Nitrogen-Vacancy (NV) centers in diamond are promising solid-state quantum emitters that can be utilized for photonic quantum applications. Various diamond nanophotonic devices have been fabricated for efficient extraction of single photons emitted from NV centers to a single guided mode. However, for constructing scalable quantum networks, further efficient coupling of single photons to a guided mode of a single-mode fiber (SMF) is indispensable and a difficult challenge. Here, we propose a novel efficient hybrid system between an optical nanofiber and a cylindrical-structured diamond nanowire. The maximum coupling efficiency as high as 75% for the sum of both fiber ends is obtained by numerical simulations. The proposed hybrid system will provide a simple and efficient interface between solid-state quantum emitters and a SMF suitable for constructing scalable quantum networks.

Diamond nanowires are fabricated on a bulk, single crystalline diamond near an edge of aluminum coating using inductively coupled plasma reactive ion etching. Two different density areas are simultaneously appeared where the dense area has 9 times higher density than that of the sparse area while keeping the size of nanowires almost uniform in these areas. The nanowire sizes realized in the dense (sparse) area are 858 +/- 22nm (876 +/- 25nm) in height and 126 +/- 6 nm (124 +/- 7 nm) in diameter, which is suitable for applications in optical quantum information processing. (C) 2017 The Japan Society of Applied Physics

We present experimental data on message transmission in a free-space optical (FSO) link at an eye-safe wavelength, using a testbed consisting of one sender and two receiver terminals, where the latter two are a legitimate receiver and an eavesdropper. The testbed allows us to emulate a typical scenario of physical-layer (PHY) security such as satellite-to-ground laser communications. We estimate information-theoretic metrics including secrecy rate, secrecy outage probability, and expected code lengths for given secrecy criteria based on observed channel statistics. We then discuss operation principles of secure message transmission under realistic fading conditions, and provide a guideline on a multi-layer security architecture by combining PHY security and upper-layer (algorithmic) security. (C) 2016 Optical Society of America

By combining a tilted-pulse-intensity-front scheme using a LiNbO3 crystal and a chirped-pulsebeating method, we generated a narrowband intense terahertz (THz) pulse, which had a maximum electric field of more than 10 kV/cm at around 2 THz, a bandwidth of similar to 50 GHz, and frequency tunability from 0.5 to 2 THz. By performing THz-pump and near-infrared-probe experiments on GaAs quantum wells, we observed that the resonant excitation of the intraexcitonic 1s-2p transition induces a clear and large Autler-Townes splitting. Our time-resolved measurements show that the splitting energy observed in the rising edge region of electric field is larger than in the constant region. This result implies that the splitting energy depends on the time-averaged THz field over the excitonic dephasing time rather than that at the instant of the exciton creation by a probe pulse. (C) 2015 AIP Publishing LLC.

Secrecy issues of free-space optical links realizing information theoretically secure communications and high transmission rates are discussed. We numerically study secrecy communication rates of optical wiretap channel based on on-off keying (OOK) modulation under typical conditions met in satellite-ground links. It is shown that, under reasonable degraded conditions on a wiretapper, information theoretically secure communications should be possible in a much wider distance range than a range limit of quantum key distribution, enabling secure optical links between geostationary Earth orbit satellites and ground stations with currently available technologies. We also provide the upper bounds on the decoding error probability and the leaked information to estimate a necessary code length for given required levels of performances. This result ensures that a reasonable length of wiretap channel code for our proposed scheme must exist.

Secrecy issues of free-space optical links realizing information theoretically secure communications and high transmission rates are discussed. We numerically study secrecy communication rates of optical wiretap channel based on on-off keying (OOK) modulation under typical conditions met in satellite-ground links. It is shown that, under reasonable degraded conditions on a wiretapper, information theoretically secure communications should be possible in a much wider distance range than a range limit of quantum key distribution, enabling secure optical links between geostationary Earth orbit satellites and ground stations with currently available technologies. We also provide the upper bounds on the decoding error probability and the leaked information to estimate a necessary code length for given required levels of performances. This result ensures that a reasonable length of wiretap channel code for our proposed scheme must exist.

