Whispering gallery mode structure and refractometric sensitivity of fluorescent capillary-type sensors

Fluorescent core microcapillary (FCM) structures present an alternative to optofluidic ring resonators for refractometric or biosensing applications. Instead of a thin-walled capillary that needs to be probed with a tuneable laser, FCMs are comprised of a fluorescent coating deposited on the channel walls of a microcapillary. The high refractive index of the coating serves to confine the fluorescence and leads to the development of whispering gallery modes (WGMs) whose field profile extends into the capillary channel. This work first investigates theoretically the conditions required to optimize the refractometric sensitivity of these structures. The optimal fluorescent coating thickness ultimately represents a trade-off between sensitivity and the mode Q factors. We then use a spectral–spatial mapping method to obtain WGM information along the length of a capillary with variable film thickness. The maps agreed well with the theoretical predictions with one main exception: the experimental sensitivities were higher than the predicted values for some modes, possibly owing to interference modulation effects.