Localization of light in an optical microcapillary introduced by a droplet

TL;DR

This paper demonstrates that whispering gallery modes in a silica microcapillary can be fully localized by a water droplet, creating a high-Q microresonator that enables precise microfluidic measurements and new photonic device designs.

Contribution

It introduces a novel method of fully localizing WGMs in microcapillaries using a water droplet, enabling high-resolution microfluidic characterization.

Findings

Achieved 4.5 nm resolution in measuring droplet evaporation

Demonstrated full localization of WGMs by a water droplet

Proposed a new microfluidic photonic device concept

Abstract

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 microcapillary to characterize the liquid medium inside it. Here we show that WGMs in a silica microcapillary can be fully localized (rather than perturbed) by evanescent coupling to a water droplet and, thus, form a high quality-factor microresonator. The spectra of this resonator, measured with a microfiber translated along the capillary, present a hierarchy of resonances which allow us to determine the size of the droplet and variation of its length due to the evaporation. The resolution of our measurements of this variation equal to 4.5 nm is only limited by the resolution of the optical spectrum analyzer used. The discovered phenomenon of complete localization of light in liquid-filled optical microcapillaries suggests a new type of microfluidic photonic devices as well as an ultraprecise method for microfluidic characterization.