We demonstrate an all-fiber cavity quantum electrodynamics system with a trapped single atom in the strong coupling regime. We use a nanofiber Fabry-Perot cavity, that is, an optical nanofiber sandwiched by two fiber-Bragg-grating mirrors. Measurements of the cavity transmission spectrum with a single atom in a state-insensitive nanofiber trap clearly reveal the vacuum Rabi splitting.

We demonstrate time-bin entanglement generation in telecom wavelength using a 7 mu m radius Si micro-ring resonator pumped by a continuous wave laser. The resonator structure can enhance spontaneous four wave mixing, leading to a photon pair generation rate of about 90-100 Hz with a laser pump power of as low as -3.92 dBm (0.41 mW). We succeed in observing time-bin entanglement with the visibility over 92%. Moreover, wavelength-tunability of the entangled photon pair is demonstrated by changing the operation temperature. (C) 2015 Optical Society of America

We propose a scheme for single-atom, quantum control of the direction of propagation of a coherent field incident, via a tapered fiber, upon a microtoroidal whispering-gallery-mode (WGM) resonator. The scheme involves overcoupling of the fiber taper to the resonator and strong coupling of an atom to the evanescent field of the WGM, i.e., an atom-field coupling that exceeds the total WGM linewidth. In contrast to previous, related schemes that operate in the bad-cavity regime, the proposed scheme can operate effectively with much stronger incident fields, while also preserving their coherent nature. It can also serve to prepare an entangled state of the atom and coherent optical pulses propagating in opposite directions along the fiber. We evaluate the fidelity of preparation of such a state taking into account absorption and atomic spontaneous emission and demonstrate that high fidelities should be possible with realistic parameters.

We design and fabricate ultra-low-loss tapered optical fibers (TOFs) with minimal lengths. We first optimize variations of the torch scan length using the flame-brush method for fabricating TOFs with taper angles that satisfy the adiabaticity criteria. We accordingly fabricate TOFs with optimal shapes and compare their transmission to TOFs with a constant taper angle and TOFs with an exponential shape. The highest transmission measured for TOFs with an optimal shape is in excess of 99.7% with a total TOF length of only 23 mm, whereas TOFs with a constant taper angle of 2 mrad reach 99.6% transmission for a 63 mm TOF length. (C) 2014 Optical Society of America

We study the elimination of the chirp of narrowband terahertz pulses generated by chirped laser pulse beating using a laser pulse stretcher with two grating pairs that cancel out the third-order spectral phase. First, we show that positively chirped terahertz pulses can be generated using a pulse stretcher with a grating pair and internal lenses. We then combine this with a second grating pair, the spectral phase of which has the opposite sign to that of the first one. By varying the separation of the second grating pair, we experimentally verify that the chirp of the generated terahertz pulses can be eliminated. (C) 2014 Optical Society of America

Single-photon sources are important elements in quantum optics and quantum information science. It is crucial that such sources be able to couple photons emitted from a single quantum emitter to a single propagating mode, preferably to the guided mode of a single-mode optical fiber, with high efficiency. Various photonic devices have been successfully demonstrated to efficiently couple photons from an emitter to a single mode of a cavity or a waveguide. However, efficient coupling of these devices to optical fibers is sometimes challenging. Here we show that up to 38% of photons from an emitter can be directly coupled to a single-mode optical fiber by utilizing the flat tip of a silica nanofiber. With the aid of a metallic mirror, the efficiency can be increased to 76%. The use of a silicon waveguide further increases the efficiency to 87%. This simple device can be applied to various quantum emitters.

