Whispering-gallery modes in photonic tubes

Abstract

Listen

Translate article

Microcavity structures are designed to enhance the interaction of light with matter. Wavelength-scale structures that confine light can be used to make highly efficient micro-lasers and sensors. Planar, spherical, and cylindrical geometries have all been developed to make efficient micro-resonators. Among these devices, the microcylindrical or microcapillary dielectric resonators have generated significant interest due to their small size and material compatibility with telecommunication optical fibers.1 The cylindrical cavity format is also compatible with a large variety of sensing modalities such as immunoassaying and molecular diagnostic assaying. Recent efforts to develop efficient micro-tube emitters focused on optical modes that are concentrated at the surface of dielectric materials. The main physical phenomenon exploited for this development is grazing-incidence total internal reflection of light resulting in ‘whispering-gallery’ modes (WGMs). In thesemodes, light propagates in planes near the surface, with integer numbers of wavelengths along closed circumferential trajectories. The high degree of confinement of light in WGM results in a high resonance quality factor (Q). Experimentally, the most widely-studied configuration of thin-wall microtube cavities is the microcapillary filled with a highly luminescent dye solution. Both diameter (typically 50-200μm) and wall thickness can be controlled by the etching of commercially-available glass samples in an HF-water solution.2 However, the short-distance evanescent field in these microcavities and the limited photostability of dye molecules are retarding factors for potential applications. In the small-size regime (with diameters less than 10μm), semiconductor microdisks of finite height—micropillars—have been widely used as a tool to control spontaneous emission and confine photons in three dimensions. The evanescent field in Figure 1. Room-temperature photoluminescence spectra of a single free-standing microtube recorded with polarizer orientation parallel to the microtube axis (red trace) and with polarizer rotated by 90 (black trace).