SPIE Newsroom 10.1117/2.120071.0585 Whispering-gallery modes in photonic tubes John Donegan, R. Alan Moore, Yury Rakovich, Yurii Gun’ko, and Tania S. Perova A new method has been developed to fabricate microtube resonators with strong whispering-gallery-mode emission and quality factors up to 3000. 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 de- veloped 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 telecommuni- cation optical fibers. 1 The cylindrical cavity format is also com- patible with a large variety of sensing modalities such as im- munoassaying and molecular diagnostic assaying. Recent ef- forts 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 these modes, light prop- agates in planes near the surface, with integer numbers of wave- lengths along closed circumferential trajectories. The high de- gree 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 microcavi- ties 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). these photonic structures extends a few micrometers into the surroundings, thus allowing efficient coupling to an external photonic device. However, fabrication of small high-Q cylindri- cal semiconductor microcavities involves complex and expen- sive processes. 3 We have recently developed a simple method for fabricat- ing highly luminescent small aluminosilicate microtubes of ∼7 - 8μm diameter using sol-gel processing and a micro- channel glass membrane as a template. 4 The microtube res- onators for our photonic experiments were fabricated by vacuum-assisted wetting and filtration of alumosilicate gel through a micro-channel glass matrix. When separated from the matrix, this type of microtube is much more optically dense than its surrounding medium. Light propagating inside can therefore be spatially constrained to Continued on next page