single heterojunction which confines two-dimensional electron gas revealing strong Rashba spin-orbit interaction. In the side-gate quantum wire QPC, the conductance quantization in unit of 0.5(2e(2)/h) was mostly observed in the various samples with different dimensions. This is in contrast to the result in the split-gate QPC, where we confirmed usual (2e(2)/h) conductance quantization. Since those results were observed under zero magnetic field, the 0.5(2e(2)/h) quantization could be attributed not to the spin splitting but to the possible spin polarized transport due to the mode coupling between the one-dimensional subbands expected in sufficiently narrow adiabatic constrictions. (C) 2003 Published by Elsevier B.V.

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    3
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  • Observation of e(2)/h conductance steps in a side-gate point contact on In0.75Ga0.25As/In0.75Al0.25As heterostructure

    T Kita, S Gozu, Y Sato, S Yamada

    JOURNAL OF SUPERCONDUCTIVITY   16 ( 2 ) 327 - 329  2003.04  [Refereed]

     View Summary

    In 1996, it was suggested by K.J. Thomas et al. (Phys. Rev. Lett. 77, 135 (1996)) that 0.7(2e(2)/h) conductance structure in Quasi-1DEG (Q1DEG) at GaAs/AlGaAs heterostructure might be related to spin polarization at zero magnetic field. We have recently studied spontaneous spin-splitting in the 2DEGs formed at normal metamorphic In0.75Ga0.25As/In0.75Al0.25As heterojunctions grown on GaAs substrates and obtained the value of less than or similar to10 meV as the zero-field splitting at Fermi level (Y. Sato et al., J. Appl. Phys. 89, 8017 (2001)). In this work, we attempted to observe spin-related phenomena in this heterojunction Q1DEGs at zero magnetic field. We observed e(2)/h conductance steps in low electron concentration side-gate point contact.

  • Conductance fluctuations in quantum wires formed at spin-splitting narrow gap heterojunctions

    S Yamada, Y Sato, T Kita, S Gozu

    COMPOUND SEMICONDUCTORS 2001   ( 170 ) 345 - 349  2002  [Refereed]

     View Summary

    Conductance fluctuations in quasi-ballistic quantum wires formed at spontaneously spin-splitted heterojunctions have been measured and discussed. The fluctuation amplitude was found to likely decrease when an applied magnetic field increased and when a gate voltage decreased. Both the applications of magnetic field and negative gate voltage are known to decrease the spin-orbit coupling constant in our heterojunction and they are expected to increase the fluctuation amplitude in anti-localization regime. The fluctuation results could, however, be explained, if we pay attentions to the fact that despite the large zero field spin-splitting,, the quantum wires am classified into those with tranport regimes of 'localization" and "weak' spin-orbit scattering due to the high mobilities.

  • Ballistic spin transport in four-terminal NiFe/In(0.7)5Ga(0.25)As structure

    Y Sato, S Gozu, T Kita, S Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   40 ( 10B ) L1093 - L1096  2001.10  [Refereed]

     View Summary

    Spin-injection experiments in NiFe/two-dimensional electron gas (2DEG)/NiFe four-terminal devices are described. The 2DEG was confined at a modulation-doped In0.75Ga0.25As/In0.75Al0.25As heterojunction interface. NiFe source-drain electrodes were located similar to1 mum apart and two voltage probes were fabricated between them. The separation between the NiFe electrodes was thus almost less than both the mean free path (similar to2 mum) and the spin-diffusion length (&gt; 1 mum) of the 2DEG. In nonlocal four-terminal measurements, where a constant current (i) was sent through one NiFe to one ohmic electrode, the two-terminal magnetoresistance (R-2t = exhibited a spin-valve-like effect, while R-4t = V-4t/i showed a resistance hysteresis. The amplitude of the latter amounted to almost 12% of the R-4t (B = 0) at 1.5 K, the extent of which was one order of magnitude larger than those so far reported. These results suggest the importance of a quasi-ballistic as well as a spin-coherent coupling between the two NiFe electrodes, which may crucial in the future operation of spin field-effect transistors (FETs).

    DOI

  • Critical layer thickness study in In0.75Ga0.25As/In0.5Al0.5As pseudomorphic resonant tunneling diode structure grown on GaAs substrates

    S Gozu, T Kita, T Kikutani, S Yamada

    JOURNAL OF CRYSTAL GROWTH   227   161 - 166  2001.07  [Refereed]

     View Summary

    We have studied critical layer thickness (CLT) in an In0.75Ga0.25As resonant tunneling diode structure grown on a GaAs substrate, ia an InAlAs step-graded buffer (SGB) with two types of inverse step(IS)-SGB. We have observed red or blue shift in photoluminescence spectra and CLT change depending on SGB condition. This change reflects from the difference of residual strain InAlAs SGBs adopted here.,(C) 2001 Elsevier Science B.V. All rights reserved.

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  • Characterization of high indium content metamorphic InGaAs/InAlAs modulation-doped heterostructures

    S Gozu, T Kita, Y Sato, S Yamada, M Tomizawa

    JOURNAL OF CRYSTAL GROWTH   227   155 - 160  2001.07  [Refereed]

     View Summary

    We have studied electronic and structural characterizations of high indium content metamorphic InGaAs/InAlAs modulation-doped heterostructures. An improved low-temperature electron mobility of mu (e) = 5.45 x 10(5) cm(2) Vs in [(1) over bar 1 0] was confirmed for a directed Hall-bar sample when Si-doped InAlAs layer was slightly etched. In addition, inplane mobility anisotropy of 40% between [(1) over bar 1 0] and [1 1 0] directions. We confirmed. This anisotropy seems to br originated fi om the different undulation period between[(1) over bar 1 0] and [1 1 0] directions. We theoretically calculated electron mobility taking both alloy disorder scattering and background impurity scattering into account. It is found that the calculated and experimental values are in good agreement. (C) 2001 Elsevier Science B.V. All rights reserved.

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    18
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  • Large spontaneous spin splitting in gate-controlled two-dimensional electron gases at normal In0.75Ga0.25As/In0.75Al0.25As heterojunctions

    Y Sato, T Kita, S Gozu, S Yamada

    JOURNAL OF APPLIED PHYSICS   89 ( 12 ) 8017 - 8021  2001.06  [Refereed]

     View Summary

    Amounts of spontaneous spin splittings were estimated from low-temperature magnetoresistances in two-dimensional electron gases created at In0.75Ga0.25As/In0.75Al0.25As heterojunctions under a gate bias. Typical sheet electron densities and mobilities in the raw wafers were similar to1.0 x 10(12)/cm(2) and 2-5 x 10(5) cm(2)/V s at 1.5 K, respectively. A maximum spin-orbit coupling constant alpha (zero) of similar to 30(x10(-12) eV m) was obtained for the van der Pauw sample. In gated Hall-bar samples, a decrease in the alpha (zero) value with decreasing gate voltage (V-g) was first confirmed in a normal heterojunction. The main origin for such a large alpha (zero), which is a few times larger than any previously reported, was found to be a structure-dependent so-called interface contribution in the Rashba term. (C) 2001 American Institute of Physics.

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  • Large spontaneous spin-splitting and enhanced effective g-factor in two-dimensional electron gases at In0.75Ga0.25As/In0.75Al0.25As metamorphic heterojunctions

    T Kita, Y Sato, S Gozu, S Yamada

    PHYSICA B   298 ( 1-4 ) 65 - 69  2001.04  [Refereed]

     View Summary

    We have estimated spin-splitting energy of two-dimensional electron gases formed at metamorphic In0.75Ga0.25As/In0.75Al0.25As heterojunctions by analyzing magneto-resistance traces up to 10 T. This heterojunction often reveals a large spontaneous spin-splitting at low fields, while at high fields, Zeeman splitting becomes dominant. We have investigated and compared the two sanples; one reveals beating oscillation and hence has a spontaneous or zero-field spin-splitting and another does not. In the former sample, if we assume the equal sign for both the spin-splittings, the dependency of the absolute splitting energy on the magnetic field is found to have a minimum (Delta = 2 meV) at about 3.5 T, in which we also confirmed a zero-field spin-splitting of Delta (0) = 11.04meV and very much enhanced g* = - 42.0 at 1.5 T. In contrast, effective g-factor (g*) obtained at high fields in the latter sample is g* = - 7.7 at 1.5 T, which is fairly smaller than that in the former. Origins of those features are discussed and the possibility of linear spin-splitting dependency on magnetic field is pointed out. (C) 2001 Published by Elsevier Science B.V.

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    (Scopus)
  • Residual strain dependence of zero field spin-splitting in high indium content metamorphic InGaAs/InAlAs modulation doped heterostructures

    S Gozu, T Kita, H Sato, Y Sato, K Fujii, T Ohyama, S Yamada

    PROCEEDINGS OF THE 10TH INTERNATIONAL CONFERENCE ON NARROW GAP SEMICONDUCTORS AND RELATED SMALL ENERGY PHENOMENA, PHYSICS AND APPLICATIONS   2   42 - 44  2001  [Refereed]

     View Summary

    In this work, we are studying the effects of interface strain on the amount of spin-orbit coupling constant of the two-dimensional electron gas formed at the heterointerface, high In-content InGaAs/InAlAs. Tuning of the interface strain is carried out by changing the step-graded buffer (SGB) layer with additional inverse steps (ISs). Estimations of photoluminescence, far-infrared absorption and magnetoresistance measurement suggest that, although a slight extent of residual strain still remains in a case of normal SGB, the residual strain is found to decrease, if an appropriate IS layer is adopted, Associating with this, the decreases of the band gap and electron effective mass were observed together with the increase of the spin-orbit coupling constant.

  • Electrical spin-injection into a two-dimensional electron gas

    Y Sato, S Gozu, T Kita, S Yamada

    PROCEEDINGS OF THE 10TH INTERNATIONAL CONFERENCE ON NARROW GAP SEMICONDUCTORS AND RELATED SMALL ENERGY PHENOMENA, PHYSICS AND APPLICATIONS   2   248 - 250  2001  [Refereed]

     View Summary

    We study the electrical spin-injection and detection of spin-polarized electrons in ferromagnetic electrode - two-dimensional electron gas (2DEG) system. The 2DEG was formed at In0.75Ga0.25As/In0.75Al0.25As heterointerface and NiFe electrodes were deposited on the side surface of the mesa containing channel layer. The NiFe electrodes (width=F1:0.5, and F2:3mum) were located 1 mum apart, which was less than the mean free-path (similar to2mum) of the 2DEG, and two voltage probes were fabricated between them. We confirmed by SdH oscillations that electrons indeed transport from F1 to F2 through the 2DEG. In non-local geometry below 1.5K, we observed a spin-valve like effect of similar to0.2% in source-drain resistance of despite including the single ferromagnet/2DEG interface in the current pass. We also observed the hysterisis behavior of similar to12% in four-terminal resistance below 1.5K, Both signals were found to disappear at 200K. This temperature dependency strongly suggests that these signals could not be explained only by the local Hall effect, since the sheet electron density, N-S, in our heterostructure is almost kept constant from 0.3 to 200K.