Single-photon sources are important elements in quantum optics and quantum information science. It is crucial that such sources be able to couple photons emitted from a single quantum emitter to a single propagating mode, preferably to the guided mode of a single-mode optical fiber, with high efficiency. Various photonic devices have been successfully demonstrated to efficiently couple photons from an emitter to a single mode of a cavity or a waveguide. However, efficient coupling of these devices to optical fibers is sometimes challenging. Here we show that up to 38% of photons from an emitter can be directly coupled to a single-mode optical fiber by utilizing the flat tip of a silica nanofiber. With the aid of a metallic mirror, the efficiency can be increased to 76%. The use of a silicon waveguide further increases the efficiency to 87%. This simple device can be applied to various quantum emitters.

We demonstrate a method for controlling the chirp of the frequency-tunable narrowband terahertz pulses that are generated by photomixing with nonlinearly chirped laser pulse pairs. We find that in a grating-based laser-pulse stretcher, the frequency sweep rates of the generated terahertz pulses can be controlled by simply changing the incident angle. This method is also applicable to other mechanisms of terahertz pulse generation. (C) 2014 AIP Publishing LLC.

We theoretically investigated a microtoroidal cavity quantum electrodynamics system in which radiation from the emitter is nearly perfectly guided into a fiber mode, and analyzed the resonance fluorescence from the emitter after pulsed excitation. We derived analytic formulas to rigorously evaluate the photon statistics of the pulse emitted into the fiber, and clarified the conditions needed for the excitation pulse to generate single- and two-photon pulses.

We show that a hemispherically shaped tip on an air-clad optical fiber simultaneously works as a high-numerical-aperture lens and efficiently collects photons from an emitter placed near the beam waist into the fundamental guided mode. Numerical simulations show that the coupling efficiency reaches about 25%. We have constructed a confocal microscope with such a lensed fiber. The measurements are in good agreement with the numerical simulation. The monolithic structure with a high-photon-collection efficiency will provide a flexible substitute for a conventional lens system in various experiments such as single-atom trapping with a tightly focused optical trap. (C) 2014 Optical Society of America

We theoretically show that it is possible to generate chirp-free terahertz (THz) pulses with a chirped-pulse beating method by using an optical fiber as a pulse stretcher. Proper choices of the core radius and the dopant fraction of the core material of a step-index single-mode optical fiber eliminate the third-order spectral phase of the fiber, thus giving the pump laser pulse a purely linear chirp. We also show that even a standard commercial single-mode optical fiber can give THz pulses of lower chirp than the lower limit for a grating pair. We perform experiments to verify our theory. © 2013 AIP Publishing LLC.

光子を用いた量子暗号・量子情報技術には単一光子源の放射する光子を高効率で集め、単一モード光ファイバーに導波する技術が必要である。本研究ではナノファイバーを切断した端を用いた新しい高効率集光デバイスを提案する。我々はナノファイバー端を用いた集光デバイスについての数値シミュレーションを行い、点光源から最大38%の効率で光子をナノファイバーの基本導波モードに集めることができることを示した。このデバイスにより、一般的な集光方法であるレンズを用いた方法では実現できないような高い効率での微小発光体からの集光・集めた光の単一モード光ファイバーへの導波を実現することができ、高効率な単一光子源への応用が期待される。

The spectral linewidth and intensity of narrowband terahertz pulses generated by photomixing with beating of nonlinearly chirped laser pulse pairs are studied. The direct relationship between the dispersion of the pump pulse stretcher and the chirp of the terahertz pulse is shown. Simple analytical expressions for the linewidth and intensity of the terahertz spectrum are obtained and their validity is confirmed by numerical simulations. Generation of narrowband terahertz pulses with minimal chirp is demonstrated. (C) 2013 The Japan Society of Applied Physics

We have performed numerical simulations of light collecting devices utilizing a silica nanofiber tip. Up to 39% of light from a point dipole source can be coupled to the fundamental guided mode of the nanofiber.

