We propose a flexibly tunable and low-loss optical burette with an all-dielectric bowtie core capillary structure, where nanoparticle arrays can be transported bidirectionally with incident light from one end. Multiple hot spots, acting as optical traps, are periodically distributed at the center of the bowtie cores along the propagation direction because of the mode interference effect of guided lights. By adjusting the beam waist position, the hot spots continuously move across the entire capillary length; thus, trapped nanoparticles also transfer with the hot spots. The bidirectional transfer can be realized simply by changing the beam waist in the forward or backward direction. We confirmed that nanosized polystyrene spheres can be bidirectionally moved along a capillary length of ≈ 20 µm. Furthermore, the magnitude of the optical force can be adjusted using the incident angle and beam waist width, whereas the trapping period can be adjusted using the incident wavelength. These results were evaluated using the finite-difference time-domain method. We believe that this new approach can be extensively used in the field of biochemical and life sciences because of the properties of an all-dielectric structure, bidirectional transportation, and single incident light.

The focus of this study was mainly on the birefringence of nanosized bowtie-shaped slot core fibers that can transport ultrasmall optical spots. Here we showed large birefringence for the fibers reaching as high as 0.33, which is three orders of magnitude greater than normal-sized birefringent fibers, the main cause of which being the impact of geometrical birefringence arising from significant differences between the orthogonal modal distributions. Moreover, we investigated the polarization maintaining property for acute-angled bending in a 5-μm fiber and demonstrated slight degradations of less than 5 dB for the extinction ratio in a 180°bend, which was several orders of magnitude lower than the ratio for the fiber without bending. We used computer simulations for the estimates presented in this study. We believe that the high-polarization maintaining performance presented herein will be required for nanosized optical sensing probes and interconnections in photonic integrated circuits.