▼display all

Presentations

  • シリコンフォトニクスチップを外部共振器とした機能性レーザの開発

    電子情報通信学会 

    Presentation date: 2017.09

  • Heterogeneous Lasers with Silicon Photonic External Cavity

    Tomohiro Kita, Shotaro Takei, Atsushi Matsumoto, Naokatsu Yamamoto, Hirohito Yamada

    応用物理学会ーOSA joint symposia 

    Presentation date: 2017.09

  • Silicon Photonic Wavelength Tunable Lasers for high-capacity optical communication system

    Optical Fiber Communication Conference and Exhibition (OFC) 2017  (United States Los Angeles) 

    Presentation date: 2017.03

  • Heterogeneous silicon photonic wavelength tunable laser diodes

    The 6th Annual World Congress of Nano Science and Technology-2016(Nano S&T-2016)  (Singapore) 

    Presentation date: 2016.10

  • Ultra-compact Wavelength Tunable Quantum Dot Laser with Silicon Photonic External Cavity

    Tomohiro Kita, Naokatsu Yamamoto, Tetsuya Kawanishi, Hirohito Yamada

    The Conference on Lasers and Electro-Optics(CLEO)  (United States サンノゼ) 

    Presentation date: 2015.05

  • InAs 量子ドット光増幅器を用いた1.2μm 帯シリコンフォトニック波長可変レーザ

    北 智洋, 山本 直克, 川西 哲也, 山田 博仁

    第62回応用物理学会春季学術講演会  (平塚) 

    Presentation date: 2015.03

  • TiO2微粒子ポリマを用いたシリコン細線光導波路のアサーマル化

    第62回応用物理学会春季学術講演会  (平塚市) 

    Presentation date: 2015.03

  • ハイブリッド集積シコンフォトニクスリングフィルタ外部共振器型波長可変レーザの狭線幅及び高出力特性

    佐藤 健二, 小林 直樹, 波若 雅彦, 山本 圭介, 渡邊 真也, 北 智洋, 山田 博仁, 山崎 裕幸

    電子情報通信学会 OPE12月研究会 

    Presentation date: 2014.12

  • Silicon Photonic Devices for a Large Capacity Optical Communication system

    Tomohiro Kita, Tang Rui, Munetoshi Soma, Hirohito Yamada

    The 4th International Symposium on Photonics and Electronics Convergence  (JAPAN 東京) 

    Presentation date: 2014.11

  • ATHERMAL SILICON PHOTONIC DEVICES USING HYBRID POLYMER CLADDING

    Freddy Susanto Tan, Okihiro Sugihara, Hirohito Yamada, Toshikuni Kaino

    The 23rd International Conference on Plastic OpticalFiber  (JAPAN 日吉) 

    Presentation date: 2014.10

  • RECENT PROGRESS OF OPTICAL INTERCONNECT DEVICES USING PHOTONIC POLYMERS

    Sugihara, T. Kita, D. Inoue, F. S. Tan, A. Hattori, A. Kawasaki, M. Sato, T. Yamashita, M. Mizuno, S. Okada, M. Kagami, M.Tsuchimori, O. Watanabe, H. Yamada, T. Kaino

    The 23rd International Conference on Plastic Optical Fibers  (JAPAN 日吉) 

    Presentation date: 2014.10

  • 高出力シリコンフォトニック波長可変レーザ

    唐 睿, 山田博仁

    電子情報通信学会2014年ソサイエティ大会  (JAPAN 徳島) 

    Presentation date: 2014.09

  • 広波長可変帯域を持つシリコンフォトニックレーザ

    唐 睿, 山田 博仁

    第75回応用物理学秋季学術講演会  (JAPAN 札幌) 

    Presentation date: 2014.09

  • 1.Wide-band Wavelength Tunable Laser Diode with Si Photonic Filter

    Tang Rui, Hirohito Yamada

    IEEE International Conference of Semiconductor Laser  (Spain パルマ) 

    Presentation date: 2014.09

  • 3. Si Photonic Wavelength Tunable Laser Diode with the Phase Shifter of Longitudinal Mode

    Tang Rui, Hirohito Yamada

    IEEE Group ⅣPhotonics  (France パリ) 

    Presentation date: 2014.08

  • 狭線幅シリコンフォトニック波長可変レーザモジュール

    唐 睿, 山田博仁

    電子情報通信学会LQE研究会  (JAPAN 小樽) 

    Presentation date: 2014.08

  • 高非線形TiO2光導波路による広波長帯域光発生

    内島晃司, 藪野正裕, 逢坂 崇, 片山竜二, 枝松圭一, 山田博仁

    電子情報通信学会LQE研究会  (JAPAN 東京) 

    Presentation date: 2014.06

  • デジタルコヒーレント通信用シリコンフォトニック波長可変レーザ

    フォトニックデバイス・応用技術研究会  (JAPAN 東京) 

    Presentation date: 2014.05

  • シリコンフォトニック波長可変レーザ

    唐 睿, 根本景太, 山田博仁

    電子情報通信学会2014年総合大会  (JAPAN 新潟) 

    Presentation date: 2014.03

▼display all

Research Projects

  • On-chip optical synthesizer

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2019.04
    -
    2022.03
     

    Tomohiro Kita

     View Summary

    Heterogeneous multi-wavelength combining a quantum dot optical amplifier and a lightwave control silicon photonics chip was fabricated. It was shown that the output light from a two-wavelength tunable heterogeneous laser can be converted into an electrical signal by a photodetector to output millimeter waves in a very wide band of 20 to 400 GHz.

  • Silicon photonic wavelength tunable laser diode for next generation optical communication systems

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2013.04
    -
    2016.03
     

    Kita Tomohiro, TANUSHI YUICHIRO

     View Summary

    Ultra compact wavelength tunable laser diode with a semiconductor optical amplifier and a silicon photonics wavelength tunable filter was developed. A 99 nm wide wavelength tunable range, less than 15 kHz narrow spectral linewidth and more than 40 mW high output power were respectively achieved. The 1.2 um wavelength tunable laser with quantum dot optical amplifier was also developed.

  • Research on hybrid Si/Silica waveguide integrated circuits

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2011.04
    -
    2014.03
     

    YAMADA Hirohito, OTERA Yasuo, KITA Tomohiro, TANUSHI Yuichiro

     View Summary

    We studied optical devices or optical integrated circuits with silicon and silica materials. We proposed hybrid silicon/silica integrated optical circuits, and demonstrated the excellent performance by both theoretical analysis and experiments. We also studied grating couplers that input/output optical signal to/from the circuits, and derived the design for compact and high coupling efficiency. We further studied the method of flip-chip mounting of laser diodes on the circuits, and demonstrated a tunable wavelength laser with the method.

  • Broad wavelength light generation using by high optical nonlinear waveguides

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2011
    -
    2012
     

    KITA Tomohiro

     View Summary

    We studied about the high optical nonlinear waveguides fabricated by ZnO and TiO_2 thin films to broaden wavelength range of propagation light. We observed the enhancement of self-phase modulation in channel waveguides due to strong light confinement.

  • Light control using photonic elements

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2008
    -
    2009
     

    KITA Tomohiro

     View Summary

    We studied about sub-micron photonic element. The characteristics of wavelength filter and spot size converter of photonic element was verified by numerical analysis. We found the unusual optical mode in the Bragg-grating filter of Si photonic-wire waveguide. We fabricated the spot size converter for the low loss connection between the Si ware and the Si slot waveguide. We also studied about ZnO photonic wire waveguide.

Misc

  • Tunable Dual-Wavelength Heterogeneous Quantum Dot Laser Diode with a Silicon External Cavity

    Tomohiro Kita, Atsushi Matsumoto, Naokatsu Yamamoto, Hirohito Yamada

    Journal of Lightwave Technology   36 ( 2 ) 219 - 224  2018.01

     View Summary

    We propose a tunable dual-wavelength heterogeneous quantum dot laser diode. The tunable dual-wavelength laser consists of a quantum dot semiconductor optical amplifier as the optical gain medium and an external cavity fabricated from silicon photonics technology as the wavelength tunable filter. We successfully demonstrated dual-wavelength lasing oscillation by tuning the difference frequency from approximately 34 to 400 GHz.

    DOI

  • Demonstration of 1-μm-band Si-photonics-based quantum dot heterogeneous tunable laser

    A. Matsumoto, K. Akahane, T. Umezawa, N. Yamamoto, H. Yamada, T. Kita

    Optics InfoBase Conference Papers   2018  2018

     View Summary

    We present a Si-photonics-based quantum dot heterogeneous tunable lasers in the 1-μm band. Optical devices using Si-photonics-based PIC in the 1-μm band probably have not been reported yet.

    DOI

  • Silicon photonic wavelength tunable laser diode with low loss direct heating phase shifter

    Tomohiro Kita, Yuto Chiba, Hirohito Yamada

    14th International Conference on Group IV Photonics, GFP 2017     125 - 126  2017.10

     View Summary

    We proposed and demonstrated a compact and low propagation loss thermo-optical phase shifter using a multi-mode interference. The calculated insertion loss of designed MMI phase shifter was approximately 0.05 dB. The fabricated phase shifter shows high speed and low power consumption. The wavelength tunable laser diode with two ring resonators loaded the MMI phase shifters was fabricated. The wavelength tuning operation with low power consumption was successfully demonstrated.

    DOI

  • Heterogeneous lasers with silicon photonic external cavity

    Tomohiro Kita, Shotaro Takei, Atsushi Matsumoto, Naokatsu Yamamoto, Hirohito Yamada

    Optics InfoBase Conference Papers   Part F78-JSAP 2017  2017.01

  • Differential frequency tunable Dual-Mode heterogeneous QD laser with Si PIC

    A. Matsumoto, T. Umezawa, K. Akahane, N. Yamamoto, H. Yamada, T. Kita

    Optics InfoBase Conference Papers   2017  2017

     View Summary

    We proposed a tunable dual-mode heterogeneous quantum dot laser diode with a Siphotonics-based photonic integrated circuit, and successfully demonstrated dual-mode lasing oscillation by tuning the differential frequency from approximately 20 GHz to 200 GHz.