We investigate the properties of a single photon generated by a solid-state emitter subject to strong pure dephasing. We employ a model in which all the elements of the system, including the propagating fields, are treated quantum mechanically. We analytically derive the density matrix of the emitted photon, which contains full information about the photon, such as its pulse profile, power spectrum, and purity. We visualize these analytical results using realistic parameters and reveal the conditions for maximizing the purity of generated photons.

Cavity quantum electrodynamics provides the setting for quantum control of strong interactions between a single atom and one photon. Many such atom-cavity systems interacting by coherent exchanges of single photons could be the basis for scalable quantum networks. However, moving beyond current proof-of-principle experiments involving just one or two conventional optical cavities requires the localization of individual atoms at distances less than or similar to 100 nm from a resonator's surface. In this regime an atom can be strongly coupled to a single intracavity photon while at the same time experiencing significant radiative interactions with the dielectric boundaries of the resonator. Here, we report using real-time detection and high-bandwidth feedback to select and monitor single caesium atoms located similar to 100 nm from the surface of a microtoroidal optical resonator. Strong radiative interactions of atom and cavity field probe atomic motion through the evanescent field of the resonator and reveal both the significant role of Casimir-Polder attraction and the manifestly quantum nature of the atom-cavity dynamics.

We report simple, reproducible methods of fabricating tapered optical fibers with subwavelength diameter and silica microtoroidal resonators, both operating at a wavelength of 850 nm. The transmission loss of tapered fibers was reduced to 0.03 dB by controlling the adiabaticity of the tapered region. The Q factor of 3 x 10(8) was obtained for a toroidal microresonator with 40 mu m diameter. (C) 2010 The Japan Society of Applied Physics

Quantum computation and communication rely on the ability to manipulate quantum states robustly and with high fidelity. To protect fragile quantum-superposition states from corruption through so-called decoherence noise, some form of error correction is needed. Therefore, the discovery of quantum error correction(1,2) (QEC) was a key step to turn the field of quantum information from an academic curiosity into a developing technology. Here, we present an experimental implementation of a QEC code for quantum information encoded in continuous variables, based on entanglement among nine optical beams(3). This nine-wave-packet adaptation of Shor's original nine-qubit scheme(1) enables, at least in principle, full quantum error correction against an arbitrary single-beam error.

Single photons from a coherent input are efficiently redirected to a separate output by way of a fiber-coupled microtoroidal cavity interacting with individual cesium atoms. By operating in an overcoupled regime for the input-output to a tapered fiber, our system functions as a quantum router with high efficiency for photon sorting. Single photons are reflected and excess photons transmitted, as confirmed by observations of photon antibunching (bunching) for the reflected (transmitted) light. Our photon router is robust against large variations of atomic position and input power, with the observed photon antibunching persisting for intracavity photon number 0.03 less than or similar to(n)over bar less than or similar to 0.7.

Beyond traditional nonlinear optics with large numbers of atoms and photons, qualitatively new phenomena arise in a quantum regime of strong interactions between single atoms and photons. By using a microscopic optical resonator, we achieved such interactions and demonstrated a robust, efficient mechanism for the regulated transport of photons one by one. With critical coupling of the input light, a single atom within the resonator dynamically controls the cavity output conditioned on the photon number at the input, thereby functioning as a photon turnstile. We verified the transformation from a Poissonian to a sub- Poissonian photon stream by photon counting measurements of the input and output fields. The results have applications in quantum information science, including for controlled interactions of single light quanta and for scalable quantum processing on atom chips.

Continuous-wave light beams with broadband quantum entanglement are created with two independent squeezed light beams generated by two periodically poled lithium niobate waveguides and a symmentric beam splitter. The quantum entanglement is confirmed with a sufficient criterion Delta(2)(A,B)=<[Delta(x(A)-x(B))](2)>+<[Delta(p(A)+p(B))](2)>< 1 and the observed Delta(2)(A,B) is 0.75 over the bandwidth of 30 MHz. Although the bandwidth is limited by that of the detector so far, it would be broadened up to 10 THz which would be only limited by the bandwidth of phase matching for the second-order nonlinear process. (c) 2007 American Institute of Physics.

