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 that 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 device as well as an ultraprecise method for microfluidic characterization.

中文翻译：

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液滴诱导的光在微毛细管中的定位

在现代微流体研究中，光学回音壁模式（WGM）感测是一种快速发展的检测方法。该方法探讨了沿光学微毛细管壁传播的WGM光谱的扰动，以表征其中的液体介质。在这里，我们显示了二氧化硅微毛细管中的WGM可以通过e逝耦合到水滴而完全定位（而不是受干扰），从而形成高质量因子的微谐振器。用沿着毛细管平移的超细纤维测量的该谐振器的光谱呈现出谐振层次，该谐振层次使我们能够确定液滴的大小以及由于蒸发而导致的液滴长度变化。我们对等于4.5 nm的这种变化的测量分辨率仅受所用光谱分析仪的分辨率限制。