    DOI

  • Silicon Photonic Wavelength-Tunable Lasers for High-Capacity Optical Communication Systems

    Tomohiro Kita, Hiroyuki Yamazaki, Naokatsu Yamamoto, Hirohito Yamada

    2017 OPTICAL FIBER COMMUNICATIONS CONFERENCE AND EXHIBITION (OFC)   Part F40-OFC 2017  2017

     View Summary

    Silicon photonic wavelength-tunable laser diodes consist of a wavelength-tunable filter with double silicon ring resonators and a compound semiconductor optical amplifier. Narrow spectral linewidth lasers for digital coherent optical communication systems and quantum dot heterogeneous laser diodes were demonstrated. The combination of silicon photonics and compound semiconductor technology leads to a breakthrough in the development of effective and compact integrated light sources for a wide range of application fields.

    DOI

  • Quantum-dot-based advanced photonic devices and its applications

    A. Matsumoto, K. Akahane, T. Umezawa, T. Kita, K. Utaka, N. Yamamoto

    Optics InfoBase Conference Papers   2017  2017

     View Summary

    Quantum dot (QD) is one of the attractive materials. In this paper, we focus on advanced photonic devices using QD structure and its applications for the next generation access networks.

    DOI

  • Silicon Photonic Wavelength-Tunable Lasers for High-Capacity Optical Communication Systems

    Tomohiro Kita, Hiroyuki Yamazaki, Naokatsu Yamamoto, Hirohito Yamada

    2017 OPTICAL FIBER COMMUNICATIONS CONFERENCE AND EXHIBITION (OFC)   Part F40-OFC 2017  2017

     View Summary

    Silicon photonic wavelength-tunable laser diodes consist of a wavelength-tunable filter with double silicon ring resonators and a compound semiconductor optical amplifier. Narrow spectral linewidth lasers for digital coherent optical communication systems and quantum dot heterogeneous laser diodes were demonstrated. The combination of silicon photonics and compound semiconductor technology leads to a breakthrough in the development of effective and compact integrated light sources for a wide range of application fields.

    DOI

  • Quantum-dot-based advanced photonic devices and its applications

    A. Matsumoto, K. Akahane, T. Umezawa, T. Kita, K. Utaka, N. Yamamoto

    Optics InfoBase Conference Papers   2017  2017

     View Summary

    Quantum dot (QD) is one of the attractive materials. In this paper, we focus on advanced photonic devices using QD structure and its applications for the next generation access networks.

    DOI

  • Differential frequency tunable Dual-Mode heterogeneous QD laser with Si PIC

    A. Matsumoto, T. Umezawa, K. Akahane, N. Yamamoto, H. Yamada, T. Kita

    Optics InfoBase Conference Papers   2017   1 - 2  2017

     View Summary

    We proposed a tunable dual-mode heterogeneous quantum dot laser diode with a Siphotonics-based photonic integrated circuit, and successfully demonstrated dual-mode lasing oscillation by tuning the differential frequency from approximately 20 GHz to 200 GHz.

    DOI

  • Instability of silicon photonic wavelength tunable laser diodes due to the nonlinear optical effect of silicon

    Tomohiro Kita, Hirohito Yamada

    Conference Digest - IEEE International Semiconductor Laser Conference    2016.12

     View Summary

    © 2016 IEICE-ES. The silicon photonic wavelength tunable laser diode is a good candidate for the integrated light source of the digital coherent optical communication system.However, optical nonlinear effects easily occur due to the strong light confinement of Si photonic wire waveguides. The impact on laser operation caused by nonlinearity was discussed.

  • Narrow Spectral Linewidth Silicon Photonic Wavelength Tunable Laser Diode for Digital Coherent Communication System

    Tomohiro Kita, Rui Tang, Hirohito Yamada

    IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS   22 ( 6 )  2016.11

     View Summary

    We demonstrated wavelength tunable laser diodes with Si-wire waveguide ring resonators as an external optical cavity. The footprint of the optical cavity including the semiconductor optical amplifier is as small as 0.8 mm x 0.6 mm which is less 1/10 of those made of silicon oxinitride material. Wavelength tuning range of approximately 55 nm was demonstrated, which covers entire L-band of the optical communication wavelength system. Furthermore, a spectral linewidth narrower than 100 kHz was obtained by optimizing the design of the external optical cavity. The optical nonlinearity of silicon is described and the influence on laser oscillation is also discussed.

    DOI

  • Heterogeneous quantum dot/silicon photonics-based wavelength-tunable laser diode with a 44 nm wavelength-tuning range

    Tomohiro Kita, Naokatsu Yamamoto, Atsushi Matsumoto, Tetsuya Kawanishi, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   55 ( 4 )  2016.04

     View Summary

    A heterogeneous wavelength-tunable laser diode combining quantum dot and silicon photonics technologies is proposed. A compact wavelength-tunable filter with two ring resonators was carefully designed and fabricated using silicon photonics technology. The tunable laser combining the wavelength-tunable filter and an optical amplifier, which includes InAs quantum dots, achieved a 44.0 nm wavelength-tuning range at around 1250 nm. The broadband optical gain of the quantum dot optical amplifier was effectively used by the optimized wavelength-tunable filter. This heterogeneous wavelength-tunable laser diode could become a breakthrough technology for high-capacity data transmission systems. (C) 2016 The Japan Society of Applied Physics

    DOI

  • Advanced photonic ICT devices and their system applications using quantum-dot technology

    Naokatsu Yamamoto, Kouichi Akahane, Toshimasa Umezawa, Atsushi Matsumoto, Atsushi Kanno, Tetsuya Kawanishi, Tetsuya Kawanishi, Tomohiro Kita, Hirohito Yamada

    Asia Communications and Photonics Conference, ACPC 2015    2015.12

     View Summary

    Quantum-dot nanotechnology is attractive for use in advanced photonic devices that will augment the available optical-frequency resources and will increase the number of wavelength channels usable by wired and wireless networks in short/middle-range communication systems. © 2015 OSA.

    DOI

  • Ultra-compact wavelength-tunable quantum-dot laser with silicon-photonics double ring filter

    Tomohiro Kita, Naokatsu Yamamoto, Tetsuya Kawanishi, Hirohito Yamada

    APPLIED PHYSICS EXPRESS   8 ( 6 )  2015.06

     View Summary

    Compact, wavelength-tunable light sources are desired for the enhancement of information communication technology and bio-imaging applications. We propose a compact, wavelength-tunable laser diode with a wide wavelength-tunable range around 1230 nm consisting of a quantum-dot optical amplifier and a silicon photonic tunable filter. High-quality InAs quantum dots grown with the sandwiched sub-nano separator technique were used as the optical gain medium. The wavelength-tunable filter was constructed with ring resonators fabricated using silicon photonics. The single-mode laser oscillation was demonstrated with a 28.5-nm wavelength-tunable range. (C) 2015 The Japan Society of Applied Physics

    DOI

  • シリコンフォトニクスを用いたコヒーレント光通信用狭線幅波長可変レーザの開発

    北 智浩

    光技術コンタクト2015年4月号   53 ( 4 ) 13 - 19  2015.04

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

    CiNii

  • Narrow-spectral-linewidth silicon photonic wavelength-tunable laser with highly asymmetric Mach-Zehnder interferometer

    Rui Tang, Tomohiro Kita, Hirohito Yamada

    OPTICS LETTERS   40 ( 7 ) 1504 - 1507  2015.04

     View Summary

    We propose a narrow-spectral-linewidth silicon photonic wavelength-tunable laser with a novel external wavelength-tunable filter, which consists of two silicon ring resonators with different circumferences and a highly asymmetric Mach-Zehnder interferometer (MZI), the two optical paths of which have significantly different lengths. Calculations and experimental results indicated that the gain difference between longitudinal modes was increased by the highly asymmetric MZI. Consequently, a narrow spectral linewidth of 12 kHz and a stable single-mode oscillation were obtained. (C) 2015 Optical Society of America

    DOI

  • Optimum waveguide-core size for reducing device property distribution of Si-wire waveguide devices

    Munetoshi Soma, Tomohiro Kita, Yuichiro Tanushi, Munehiro Toyama, Miyoshi Seki, Nobuyuki Yokoyama, Minoru Ohtsuka, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   54 ( 4 )  2015.04

     View Summary

    We investigated the waveguide-core size distribution of ring resonators fabricated on a 300mm silicon-on-insulator (SOI) wafer using a CMOS-compatible process featuring ArF immersion lithography. These ring resonators were constructed in a Si-wire waveguide with a standard core size of 400nm width and 220nm height. The group refractive indices of the waveguide were derived from the transmission spectra of the ring resonators. From the deviation of these group refractive indices, the waveguide-core width distribution was estimated to be 5 nm, and the waveguide-core height distribution was estimated to be 1 nm. Moreover, the device property distribution of various Si-wire waveguide depended on the estimated fabrication error was calculated. The waveguide core with the smallest device property distribution had a 540nm width and a 160nm height, and this waveguide has a device property distribution of 2/3 value compared with the standard core size. (C) 2015 The Japan Society of Applied Physics

    DOI

  • C-3-25 Narrow Spectral Linewidth Silicon Photonic Wavelength Tunable Laser with Highly Asymmetric Mach-Zehnder Interferometer

    Tang Rui, Kita Tomohiro, Yamada Hirohito

    Proceedings of the IEICE General Conference   2015 ( 1 ) 149 - 149  2015.02

    CiNii

  • Ultra-compact Wavelength Tunable Quantum Dot Laser with Silicon Photonic External Cavity

    Tomohiro Kita, Naokatsu Yamamoto, Tetsuya Kawanishi, Hirohito Yamada

    2015 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO)     2267  2015

     View Summary

    Ultra-compact wavelength tunable laser diode with wide tunability was successfully developed with combining quantum dot optical amplifier and silicon micro-ring filters. The single mode laser oscillation was demonstrated with 25 nm wavelength tuning range.

    DOI

  • Ultra-compact Wavelength Tunable Quantum Dot Laser with Silicon Photonic External Cavity

    Tomohiro Kita, Naokatsu Yamamoto, Tetsuya Kawanishi, Hirohito Yamada

    2015 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO)   2015-August   2267  2015

     View Summary

    Ultra-compact wavelength tunable laser diode with wide tunability was successfully developed with combining quantum dot optical amplifier and silicon micro-ring filters. The single mode laser oscillation was demonstrated with 25 nm wavelength tuning range.

    DOI

  • Demonstration of Silicon Photonic Hybrid Ring-Filter External Cavity Wavelength Tunable Lasers

    K. Sato, N. Kobayashi, M. Namiwaka, K. Yamamoto, S. Watanabe, T. Kita, H. Yamada, H. Yamazaki

    ECOC 2015 41ST EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION   2015-November  2015

     View Summary

    Silicon-photonic hybrid ring-filter external cavity (SHREC) tunable lasers fabricated by passive alignment techniques are demonstrated. High power, narrow linewidth and highly reliable characteristics are achieved thanks to drastically reduced losses in the laser cavity toward commercialization.