Over the past decade, strong interactions of light and matter at the single-photon level have enabled a wide set of scientific advances in quantum optics and quantum information science. This work has been performed principally within the setting of cavity quantum electrodynamics(1-4) with diverse physical systems(5), including single atoms in Fabry-Perot resonators(1,6), quantum dots coupled to micropillars and photonic bandgap cavities(7,8) and Cooper pairs interacting with superconducting resonators(9,10). Experiments with single, localized atoms have been at the forefront of these advances(11-15) with the use of optical resonators in high-finesse Fabry-Perot configurations(16). As a result of the extreme technical challenges involved in further improving the multilayer dielectric mirror coatings(17) of these resonators and in scaling to large numbers of devices, there has been increased interest in the development of alternative microcavity systems(5). Here we show strong coupling between individual caesium atoms and the fields of a high-quality toroidal microresonator. From observations of transit events for single atoms falling through the resonator's evanescent field, we determine the coherent coupling rate for interactions near the surface of the resonator. We develop a theoretical model to quantify our observations, demonstrating that strong coupling is achieved, with the rate of coherent coupling exceeding the dissipative rates of the atom and the cavity. Our work opens the way for investigations of optical processes with single atoms and photons in lithographically fabricated microresonators. Applications include the implementation of quantum networks(18,19), scalable quantum logic with photons(20), and quantum information processing on atom chips(21).

We report generation of squeezed vacuum in sideband modes of continuous-wave light at 946nm using a periodically poled KTiOPO4 crystal in an optical parametric oscillator. At the pump power of 250mW, we observe the squeezing level of -5.6 +/- 0.1dB and the anti- squeezing level of + 12.7 +/- 0.1dB. The pump power dependence of the observed squeezing/anti- squeezing levels agrees with theoretically calculated values when phase fluctuation of locking is taken into account. (c) 2006 Optical Society of America.

We demonstrate unconditional telecloning for the first time. In particular, we symmetrically and unconditionally teleclone coherent states of light from one sender to two receivers, achieving a fidelity for each clone of F=0.58 +/- 0.01, which surpasses the classical limit. This is a manipulation of a new type of multipartite entanglement whose nature is neither purely bipartite nor purely tripartite.

Quantum teleportation of a squeezed state is demonstrated experimentally. Due to some inevitable losses in experiments, a squeezed vacuum necessarily becomes a mixed state which is no longer a minimum uncertainty state. We establish an operational method of evaluation for quantum teleportation of such a state using fidelity and discuss the classical limit for the state. The measured fidelity for the input state is 0.85 +/- 0.05, which is higher than the classical case of 0.73 +/- 0.04. We also verify that the teleportation process operates properly for the nonclassical state input and its squeezed variance is certainly transferred through the process. We observe the smaller variance of the teleported squeezed state than that for the vacuum state input.

We experimentally demonstrate continuous-variable quantum teleportation beyond the no-cloning limit. We teleport a coherent state and achieve the fidelity of 0.70 +/- 0.02 that surpasses the no-cloning limit of 2/3. Surpassing the limit is necessary to transfer the nonclassicality of an input quantum state. By using our high-fidelity teleporter, we demonstrate entanglement swapping, namely, teleportation of quantum entanglement, as an example of transfer of nonclassicality.

We demonstrate quantum teleportation of a coherent state and achieve the fidelity of 0.70 that surpasses the no-cloning limit of 2/3. We also perform entanglement swapping using our high-fidelity teleporter.

