Evanescent coupling between two parallel nanowires is investigated using the finite-difference time-domain method. Silica, tellurite, and silicon nanowires are used as typical materials in the simulation. Compared with weakly coupled waveguides, strongly coupled nanowires show much smaller transfer lengths without sacrificing high coupling efficiencies, suggesting possibilities for developing highly compact evanescent-coupling-based nanowire photonic devices, as well as for achieving high-efficiency interconnection between nanowires and external optical systems. Meanwhile, the polarization-dependent coupling properties, the considerably high minimum coupling efficiency, and the supermode-cutoff-like behavior are also observed. Evanescent coupling properties of strongly coupled nanowires demonstrated may provide valuable references for practical applications of optical nanowires.
Endface output patterns of micro/nanofibers (MNFs) are simulated using a Three-Dimension Finite-Difference Time-Domain (3D-FDTD) method. The intensity distribution and beam widths of near- or farfield output patterns of freestanding silica and tellurite MNFs with flat, angled, spherical and tapered endfaces in air and/or water are obtained. It shows that, for a subwavelength-diameter MNF, highly confined output beam can be obtained in the near field, and the beam width can be tuned by the ratio of fiber diameter and light wavelength with a minimum width smaller than the wavelength. Meanwhile, MNFs with shaped endfaces behave differently from standard fibers in reflection, redirection and focus of light beam at the endfaces. These results may offer valuable references for practical evaluation and application of terminated MNFs with wavelength- or subwavelength-scale endfaces.
We demonstrated the evanescent wave of an optical microfiber taper of µm level with the high sensitivity spectroscopic methods such as the difference spectroscopy and the optical heterodyne spectroscopy to observe the acetylene absorption transition.
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