    DOI

  • Silicon Photonic Wavelength Tunable Laser with Highly Asymmetric Mach-Zehnder Interferometer

    Tang Rui, Kita Tomohiro, Yamada Hirohito

    Technical report of IEICE. OPE   114 ( 377 ) 7 - 10  2014.12

     View Summary

    Utilizing tunable silicon ring resonator filter as an external resonator, the wavelength tunable laser has demonstrated excellent wavelength tunability and less than 100 kHz narrow spectral linewidth, thus is promising to be applied to digital coherent optical communication systems. In order to apply to higher-order modulation formats, further narrower spectral linewidth is desired. However, with the previous filter structure, because of the trade-off relationship between spectral linewidth and gain difference of longitudinal modes, it is difficult to obtain both narrower spectral linewidth and stable single mode oscillation. In this paper, we demonstrate that by utilizing a highly asymmetric Mach-Zehnder interferometer which has a very long optical delay line, the newly designed filter can have both long cavity length and large longitudinal mode gain difference. From our experimental results, 49 dB high side mode suppression ratio(SMSR) and about 12 kHz narrow spectral linewidth were confirmed.

    CiNii

  • Silicon Photonic Wavelength Tunable Laser with Highly Asymmetric Mach-Zehnder Interferometer

    Tang Rui, Kita Tomohiro, Yamada Hirohito

    Technical report of IEICE. LQE   114 ( 378 ) 7 - 10  2014.12

     View Summary

    Utilizing tunable silicon ring resonator filter as an external resonator, the wavelength tunable laser has demonstrated excellent wavelength tunability and less than 100 kHz narrow spectral linewidth, thus is promising to be applied to digital coherent optical communication systems. In order to apply to higher-order modulation formats, further narrower spectral linewidth is desired. However, with the previous filter structure, because of the trade-off relationship between spectral linewidth and gain difference of longitudinal modes, it is difficult to obtain both narrower spectral linewidth and stable single mode oscillation. In this paper, we demonstrate that by utilizing a highly asymmetric Mach-Zehnder interferometer which has a very long optical delay line, the newly designed filter can have both long cavity length and large longitudinal mode gain difference. From our experimental results, 49 dB high side mode suppression ratio(SMSR) and about 12 kHz narrow spectral linewidth were confirmed.

    CiNii

  • Wide-band wavelength tunable laser diode with si photonic filter

    Tomohiro Kita, Rui Tang, Hirohito Yamada

    Conference Digest - IEEE International Semiconductor Laser Conference     149 - 150  2014.12

     View Summary

    We investigated the wide-band wavelength tuning operation to increase the possibility of silicon photonic wavelength tunable laser diodes which is composed semiconductor optical amplifier and silicon photonic waveguide filter. The wide-band wavelength tuning operation of 94.1 nm was demonstrated.

    DOI

  • Narrow spectral linewidth Si photonic wavelength tunable laser diode for digital coherent optical communication

    Rui Tang, Tomohiro Kita, Hirohito Yamada

    Conference Digest - IEEE International Semiconductor Laser Conference     96 - 97  2014.12

     View Summary

    A compact wavelength tunable laser diode for digital coherent optical communication was demonstrated with a wavelength tunable filter consisting of silicon photonic wire waveguides. Stable single-mode operation with 17.8 dBm maximum output power as well as less than 100 kHz spectral line width were obtained.

    DOI

  • C-3-24 Narrow Spectral Linewidth and High Output Power Silicon Photonic Wavelength Tunable Laser Diode

    Kita Tomohiro, Tang Rui, Yamada Hirohito

    Proceedings of the Society Conference of IEICE   2014 ( 1 ) 121 - 121  2014.09

    CiNii

  • Narrow spectral linewidth Silicon photonic wavelength tunable laser diode module

    Kita Tomohiro, Tang Rui, Yamada Hirohito

    IEICE technical report. Component parts and materials   114 ( 185 ) 83 - 86  2014.08

     View Summary

    We fabricated the narrow spectral linewidth wavelength tunable laser diodes with Si photonic wire waveguide ring resonators as an external optical cavity. The footprint of the optical cavity including the semiconductor optical amplifier is 2.6 mm × 0.5 mm, which is about 1/9 that of those made of SiON waveguide. More than 40 mW optical output power, wide wavelength range tuning operation which covers C-band or L-band of optical communication wavelength range and less than 100 kHz narrow spectral linewidth were obtained by optimizing design of the external optical cavity. The 1-chip wavelength tunable laser diode was fabricated by flip-chip bonding technique.

    CiNii

  • Wide Band Wavelength Generation in Nonlinear TiO_2 Waveguide

    Kita Tomohiro, Uchijima Koji, Yabuno Masahiro, Aisaka Takashi, Katayama Ryuji, Edamatsu Keiichi, Yamada Hirohito

    Technical report of IEICE. OPE   114 ( 97 ) 11 - 14  2014.06

     View Summary

    We studied the nonlinear optical waveguide with TiO_2 core which has large nonlinear optical constant and wide band-gap energy. We fabricated the TiO_2 channel waveguides and the spectral broadening due to self phase modulation was observed. The ultra wide band wavelength generation was also demonstrated in TiO_2 waveguides by using numerical analysis.

    CiNii

  • Long external cavity Si photonic wavelength tunable laser diode

    Tomohiro Kita, Keita Nemoto, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   53 ( 4 )  2014.04

     View Summary

    We fabricated wavelength-tunable laser diodes with external cavity consist of Si photonic wire waveguide ring resonators. About 51.5 nm wavelength tuning operation, which covers the entire L-band of the optical communication wavelength range, was obtained. The cavity length dependence of the spectral line width was verified to obtain narrower spectral linewidth. The observed spectral linewidth of 3.78 mm long cavity and 8.19 mm long cavity are 131.0 and 64.8 kHz, respectively. The advantage of long external cavity to design the narrow spectral linewidth Si photonic wavelength tunable laser diode was confirmed. (C) 2014 The Japan Society of Applied Physics

    DOI

  • CI-2-3 Optical Coupler in Siliconphotonics

    Yamada Hirohito, Nara Masaki, Kita Tomohiro

    Proceedings of the IEICE General Conference   2014 ( 1 ) "SS - 13"-"SS-14"  2014.03

    CiNii

  • C-3-79 Si-wire Waveguide Ring Resonator Type Tunable Laser Diodes with Phase Control Mechanism

    Tang Rui, Kita Tomohiro, Nemoto Keita, Sawada Nobuyuki, Yamada Hirohito

    Proceedings of the IEICE General Conference   2014 ( 1 ) 234 - 234  2014.03

    CiNii

  • C-4-18 Silicon Photonic Wavelength Tunable Laser Diode

    Kita Tomohiro, Tang Rui, Nemoto Keita, Yamada Hirohito

    Proceedings of the IEICE General Conference   2014 ( 1 ) 254 - 254  2014.03

    CiNii

  • High Output Power and Narrow Linewidth Silicon Photonic Hybrid Ring-Filter External Cavity Wavelength Tunable Lasers

    K. Sato, N. Kobayashi, M. Namiwaka, K. Yamamoto, T. Kita, H. Yamada, H. Yamazaki

    2014 EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION (ECOC)    2014

     View Summary

    Si-photonic Hybrid Ring-filter External Cavity (SHREC) wavelength tunable lasers by passive alignment techniques with over 100-mW fiber-coupled power and linewidth narrower than 15 kHz along the whole C-band are demonstrated. Obtained results show excellent features of Si-photonics towards commercial products.

    DOI

  • Athermal Silicon photonic devices using hybrid polymer cladding

    Tomohiro Kita, Freddy Susanto Tan, Okihiro Sugihara, Hirohito Yamada, Toshikuni Kaino

    POF 2014 - 23rd International Conference on Plastic Optical Fibers, Proceedings     272 - 275  2014.01

     View Summary

    © 2014 ICPOF. The compensation of sensitivity for ambient temperature is big issue to realize Silicon photonic integrated circuits. The large positive thermo-optic coefficient of Silicon core was cancelled by the negative thermooptic coefficient of hybrid polymer cladding including rutile TiO2nanoparticles. The athermal waveguide structure with Silicon core and Hybrid polymer cladding was designed by numerical calculation.

  • Si Photonic Wavelength Tunable Laser Diode with the Phase Shifter of Longitudinal Mode

    Tomohiro Kita, Rui Tang, Keita Nemoto, Hirohito Yamada

    2014 IEEE 11TH INTERNATIONAL CONFERENCE ON GROUP IV PHOTONICS (GFP)     209 - 210  2014

     View Summary

    We fabricated wavelength tunable laser diodes with Si photonic wire waveguide ring resonators as an external optical cavity. The continuous wavelength tuning operation was demonstrated using by the micro heaters. Our wavelength tunable laser diode is suitable for digital coherent optical transmission systems based on wavelength division multiplexing.

    DOI

  • Recent progress of optical interconnect devices using photonic polymers

    O. Sugihara, T. Kita, D. Inoue, F. S. Tan, A. Hattori, A. Kawasaki, M. Sato, T. Yamashita, M. Mizuno, S. Okada, M. Kagami, M. Tsuchimori, O. Watanabe, H. Yamada, T. Kaino

    POF 2014 - 23rd International Conference on Plastic Optical Fibers, Proceedings     87  2014.01

     View Summary

    © 2014 ICPOF. We present recent progress of photonic polymers and waveguide-type device fabrication for high performance optical integrated circuit modules.

  • Si photonic wavelength tunable laser diode for digital coherent optical communication

    T. Kita, K. Nemoto, K. Watanabe, H. Yamazaki, H. Yamada

    2013 18th OptoElectronics and Communications Conference Held Jointly with 2013 International Conference on Photonics in Switching, OECC/PS 2013    2013.10

     View Summary

    We fabricated wavelength tunable laser diodes with Si photonic wire waveguide ring resonators as an external optical cavity. Less than 100 kHz narrow spectral linewidth was obtained with small footprint. Our wavelength tunable laser diodes with narrow spectral linewidth are promising light source for practical use in digital coherent optical transmission systems. © 2013 IEICE.

  • Fabrication of Narrow Spectral Linewidth Wavelength Tunable Laser Diode with Si Photonic Wire Waveguide

    KITA Tomohiro, NEMOTO Keita, YAMADA Hirohito

    Technical report of IEICE. LQE   113 ( 264 ) 85 - 88  2013.10

     View Summary

    We fabricated the narrow spectral line width wavelength tunable laser diodes with Si photonic wire waveguide ring resonators as an external optical cavity. The footprint of the optical cavity including the semiconductor optical amplifier is 2.6mm×0.5mm, which is about 1/9 that of those made of SiON waveguide. About 34.9 mW optical output power, 67.6 nm wavelength tuning operation which covers entire L-band of optical communication wavelength range and less than 100 kHz narrow spectral line width were obtained by optimizing design of the external optical cavity.