Quantum teleportation(1-8) involves the transportation of an unknown quantum state from one location to another, without physical transfer of the information carrier. Although quantum teleportation is a naturally bipartite process, it can be extended to a multipartite protocol known as a quantum teleportation network(9). In such a network, entanglement is shared between three or more parties. For the case of three parties ( a tripartite network), teleportation of a quantum state can occur between any pair, but only with the assistance of the third party. Multipartite quantum protocols are expected to form fundamental components for larger-scale quantum communication and computation(10,11). Here we report the experimental realization of a tripartite quantum teleportation network for quantum states of continuous variables ( electromagnetic field modes). We demonstrate teleportation of a coherent state between three different pairs in the network, unambiguously demonstrating its tripartite character.

A continuous-variable tripartite entangled state is experimentally generated by combining three independent squeezed vacuum states, and the variances of its relative positions and total momentum are measured. We show that the measured values violate the separability criteria based on the sum of these quantities and prove the full inseparability of the generated state.

We propose a simple method to estimate the biexcitonic contribution to the excitonic non-linearity. The method is based on the time integrated four-wave mixing (TI FWM) with picosecond pulses. The TI FWM signal, which is measured as a function of the delay between pump and test pulses, shows shift towards positive and negative time delay when the laser is tuned at the exciton and biexciton resonance, respectively. We show theoretically that the magnitude of the shift allows us to estimate the biexcitonic contribution to the third-order non-linearity at the fundamental band edge. (C) 2003 Elsevier Science Ltd. All rights reserved.

Spectrally resolved and time-integrated four-wave mixing are used to measure the polarization dependence of biexcitonic signals and quantum beats between two-A-exciton (XAXA*) and A-biexciton (XAXA) states in a high-quality GaN epilayer. Mixed beats with two periods are observed: the first beating period corresponds to the energy splitting between XAXA* and XAXA; the second period corresponds to beating between A excitons (X-A) and donor bound excitons ((DX)-X-0). We also measure the polarization-dependent B-biexciton (XBXB) signal. The effective masses for the A and B holes are deduced from the binding energy.

The polarization dependence of biexcitonic signals and quantum beats between A-excitons (X-A) and A-biexcitons (XAXA) in a high-quality GaN epilayer is measured by spectrally-resolved and time-integrated four-wave mixing. With cross-linear polarised light, mixed beats with two periods are observed: the first beating period corresponds to the energy splitting between X-A and XAXA, and agrees well with the calculated XAXA binding energy, while the second beating period corresponds to that between X-A and donor bound excitons ((DX)-X-0). The temperature-dependent homogeneous linewidth shows that the (DX)-X-0 has a larger acoustic phonon coupling coefficient than the XAXA. We also measured the polarization dependent B-biexciton (XBXB) signal. The effective masses for the A- and B-hole were deduced from the binding energy.

We present spectrally resolved and time-integrated four-wave mixing measurements at coherent dynamics of bound excitons in a high-quality GaN epilayer. Coherent excitation, with co-circular polarized light, of the neutral donor-bound excitons (D X-0) and A excitons (X-A) results in quantum beats, corresponding to the energy splitting between D X-0 and X-A. The temperature-dependent dephasing rate is used to deduce the strength of the D X-0-acoustic-phonon interaction via the homogeneous linewidth. (C) 2001 American Institute of Physics.

We report on the study of quantum beats in the time-integrated four-wave mixing signal in reflection geometry from a pair of excitons with different valence orbitals in GaN and ZnSe. We observe a pi -phase shift between quantum beats in the co- and cross-linear polarization configuration, and nearly 100% modulation of the signal for both materials. In the co-circular polarization configuration, the observed phases of the quantum beats at the frequency of the A exciton in GaN and heavy-hole exciton in ZnSe are 0.2 pi and 0.1 pi, respectively. The phases of the quantum beats change sign for co- and counter-circular polarization configurations and also for A- (heavy-hole) and B-(light-hole) excitons in GaN(ZnSe). We describe the third-order coherent optical response of the exciton pair with different valence orbitals by taking into account the finite memory depth of the four-particle correlation. In particular, our experimental findings indicate that excitons with different valence orbitals and equal angular momentum attract each other similar to excitons with the same valence orbitals but opposite angular momentum. Excitons with different orbitals and opposite angular momenta repel one another. The comparison between experimental and theoretical results allows us to develop a quantitative analysis of the four-particle correlation in the presence of an exciton pair with different valence orbitals. We show that the observed pi -phase shift and nearly 100% modulation of the signal in the co- and cross-linear polarization configuration impose restraints on the memory functions. which describe the exciton-exciton interaction. These restraints imply, in particular, that electron spins play a more important role in the exciton-exciton interaction in comparison to hole spins. We show that a striking similarity, which we observe in the quantum beat signal from GaN and ZnSe, originates from the strong four-particle correlation contribution to the third-order excitonic nonlinearity.

