Reflection Properties Of Micro-/nanofibers And Their Applications In Photonics Devices

Recent years witness one of the most important trends in photonics in the miniaturization of photonic components and devices with smaller footprints, better performance and higher integration density. As one of the typical micro-/nanoscale waveguiding structures, optical micro-/nanofibers (MNFs) have attracted more and more attentions for their high potentials in realizing micro-/nanophotonic components and devices, due to their easy fabrication, low loss, high diameter uniformity, low surface roughness, excellent mechanical property, strong evanescent field and tight optical confinement.In the first part of this work, based on waveguiding properties of the MNFs including electromagnetic fields, Poynting vector, and the fractional power inside/outside the fiber core, we theoretically investigate the possibility of nanoparticle sensing with evanescent fields of a waveguiding MNF. Calculated Rayleigh-Gans scattering of spherical particles suggested that, this kind of MNF sensor is possible to detect nanoparticles/molecules located in the vicinity of a MNF surface to the single particle level.While the above-mentioned studies assuming that MNFs are infinitely long without endface effects, more and more micro-/nanophotonics components and devices rely on endface effects of MNFs or nanowires, e.g., subwavelength-dimension point sources, nanolasers, and all-optical switches, in which endface effects play critical roles.In the second part of this work, we theoretically investigate the endface effects, including the endface output patterns, FWHM of output beam in the near field and endface reflectivities (ERs), of MNFs with flat, inclined and shaped endfaces. Numerical simulations using a three-dimensional finite-difference time-domain method show that for a subwavelength-diameter MNF, highly confined output beam can be obtained in the near field, even within the wavelength scale. Meanwhile, MNFs with shaped endfaces behave differently from standard fibers in reflection, redirection and focus of light beam at the endfaces. ERs of MNFs or nanowires are usually lower than 10% under the single-mode operations, even for MNFs with high indices. Additionally, the ERs decrease with the decreasing diameters of MNFs, which prevents very thin MNFs or nanowires from being used in nanolasers based on endface reflection.In view of the low ERs of MNFs, in the third part of this paper, we propose a miniaturized Sagnac loop mirror assembled with MNFs. By tuning the coupling length of the evanescently coupled loop, it is possible to tune the reflectivity of the loop mirror. Experimentally, an all-fiber Fabry-Perot resonator based on two MNF Sagnac loop mirrors is demonstrated, which shows clear resonant responses with typical quality factor (Q factor) of 5700, free spectral range (FSR) of about 0.88 nm, and a maximum extinction ratio of 18 dB. ER of 48% is achieved in such kind of Sagnac loop mirror, which is much higer than that of a MNF or nanowire. The possibility of tuning the Q factor, FSR, and extinction ratio by micromanipulation is also verified. The loop-mirror-based resonator demonstrated here offers advantages of small footprints, easy fabrication, high compactness, tunability, and easy integration with fiber system, which may find applications in micro-/nanophotonic components or devices including filters, lasers and sensors.