    CiNii

  • Fabrication of Narrow Spectral Linewidth Wavelength Tunable Laser Diode with Si Photonic Wire Waveguide

    KITA Tomohiro, NEMOTO Keita, YAMADA Hirohito

    Technical report of IEICE. OPE   113 ( 263 ) 85 - 88  2013.10

     View Summary

    We fabricated the narrow spectral line width wavelength tunable laser diodes with Si photonic wire waveguide ring resonators as an external optical cavity. The footprint of the optical cavity including the semiconductor optical amplifier is 2.6mm×0.5mm, which is about 1/9 that of those made of SiON waveguide. About 34.9 mW optical output power, 67.6 nm wavelength tuning operation which covers entire L-band of optical communication wavelength range and less than 100 kHz narrow spectral line width were obtained by optimizing design of the external optical cavity.

    CiNii

  • Narrow spectral linewidth wavelength tunable laser diode with Si-wire waveguide ring resonators and a MZI filter

    Nemoto Keita, Kita Tomohiro, Yamada Hirohito

    Proceedings of the Society Conference of IEICE   2013 ( 1 ) 172 - 172  2013.09

    CiNii

  • Silicon photonics devices on 300 mm wafer fabricated by using ArF immersion lithography

    KITA Tomohiro, TANUSHI Yuichiro, NARA Masaki, HIRANO Syu, TOYAMA Munehiro, SEKI Miyoshi, KOSHINO Keiji, YOKOYAMA Nobuyuki, OHTSUKA Minoru, SUGIYAMA Akinobu, ISHITSUKA Eiichi, SANO Tsukuru, HORIKAWA Tsuyoshi, YAMADA Hirohito

    Technical report of IEICE. LQE   113 ( 100 ) 1 - 5  2013.06

     View Summary

    We have investigated optical characteristics of Si photonics devices formed on 300 mm SOI wafers by using ArF immersion lithography process. The measurement results were in good agreement with the calculated results for designed structure and the process accuracy for silicon photonics devices were verified.

    CiNii

  • Nonlinear optical photonic crystal waveguide with TiO2 material

    Koji Uchijima, Tomohiro Kita, Hirohito Yamada

    OXIDE-BASED MATERIALS AND DEVICES IV   8626  2013

     View Summary

    Titanium dioxide (TiO2) has several attractive properties such as high linear and nonlinear refractive indices and the wide bandgap for applying the nonlinear optical devices. We have been studied channel waveguides with TiO2 as the submicron size core to verify their feasibility as the nonlinear optical devices. In this study, we analyzed optical propagation in the TiO2 photonic crystal waveguide and demonstrated the enhanced optical nonlinearity due to slow light effect.

    DOI

  • Anomalous localization modes in Bragg-grating based on high index-difference waveguide

    Tomohiro Kita, Koji Uchijima, Hirohito Yamada

    SILICON PHOTONICS VIII   8629  2013

     View Summary

    We observed photonic band-gap disappearance on Bragg-grating wavelength filters with Si photonic-wire waveguides, and studied the physical mechanism with theoretical and numerical analyses. This is a unique phenomenon observed in channel waveguides with very high index-contrast between the waveguide core and cladding materials. The photonic band-gap disappearance was observed in structures where two different optical field distributions of standing wave degenerate.

    DOI

  • Narrow spectral linewidth and high output power Si photonic wavelength tunable laser diode

    Tomohiro Kita, Keita Nemoto, Hirohito Yamada

    IEEE International Conference on Group IV Photonics GFP     152 - 153  2013

     View Summary

    We fabricated wavelength tunable laser diodes with Si photonic wire waveguide ring resonators as an external optical cavity. Less than 100 kHz narrow spectral linewidth was obtained with small footprint. Our wavelength tunable laser diode shows sufficient good performance for practical use in digital coherent optical transmission systems. © 2013 IEEE.

    DOI

  • Uniform characteristics of Si-wire waveguide devices fabricated on 300 mm SOI wafers by using ArF immersion lithography

    Yuichiro Tanushi, Tomohiro Kita, Munehiro Toyama, Miyoshi Seki, Keiji Koshino, Nobuyuki Yokoyama, Minoru Ohtsuka, Akinobu Sugiyama, Eiichi Ishitsuka, Tsukuru Sano, Tsuyoshi Horikawa, Hirohito Yamada

    IEEE International Conference on Group IV Photonics GFP     105 - 106  2013

     View Summary

    We have investigated characteristics uniformity of Si-wire waveguide devices formed on 300 mm SOI wafers by using ArF immersion lithography process. Very low dispersion of group indices within wafers was confirmed from measurements of asymmetric Mach-Zhender interferometers. © 2013 IEEE.

    DOI

  • Fabrication of Narrow Spectral Linewidth Wavelength Tunable Laser Diode with Si Photonic Wire Waveguide

    KITA Tomohiro, NEMOTO Keita, YAMADA Hirohito

    IEICE technical report. Reliability   112 ( 180 ) 111 - 114  2012.08

     View Summary

    We fabricated the narrow spectral linewidth wavelength tunable laser diodes with Si photonic wire waveguide ring resonators as an external optical cavity. The footprint of the optical cavity including the semiconductor optical amplifier is 2.73 mm × 0.89 mm, which is about 1/3 that of those made of SiON waveguide. About 44 nm wavelength tuning operation which covers entire L-band of optical communication wavelength range and less than 100 kHz narrow spectral linewidth were obtained by optimizing design of the external optical cavity.

    CiNii

  • Narrow-Spectral-Linewidth Wavelength-Tunable Laser Diode with Si Wire Waveguide Ring Resonators

    Keita Nemoto, Tomohiro Kita, Hirohito Yamada

    APPLIED PHYSICS EXPRESS   5 ( 8 )  2012.08

     View Summary

    We fabricated wavelength-tunable laser diodes with Si wire waveguide ring resonators as an external optical cavity. The footprint of the optical cavity including the semiconductor optical amplifier is as small as 2.73 x 0.89 mm(2), which is about 1/5 of those made of silicon oxinitride material. About 44-nm-wavelength tuning operation, which covers the entire L-band of the optical communication wavelength range, was obtained. Furthermore, spectral linewidth narrower than 100 kHz was obtained by optimizing the design of the external optical cavity. The tunable laser diodes with narrow spectral linewidth are suitable as light sources for digital coherent optical transmission systems. (C) 2012 The Japan Society of Applied Physics

    DOI

  • Analysis of Phase Matching Conditions for Generating Second Harmonic in ZnO Channel Waveguides

    Yuta Taira, Tomohiro Kita, Edgar Yoshio Morales Teraoka, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   51 ( 4 )  2012.04

     View Summary

    We describe a phase-matching scheme for generating the second harmonic in ZnO channel waveguides using a modal phase-matching (MPM) method. We found that MPM can be achieved between the TM00 mode of the fundamental wave and the TM02 mode of the second-harmonic wave by using structural dispersion of the waveguide mode. Furthermore, we calculated the normalized conversion efficiency by taking the overlap integral of each waveguide mode and the effective area of the fundamental wave into account, and obtained a sufficiently high conversion efficiency. These results show that ZnO channel waveguides are very attractive for generating second-harmonic devices. (C) 2012 The Japan Society of Applied Physics

    DOI

  • C-3-44 Narrow spectral linewidth wavelength tunable laser diodes consisting of Si-wire waveguide ring resonators wavelength filter

    Nemoto Keita, Kita Tomohiro, Yamada Hirohito

    Proceedings of the IEICE General Conference   2012 ( 1 ) 193 - 193  2012.03

    CiNii

  • シリコン細線光導波路を用い/ T 狭線幅波長可変レーザ一の開発

    北智洋

    信学技報   112 ( 183 ) 111 - 114  2012

    CiNii

  • Narrow spectral linewidth wavelength tunable laser with Si photonic-wire waveguide ring resonators

    Keita Nemoto, Tomohiro Kita, Hirohito Yamada

    IEEE International Conference on Group IV Photonics GFP     216 - 218  2012

     View Summary

    We fabricated wavelength tunable laser diodes with Si-wire waveguide ring resonators as an external optical cavity. Less than 100 kHz narrow spectral linewidth was obtained by optimizing design of the ring resonators. The wavelength tunable laser diodes with narrow spectral linewidth are suitable as light sources for digital coherent optical transmission systems. © 2012 IEEE.

    DOI

  • C-3-48 Analysis of ring-resonator wavelength filters with silicon-wire waveguides for narrow-spectrum wavelength-tunable laser diodes

    Kikuchi Kenji, Suzuki Ken, Kita Tomohiro, Yamada Hirohito

    Proceedings of the IEICE General Conference   2011 ( 1 ) 223 - 223  2011.02

    CiNii

  • Wavelength Tunable Laser Diodes with Si-Wire Waveguide Ring Resonator Wavelength Filters

    Ken Suzuki, Tomohiro Kita, Hirohito Yamada

    SILICON PHOTONICS VI   7943  2011

     View Summary

    We fabricated wavelength tunable laser diodes consisting of ring resonator wavelength filters with Si-wire waveguides. More than 45 nm wavelength tuning operation was obtained by about 100 mW heating power. We also measured spectral linewidth of the tunable laser, and observed about 200 kHz of spectral linewidth.

    DOI

  • Variation of optical properties by the crystalline phase transition of polycrystalline silicon

    Hidenori Iwata, Tomohiro Kita, Hirohito Yamada

    SILICON PHOTONICS VI   7943  2011

     View Summary

    We studied a characteristics trimming technique in Si photonic-wire waveguide devices. In order to trimming device properties, we utilized refractive index change of amorphous silicon when it crystallizes by annealing. We fabricated MZI devices with amorphous silicon waveguides, and demonstrated the trimming of the transmission spectra by thermal annealing and also laser annealing after finished the device fabrication process. We observed 5.2 % of the refractive index change owing to crystallization by annealing in a nitrogen atmosphere and 5.8% of change by laser crystallization.

    DOI

  • Fabrication of ZnO channel waveguides for nonlinear optical applications

    Edgar Yoshio Morales Teraoka, Tomohiro Kita, Atsushi Tsukazaki, Masashi Kawasaki, Hirohito Yamada

    OXIDE-BASED MATERIALS AND DEVICES II   7940  2011

     View Summary

    We present a fabrication procedure for ZnO channel waveguides intended for nonlinear optical applications. Ar ion milling was used to etch the single crystal thin film samples, and the effects of bias power, chamber pressure and Ar flow rate were investigated, finding optimal parameters for waveguide fabrication. The effect of sidewall roughness was estimated by comparing the results of cut-back measurements and an analytical model. We show an easy and effective method for the fabrication of ZnO channel waveguides.