We report on photoluminescence properties of individual macroscopically sized InGaN clusters that were formed in InGaN multiple quantum wells. Phase separation in InGaN results in the formation of clusters with a size of 1-2 mum with three different indium compositions. A small fraction (one in 100-1000) of the clusters shows random telegraph noise in luminescence at room temperature. Superlinear dependence of the luminescence switching rate on excitation intensity indicates that the switching is induced by the cooperation of multiple carriers. (C) 2001 American Institute of Physics.

We employ a pump-induced polarization rotation scheme for the quantitative study of four-particle (exciton-exciton) correlations in a strongly coupled semiconductor microcavity. From the measured rotation angle in the chi((3))-regime, in which the nonlinear response is linearly proportional to pump intensity (I-pump), we evaluate the absolute value of anharmonicity of two-dimensional excitons. Deviation from the chi((3))-regime is observed beyond I-pump = 0.2 kW/cm(2).

Weakly interacting boson model with the account for the bound biexciton is employed to obtain the evolution equations for the macroscopic exciton polarization. The polarization-sensitive degenerate four-wave mixing measurements in frequency domain enable us to estimate the relative contribution of the bound biexciton stale on the nonlinear optical response. In the time-domain measurements, we study the amplitude and phase of the beating in the time-integrated four-wave mixing signal. On the basis of the developed evolution equations, we show that the phase shift of the beating signal is polarization sensitive and is determined by the interplay between excitation-induced dephasing, exciton-exciton interaction and other mechanisms of the nonlinearity, (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Spectrally resolved four-wave mixing measurements in wurtzite GaN show a dependence of the quantum beats' phase on incident polarizations. We observe different phases at the A-exciton and the B-exciton resonances when exciting with circular polarized light. The observed phase difference indicates that exciton-exciton interaction plays a major role in the quantum beat process. The developed analysis allows us to conclude that the spins of the electrons rather than the holes give the major contribution to the exciton-exciton interaction in GaN.

We investigate the gain mechanism in a current GaN based laser diode design using the variable stripe length method and optical nanosecond excitation. The combination of absorption measurements, gain spectra and photoluminescence excitation data allows us to understand the fundamental mechanisms responsible for gain in these devices. We conclude that optical amplification is due to a conventional two-dimensional electron-hole plasma in contrast to the spontaneous emission mechanism in similar light emitting diode structures. (C) 1998 Elsevier Science Ltd. All rights reserved.

The polarization dependence of quantum beats from the A-exciton and the B-exciton in a GaN sample of exceptional quality is studied with four-wave mixing experiments. When changing the incident polarizations from collinear to crossed linear, a pi-phase shift of the beats is observed while the decay rate remains unchanged. This confirms previous theoretical predictions. (C) 1997 Optical Society of America

The optical gain in a Nichia(R) group-III nitride based blue laser diode has been measured at room temperature using the variable stripe length method and nanosecond excitation. Except for stripe lengths less than 50 mu m the device shows considerable deviation from the expected exponential intensity behavior. However, for short stripe lengths large gain values of up to 650 cm(-1) are observed when exciting with 20 MW cm(-2). Plasma recombination from a quantum confined level is suggested as the gain mechanism. (C) 1997 Elsevier Science Ltd.