Surface Nanoscale Axial Photonics (SNAP) for Optofluidics

Sensing with optical whispering gallery modes (WGMs) is a rapidly developing detection method in modern microfluidics research. This method explores the perturbations of spectra of WGMs propagating along the wall of an optical microresonator to characterize the liquid medium inside it. Out of the many available types of WGM microresonators, the surface nanoscale axial photonics (SNAP) platform enables fabrication of resonant ultralow loss photonics structures at the surface of an optical fiber with unprecedented precision currently approaching 0.1 angstroms. In this work, first we explore a new technique for the creation of SNAPs, by using a regular hydrogen-oxygen torch, which requires less equipment than current techniques. The transmission spectra shows that light can be fully localized by pulling a fiber, with very low loss resonant modes. We then present the first demonstration of a platform with potential for microfluidic sensing based on SNAP microresonators fabricated in silica capillary fiber with ultra-thin walls by local annealing with a focused CO2 laser and internal etching with hydrofluoric acid. This demonstration paves the groundwork for advanced microfluidic sensing with SNAP microresonators. Finally, we show that light circulating in a silica microcapillary can be fully localized by evanescent coupling to a water droplet forming a high Q-factor microresonator. The discovered phenomenon suggests a novel method for microfluidics sensing and a new type of tunable resonant microfluidic-based photonic devices