    DOI

  • Nonlinear optical waveguides with rutile TiO(2)

    Koichi Abe, Edgar Yoshio Morales Teraoka, Tomohiro Kita, Hirohito Yamada

    OXIDE-BASED MATERIALS AND DEVICES II   7940  2011

     View Summary

    We investigated a possibility of making rutile TiO(2) channel waveguides for nonlinear optical applications. Single mode conditions, the group velocity dispersion and the nonlinear-optical parameters for the rutile TiO(2) channel waveguides were calculated using the finite-element method. We also fabricate channel waveguides with rutile TiO(2) using reactive ion etching. The propagation loss measured by the cut-back method was around 6 dB/mm. According to our simulation, around 70 nm of spectral broadening owing to the self-phase modulation will be expected by a pumping optical pulse with 360 W peak power.

    DOI

  • Second and Third-Order Nonlinear Optical Effects in ZnO Channel Waveguides

    Tomohiro Kita, Yoshio Morales Teraoka, Yoshitomo Okawachi, Atsushi Tsukazaki, Masashi Kawasaki, Alexander L. Gaeta, Hirohito Yamada

    2011 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO)    2011

     View Summary

    We demonstrate simultaneous self-phase modulation and second harmonic generation in ZnO channel waveguides. We estimate a value of gamma = 13.9 +/- 3.0 W(-1)m(-1), indicating that ZnO offers promise as a platform for nonlinear devices. (C)2011 Optical Society of America

  • Zero photonic band-gap structure in Si photonic-wire Bragg-grating

    Tomohiro Kita, Hirohito Yamada

    IEEE International Conference on Group IV Photonics GFP     133 - 135  2011

     View Summary

    We have systematically studied Bragg-grating filters in Si photonic-wire waveguides. We found an abnormal localized optical mode on the numerical calculations and verified it on the experimental measurements. This phenomenon originates from high index-contrast between the waveguide materials. Zero photonic band-gap structure was experimentally realized at transition structure of abnormal localized optical mode in 1-D photonic crystal. © 2011 IEEE.

    DOI

  • Optical crossing by hybrid Si-wire/silica waveguides

    Younosuke Wakayama, Tomohiro Kita, Hirohito Yamada

    IEEE International Conference on Group IV Photonics GFP     305 - 307  2011

     View Summary

    We are studying the feasibility of an integrated optical circuit consisting of Si-wire waveguides and silica-based waveguides. In order to realize such a hybrid waveguide optical circuit, we proposed a simple optical crossing structure consisting of heterogeneous waveguides, and investigated the characteristics by numerical analysis and measurement. We found that the insertion loss of the waveguide crossing is low enough, and it was largely improved by making crossing with Si-taper structure. © 2011 IEEE.

    DOI

  • Second and Third-Order Nonlinear Optical Effects in ZnO Channel Waveguides

    Tomohiro Kita, Yoshio Morales Teraoka, Yoshitomo Okawachi, Atsushi Tsukazaki, Masashi Kawasaki, Alexander L. Gaeta, Hirohito Yamada

    2011 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO)    2011

     View Summary

    We demonstrate simultaneous self-phase modulation and second harmonic generation in ZnO channel waveguides. We estimate a value of gamma = 13.9 +/- 3.0 W(-1)m(-1), indicating that ZnO offers promise as a platform for nonlinear devices. (C)2011 Optical Society of America

  • C-3-14 Analysis of Si photonic-wire waveguide ring resonators filter for wavelength tunable laser

    Suzuki Ken, Kita Tomohiro, Yamazaki Hiroyuki, Yamada Hirohito

    Proceedings of the Society Conference of IEICE   2010 ( 1 ) 135 - 135  2010.08

    CiNii

  • Self-phase modulation at visible wavelengths in nonlinear ZnO channel waveguides

    Edgar Yoshio Morales Teraoka, Daniel H. Broaddus, Tomohiro Kita, Atsushi Tsukazaki, Masashi Kawasaki, Alexander L. Gaeta, Hirohito Yamada

    APPLIED PHYSICS LETTERS   97 ( 7 )  2010.08

     View Summary

    We report the observation of nonlinear optical effects in the visible spectrum using ZnO channel waveguides. We demonstrate sixfold spectral broadening and a 2 pi phase shift due to self-phase modulation in ZnO channel waveguides using femtosecond optical pulses with a center wavelength of 840 nm. We measured a value for the nonlinear parameter gamma of 13.9 +/- 3.0 W-1 m(-1), which is more than 1300 times that of a highly nonlinear fiber. The calculated intensity-dependent refractive index is found to be consistent with previously reported values of bulk single-crystal ZnO. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3480422]

    DOI

  • ZnO Channel Waveguides for Nonlinear Optical Applications

    Edgar Yoshio Morales Teraoka, Tomohiro Kita, Atsushi Tsukazaki, Masashi Kawasaki, Yasuo Ohtera, Hirohito Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS   49 ( 4 )  2010

     View Summary

    ZnO channel waveguides were fabricated for the first time, for the purpose of being used in nonlinear optical applications. Wide-bandgap ZnO was chosen for the core material to avoid two-photon absorption at wavelengths above 800nm which are used for biomedical applications. We calculated the single-mode conditions and group-velocity dispersion of the propagating modes for the waveguides. We also fabricated channel waveguides using single-crystal ZnO thin film grown on sapphire substrate. Measured propagation losses with a cut-back method were approximately 3 to 6 dB/mm for single-mode waveguides. (C) 2010 The Japan Society of Applied Physics

    DOI

  • Experimental and numerical analysis study of 1-D photonic crystal in Si photonic-wire waveguides

    Tomohiro Kita, Hirohito Yamada

    Proceedings of SPIE - The International Society for Optical Engineering   7606  2010

     View Summary

    We have systematically studied Bragg Grating Filters in Si photonic-wire waveguides. Highly uniform samples have been fabricated in a specialized foundry service for Si photonics. The reflected wavelength has been controlled over an 180 nm range. By changing the design parameters we controlled the photonic band-gap to be between 6 to 16 nm. Thermal tuning of the reflected wavelength was also measured, resulting in a wavelength shift of more than 3 nm. © 2010 Copyright SPIE - The International Society for Optical Engineering.

    DOI

  • Photonic band-gap anomaly in SOI photonic-wire Bragg-grating filter

    Tomohiro Kita, Ryota Ishikawa, Hirohito Yamada

    2010 7TH IEEE INTERNATIONAL CONFERENCE ON GROUP IV PHOTONICS (GFP)     237 - 239  2010

     View Summary

    We have systematically studied Bragg Grating Filters in Si photonic-wire waveguides. The reflected wavelength and the photonic band-gap were controlled over a wide range by changing the design parameters. We found a photonic band-gap anomaly on the numerical calculations and verified it on the experimental measurements. This phenomenon originates from the abnormal localized optical mode in 1-D photonic crystals.

    DOI

  • Practical integration method of active devices on SOI photonic integrated circuits.

    Tomohiro Kita, Masahiro Abe, Yasuo Ohtera, Hirohito Yamada

    2010 7TH IEEE INTERNATIONAL CONFERENCE ON GROUP IV PHOTONICS (GFP)     275 - 277  2010

     View Summary

    We propose a method of mounting LD chips on SOI substrate and discuss about the optical coupling loss between LDs and Si photonic-wire waveguides. In our scheme, light output from an LD is coupled to the optical waveguides through spot-size-converters (SSC). The simulations show that a minimum optical coupling loss of 1.6 dB and an offset tolerance of +/- 0.7 mu m for the LD chip positioning can be achieved. We measured the optical coupling loss, confirming the validity of the simulations.

    DOI

  • Numerical analysis of bending loss for Si photonic-wire waveguides

    HOKAMA Yohei, KITA Tomohiro, OHTERA Yasuo, YAMADA Hirohito

    Technical report of IEICE. OME   109 ( 283 ) 49 - 54  2009.11

     View Summary

    Bending loss of Si photonic-wire waveguides were analyzed. We introduced a simple method to estimate the bending loss by separating them into three fundamental loss factors. We also confirmed the total bending loss can be calculated with the loss values of the three fundamental loss factors by comparing with measurement results.

    CiNii

  • Study of optical coupling between Si photonic-wire waveguides and laser diode on SOI

    ABE Masahiro, MIYAMURA Satoshi, NAKAMURA Koji, KITA Tomohiro, OHTERA Yasuo, YAMADA Hirohito

    Technical report of IEICE. OME   109 ( 283 ) 55 - 60  2009.11

     View Summary

    Integration technique of laser diodes or photodiodes on Si substrates with SOI optical waveguides is required for realizing future opto-electronic integrated circuits. In this paper, we propose a mounting method of LD chips on the substrate and discuss about the optical coupling loss between LDs and Si photonic-wire waveguides. In the method, light output from LDs is butt joint coupled to the optical waveguides with spot-size-converters. As a result of calculation, we found that about 2dB of minimum optical coupling loss can be attained and the method has enough tolerance for the misalignment of LD chip. We also compared with measured optical coupling loss and confirmed the validity of the calculation.

    CiNii

  • ZnO/MgZnO界面における量子ホール効果

    塚崎 敦, 大友 明, 北 智洋, 大野 裕三, 大野 英男

    固体物理   499 ( 42 ) 559 - 567  2007.09

    Article, review, commentary, editorial, etc. (scientific journal)  

    CiNii

  • ZnOの量子ホール効果

    大友明, 塚崎敦, 北智洋, 大野裕三, 大野英男, 川崎雅司

    応用電子物性分科会誌   13 ( 2 ) 73 - 78  2007.02

    Article, review, commentary, editorial, etc. (other)  

  • (In,Ga)As gated-vertical quantum dot with an Al2O3 insulator

    T. Kita, D. Chiba, Y. Ohno, H. Ohno

    APPLIED PHYSICS LETTERS   90 ( 6 )  2007.02

     View Summary

    The authors fabricated a gated-vertical (In,Ga)As quantum dot with an Al2O3 gate insulator deposited using atomic layer deposition and investigated its electrical transport properties at low temperatures. The gate voltage dependence of the dI/dV-V characteristics shows clear Coulomb diamonds at 1.1 K. The metal-insulator gate structure allowed the authors to control the number of electrons in the quantum dot from 0 to a large number estimated to be about 130. (c) 2007 American Institute of Physics.

    DOI

  • Bias voltage dependence of the electron spin injection studied in a three-terminal device based on a (Ga,Mn) As/ n+-GaAs Esaki diode

    M. Kohda, T. Kita, T. Kita, Y. Ohno, Y. Ohno, F. Matsukura, F. Matsukura, H. Ohno, H. Ohno

    Applied Physics Letters   89  2006.07

     View Summary

    We investigated injection of spin polarized electrons in a (Ga,Mn)As/n+-GaAs Esaki diode (ED) by using a three-terminal device integrating a (Ga,Mn)As ED and a light emitting diode (LED). Electroluminescence polarization (PEL) from the LED was measured under the Faraday configuration as a function of bias voltages applied independently to the Esaki diode and to the LED. The maximum PELof 32.4% was observed when the valence electrons near the Fermi energy of (Ga,Mn)As are ballistically injected into the LED. © 2006 American Institute of Physics.

    DOI

  • Pulse-width and magnetic-field dependences of current-induced magnetization switching in a (Ga,Mn)As magnetic tunnel junction

    D. Chiba, T. Kita, F. Matsukura, H. Ohno

    Journal of Applied Physics   99  2006.05

     View Summary

    We have investigated the pulse-width and magnetic-field dependences of current-induced magnetization switching in a (Ga,Mn) AsGaAs (Ga,Mn) As magnetic tunnel junction. Critical current to induce magnetization switching shows a linear dependence on pulse width from 10 to 1000 μs. The magnetic-field dependence appears to indicate that the observed current-induced magnetization switching proceeds through metastable magnetization structures. © 2006 American Institute of Physics.

    DOI

  • Observation of e2/h conductance steps in a side-gate point contact on In<inf>0.75</inf>Ga<inf>0.25</inf>As/In<inf>0.75</inf>Al<inf>0.25</inf>As heterostructure

    T. Kita, S. Gozu, Y. Sato, S. Yamada

    Journal of Superconductivity and Novel Magnetism   16   327 - 329  2003.12

     View Summary

    In 1996, it was suggested by K. J. Thomas et al. (Phys. Rev. Lett. 77,135 (1996)) that 0.7(2e2/h) conductance structure in Quasi-1 DEG (Q1DEG) at GaAs/AlGaAs heterostructure might be related to spin polarization at zero magnetic field. We have recently studied spontaneous spin-splitting in the 2DEGs formed at normal metamorphic In0.75Ga0.25As/n0.75Al0.25As heterojunctions grown on GaAs substrates and obtained the value of ≲10 meV as the zero-field splitting at Fermi level (Y. Sato et al., J. Appl. Phys. 89, 8017 (2001)). In this work, we attempted to observe spin-related phenomena in this heterojunction Q1DEGs at zero magnetic field. We observed e2/h conductance steps in low electron concentration side-gate point contact. © 2003 Plenum Publishing Corporation.

  • Rashba Effects in diffusive quantum wires made at high In-content InGaAs/InAlAs

    Kakegawa T., Kita T., Akabori S., Yamada S.

    Meeting abstracts of the Physical Society of Japan   58 ( 2 ) 575 - 575  2003.08

    CiNii

  • Spontaneous spin-splitting observed in resonant tunneling diode with narrow band-gap asymmetric quantum well

    S Yamada, T Kikutani, S Gozu, Y Sato, T Kita

    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES   13 ( 2-4 ) 815 - 818  2002.03

     View Summary

    This paper deals with the experiments to observe the spontaneous spin-splitting (SSS) in the resonant tunneling diode (RTD), which was specially designed to have a narrow band gap as well as an asymmetric potential well. In current-voltage (I-V) characteristics. there observed a current peak doubly splitted (denoted as P1-1 and P1-2) at low bias field. The difference of the derivative peak height of P1-1 between those with and without the parallel magnetic field (dI/dV(B)-dI/dV(B = 0)), which was applied along the easy axis of the emitter electrode. increased with increasing field strength, while the quantity of P1-2 stayed almost zero, The increase of the difference of the derivative peak height of P1-1 was found to saturate almost at the coercive field of the electrode. It was also found that those variations of the derivative peak height were observed only in the case of electron injection via the ferromagnetic (NiFe) emitter electrode. The amount of splitting (similar to 15 meV) observed was almost the same as those estimated from magnetoresistance measurements. These results suggest that the observed splitted peak is intimately related to the SSS expected at the center well in our unique RTDs. (C) 2002 Elsevier Science B.V. All rights reserved.

    DOI

  • Study for realization of spin-polarized field effect transistor in In0.75Ga0.25As/In0.75Al0.25As heterostructure

    Y Sato, S Gozu, T Kita, S Yamada

    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES   12 ( 1-4 ) 399 - 402  2002.01

     View Summary

    For realization of the spin-polarized field effect transistor (spin-FET), we investigated a gated modulation of spin-orbit interaction and a spin-injection from a ferromagnetic metal (FM) to a two-dimensional electron gas (2DEG). Prepared samples were an In0.75GaO0.25As/In0.75Al0.25As modulation-doped narrow-gap heterojunction grown by molecular beam epitaxy (MBE). For the determination of spin-orbit interaction parameter, alpha we measured Shubnikov-de Haas (SdH) oscillations at 1.5 K. We confirmed a modulation of D when a gate voltage was applied. We also carried out spin-injection experiments using a multi-terminal geometry sample, which had two different widths Ni40Fe60 electrodes. We observed a spin-valve like effect in a source-drain resistance of similar to 0.1% as well as a resistance hysteresis behavior of similar to 12% in non-local geometry below 20 K. These results are the first step to realize an active spintronic device, such as spin-FET. (C) 2002 Elsevier Science B.V. All rights reserved.

    DOI

  • Ballistic spin transport in four-terminal NiFe/In(0.7)5Ga(0.25)As structure

    Y Sato, S Gozu, T Kita, S Yamada

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   40 ( 10B ) L1093 - L1096  2001.10

     View Summary

    Spin-injection experiments in NiFe/two-dimensional electron gas (2DEG)/NiFe four-terminal devices are described. The 2DEG was confined at a modulation-doped In0.75Ga0.25As/In0.75Al0.25As heterojunction interface. NiFe source-drain electrodes were located similar to1 mum apart and two voltage probes were fabricated between them. The separation between the NiFe electrodes was thus almost less than both the mean free path (similar to2 mum) and the spin-diffusion length (&gt; 1 mum) of the 2DEG. In nonlocal four-terminal measurements, where a constant current (i) was sent through one NiFe to one ohmic electrode, the two-terminal magnetoresistance (R-2t = exhibited a spin-valve-like effect, while R-4t = V-4t/i showed a resistance hysteresis. The amplitude of the latter amounted to almost 12% of the R-4t (B = 0) at 1.5 K, the extent of which was one order of magnitude larger than those so far reported. These results suggest the importance of a quasi-ballistic as well as a spin-coherent coupling between the two NiFe electrodes, which may crucial in the future operation of spin field-effect transistors (FETs).

  • Characterization of high indium content metamorphic InGaAs/InAlAs modulation-doped heterostructures

    S Gozu, T Kita, Y Sato, S Yamada, M Tomizawa

    JOURNAL OF CRYSTAL GROWTH   227   155 - 160  2001.07

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    We have studied electronic and structural characterizations of high indium content metamorphic InGaAs/InAlAs modulation-doped heterostructures. An improved low-temperature electron mobility of mu (e) = 5.45 x 10(5) cm(2) Vs in [(1) over bar 1 0] was confirmed for a directed Hall-bar sample when Si-doped InAlAs layer was slightly etched. In addition, inplane mobility anisotropy of 40% between [(1) over bar 1 0] and [1 1 0] directions. We confirmed. This anisotropy seems to br originated fi om the different undulation period between[(1) over bar 1 0] and [1 1 0] directions. We theoretically calculated electron mobility taking both alloy disorder scattering and background impurity scattering into account. It is found that the calculated and experimental values are in good agreement. (C) 2001 Elsevier Science B.V. All rights reserved.

    DOI

  • Large spontaneous spin splitting in gate-controlled two-dimensional electron gases at normal In0.75Ga0.25As/In0.75Al0.25As heterojunctions

    Y Sato, T Kita, S Gozu, S Yamada

    JOURNAL OF APPLIED PHYSICS   89 ( 12 ) 8017 - 8021  2001.06

     View Summary

    Amounts of spontaneous spin splittings were estimated from low-temperature magnetoresistances in two-dimensional electron gases created at In0.75Ga0.25As/In0.75Al0.25As heterojunctions under a gate bias. Typical sheet electron densities and mobilities in the raw wafers were similar to1.0 x 10(12)/cm(2) and 2-5 x 10(5) cm(2)/V s at 1.5 K, respectively. A maximum spin-orbit coupling constant alpha (zero) of similar to 30(x10(-12) eV m) was obtained for the van der Pauw sample. In gated Hall-bar samples, a decrease in the alpha (zero) value with decreasing gate voltage (V-g) was first confirmed in a normal heterojunction. The main origin for such a large alpha (zero), which is a few times larger than any previously reported, was found to be a structure-dependent so-called interface contribution in the Rashba term. (C) 2001 American Institute of Physics.

    DOI

  • An investigation of tunable spin-orbit interactions in front-gated In0.75Ga0.25As/In0.75Al0.25As heterojunctions

    Y Sato, S Gozu, T Kita, S Yamada

    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES   10 ( 1-3 ) 77 - 80  2001.05

     View Summary

    We investigated zero-field spin-splitting in normal-type In0.75Ga0.25As/In0.75Al0.25As heterostructure by magnetoresistance measurements at 1.5 K. The maximum value of spin-orbit interaction parameter, alpha (zero) obtained here is 32x10(-12) eVm. We also confirmed a tuning of alpha (zero) by applying gate biases. On the other hand, we observed no beat oscillation when the In0.75Ga0.25As well width decreased from 30 to 10nm. These results suggested that interface contribution related to the asymmetry of wave function penetration into the barriers could be enhanced in our heterojunction (C) 2001 Published by Elsevier Science B.V.

    DOI

  • Large spontaneous spin-splitting and enhanced effective g-factor in two-dimensional electron gases at In0.75Ga0.25As/In0.75Al0.25As metamorphic heterojunctions

    T Kita, Y Sato, S Gozu, S Yamada

    PHYSICA B   298 ( 1-4 ) 65 - 69  2001.04

     View Summary

    We have estimated spin-splitting energy of two-dimensional electron gases formed at metamorphic In0.75Ga0.25As/In0.75Al0.25As heterojunctions by analyzing magneto-resistance traces up to 10 T. This heterojunction often reveals a large spontaneous spin-splitting at low fields, while at high fields, Zeeman splitting becomes dominant. We have investigated and compared the two sanples; one reveals beating oscillation and hence has a spontaneous or zero-field spin-splitting and another does not. In the former sample, if we assume the equal sign for both the spin-splittings, the dependency of the absolute splitting energy on the magnetic field is found to have a minimum (Delta = 2 meV) at about 3.5 T, in which we also confirmed a zero-field spin-splitting of Delta (0) = 11.04meV and very much enhanced g* = - 42.0 at 1.5 T. In contrast, effective g-factor (g*) obtained at high fields in the latter sample is g* = - 7.7 at 1.5 T, which is fairly smaller than that in the former. Origins of those features are discussed and the possibility of linear spin-splitting dependency on magnetic field is pointed out. (C) 2001 Published by Elsevier Science B.V.

    DOI

  • Effect of strain relaxation layers in high Indium content metamorphic InGaAs/InAlAs modulation doped heterostructures

    S Gozu, T Kita, H Sato, S Yamada

    2001 INTERNATIONAL CONFERENCE ON INDIUM PHOSPHIDE AND RELATED MATERIALS, CONFERENCE PROCEEDINGS     55 - 58  2001

     View Summary

    We have investigated effect of residual strain appeared in the transport and optical properties of high indium content metamorphic In0.75Ga0.25As/In0.75Al0.25As modulation doped heterostructures grown via (inverse step: IS-) step graded buffer (SGB). The effect of residual strain was controlled by IS structure and a drastic decrease of residual strain (about one third) was observed. We have also observed an improved low temperature (77K) electron mobility, when the IS buffer was used. We found that the residual strain is one of the important interface condition which determines the low temperature (77K) electron mobility.

    DOI

  • Critical layer thickness study in In0.75Ga0.25As/In0.5Al0.5As pseudomorphic resonant tunneling diode structure grown on GaAs substrates

    Shin-Ichiro Gozu, Tomohiro Kita, Tomoyuki Kikutani, Syoji Yamada

    Journal of Crystal Growth   227 ( 228 ) 161 - 166  2001

     View Summary

    We have studied critical layer thickness (CLT) in an In0.75Ga0.25As resonant tunneling diode structure grown on a GaAs substrate via an InAlAs step-graded buffer (SGB) with two types of inverse step(IS)-SGB. We have observed red or blue shift in photoluminescence spectra and CLT change depending on SGB condition. This change reflects from the difference of residual strain InAlAs SGBs adopted here. © 2001 Elsevier Science B.V.

    DOI

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Other

  • 高い環境耐性を有するキャリアコンバータ技術の研究開発

    2016.05
     
     

     View Summary

    5G時代に対応した大容量・低遅延・シームレスな光/ミリ波変換デバイ
    スの開発と実証評価

  • ナノハイブリッド電気光学ポリマーを用いた光インターコネクト技術

    2016.04
     
     

     View Summary

    本課題では、電気光学ポリマーとシリコン光導波路を複合化する事で、これまでにない機能性を有する導波路デバイスを作製する。本研究の目的は、車載通信に用いるインターコネクトデバイスへの製品化を目的として高機能・小型光導波路を作製する。

  • 狭線幅波長可変レーザの開発

    2015.12
     
     

     View Summary

    次世代の大容量光通信方式として実用化の始まっているデジタル・コヒーレント光通信用狭線幅波長可変レーザーをシリコンフォトニクスによって独自に開発した超小型波長可変フィルターを用いて実現する。波長可変動作、狭線幅発振といった個々の特性に関しては、すでに十分な特性が得られているが本技術が実用化に耐えうる特性を持つことを証明するために高出力、省スペース、安定した波長可変動作といった特性を満足する波長可変レーザーモジュールを開発する。

  • シリコンフォトニック波長可変レーザを用いた医療イメージング用光源の開発

    2015.04
     
     

     View Summary

    光断層イメージングに必要な広波長帯域可変レーザをシリコンフォトニクスを用いて作製した波長可変フィルタと量子ドット光増幅器を用いて実現する。

  • 大容量光通信用高機能シリコンフォトニック波長可変レーザの開発

    2014.08
     
     

     View Summary

    次世代の大容量光通信技術であるデジタル・コヒーレント光通信、フォトニックネットワーク、超広周波数帯域波長分割多重といった通信方式において使用できる実用的な特性を持つ高機能波長可変レーザをシリコンフォトニクスを用いて開発する。

  • ナノハイブリッド電気光学ポリマーを用いた光インターコネクト技術

    2014.04
     
     

     View Summary

    本課題では、電気光学ポリマーとシリコン光導波路を複合化する事で、これまでにない機能性を有する導波路デバイスを作製する。本研究の目的は、車載通信に用いるインターコネクトデバイスへの製品化を目的として高機能・小型光導波路を作製する。

  • 次世代コヒーレント光通信用 超小型・狭線幅シリコンフォトニクス波長可変レーザモジュールの開発

    2014.02
     
     

     View Summary

    次世代の大容量光通信方式として実用化の始まっているデジタル・コヒーレント光通信用狭線幅波長可変レーザーをシリコンフォトニクスによって独自に開発した超小型波長可変フィルターを用いて実現する。波長可変動作、狭線幅発振といった個々の特性に関しては、すでに十分な特性が得られているが本技術が実用化に耐えうる特性を持つことを証明するために高出力、省スペース、安定した波長可変動作といった特性を満足する波長可変レーザーモジュールを開発する。

  • シリコンフォトニクスを用いたデジタル・コヒーレント光通信用波長可変レーザの開発

    2012.12
     
     

     View Summary

    本波長可変レーザは、すでに波長可変幅、狭線幅動作に関しては十分な特性を示しており、素子の安定性を向上させ出力光強度を大きくできれば技術移転が可能である。本研究によってシリコン細線光導波路チップへのSOAのフリップチップ実装技術や反射戻り光低減のためのスポットサイズコンバータの構造最適化手法などを開発し、実システムで使用可能出力光強度を得る事を目標とする。

  • 局所レーザー加熱を用いたSi細線光導波路デバイスのトリミング技術の開発

    2011.09
     
     

     View Summary

    Siをコア、SiO2をクラッドとして用いる超High-ΔのSi細線光導波路は、シリカ系の光導波路で作製されてきた従来の光回路を大幅に小型・低消費電力化できるため、通信用光デバイスやLSIチップ内光配線、バイオセンシングなどの各種センサーといった広範な領域で様々な応用デバイスの実現を期待されている。このように様々な分野で活発な研究が行われているにも関わらず、Si光導波路デバイスは未だあまり実用化されていない。この実用化にあたっての最大の障壁は、僅かな製造誤差によって特性が大きくばらつくことにある。Si細線導波路デバイスのサイズは極めて小さい(マッハ・ツェンダー干渉計型熱光学光スイッチでも100μm以下)ために、数nm程度の製造誤差でも光共振器や光干渉計等を有するデバイス特性には大きな影響がでてしまう。本研究ではこの問題を解決するために、光導波路をアモルファスSiによって形成し、局所的なレーザー照射によってアモルファスSiの光導波路コアの一部分を加熱し、部分的に多結晶Siを析出させることで屈折率を変化させデバイス特性をトリミングする技術を開発する。

  • 省電力ネットワーク実現のためのスロット型Si細線光導波路の機能集積化の研究

    2009.07
     
     

     View Summary

    Si細線を用いた光導波路によって構成される光集積回路は、現行のシステムを大幅に小型化、低消費電力化することが期待されている。本研究ではSi細線にスロット型光導波路を導入することで更なる高機能化を図る。

  • Si光導波路の界面化学反応を利用した機能性光学デバイスの研究

    2009.04
     
     

     View Summary

    本研究課題では、近年発展が著しいSiをベースとした光学デバイス(Siフォトニクス)の研究に、従来は用いられなかった電気化学的な手法を応用する。Siフォトニクスの持つ可能性と電気化学において蓄積された材料物性制御技術を融合することで、より高機能・低消費電力なデバイス応用を可能にする微小領域の光波制御手法を提案、実証していく。

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Syllabus

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Sub-affiliation

  • Faculty of Science and Engineering   Graduate School of Advanced Science and Engineering

Research Institute

  • 2022
    -
    2024

    Waseda Research Institute for Science and Engineering   Concurrent Researcher

Internal Special Research Projects

  • ハイブリッド波長可変レーザを用いたFMCW計測の高精度化

    2022  

     View Summary

    シリコンフォトニクスチップを用いて作製した波長可変フィルタと化合物半導体光増幅器を結合したハイブリッド波長可変レーザに電流注入型高速位相シフタを装荷した直接位相変調レーザ光源を作製し、1 GHz以上の帯域を持つ周波数チャープ光を1MHzの高繰り返しで出力可能なことを確認した。本レーザを用いてFrequency modulated cotinuous wave (FMCW)距離計測を行い、4 cm程度の高精度での距離計測が可能なことを示した。

  • ハイブリッドレーザを用いたFMCW測距技術の開発

    2021  

     View Summary

    シリコンフォトニクスチップ波長可変フィルタと化合物半導体光増幅器を結合したハイブリッド波長可変レーザを用いて、FMCW(Frequency modulated continuous wave)方式により距離計測を技術を開発した。高速な周波数変調が可能な電流注入型熱光学式位相シフタの利用によって、25 KHzの変調周波数で2.5 cm以下の距離精度を実証した。

  • 1チップレーザービームスキャナの作製

    2020  

     View Summary

    経路差を用いたシリコンアレイ導波路及びブラッグ回折格子構造によってレーザ光の波長によってレーザビームの出射方向を二次元的に制御できることを確認した。さらに光増幅チップとシリコンフォトニクスチップとを光学接着剤を用いて結合し良好なレーザ発振特性が得られた。これらの成果によって1チップ二次元ビームスキャナ実現への道筋を得ることができた。

  • 光波制御ユニットの高精度制御に関する研究

    2019  

     View Summary

    シリコンフォトニクスチップ上に試作した多数のヒーターを同時に制御する計測システムを構築した。複数の直流安定化電源、ファンクションジェネレータ等を独立に制御することでハイブリッドレーザーから出力した二波長のレーザー光を用いてマイクロ波を発生させることに成功した。

  • 非常に広い可変波長域を有するヘテロジニアス波長可変レーザの開発

    2018  

     View Summary

    シリコンフォトニクスによって作製した外部共振器と化合物半導体光増幅器とを組み合わせたヘテロジニアス波長可変レーザをより広い波長範囲に適用するために波長依存性の小さな導波路型波長可変フィルタを開発した。本研究においては、リング共振器とバス導波路とを結合する方向性結合器に曲がり導波路構造を用いることで、従来型の波長可変フィルタの1.5倍程度の波長可変範囲で使用可能な波長フィルタの設計、試作した。方向性結合の結合効率の波長依存性が小さいためリング共振器のQ値といった特性の波長依存性が低減されることが確認できた。