High-Q Whispering Gallery Modes in dynamic liquid microsphere cavities coupled to sub-micron tapered fibers

Whispering Gallery Modes (WGMs) are the eigenmodes of circularly symmetric cavities which get excited when any kind of waves, like acoustic or electromagnetic, get trapped due to continuous total internal reflections and after orbiting the cavity return to the starting point in phase. WGMs are observed in circularly enclosed buildings with dome-shaped roofs, spherical, toroidal and cylindrical optical or acoustical resonators, rapidly rotating stars, etc. WGMs in optical microcavities have very high Quality factor and the ability to confine high optical power in a very small mode volume, and thus have found many applications in non- linear optics, optomechanics, cavity QED, sensing, integrated photonic circuits, etc. In this talk, I will be presenting my thesis work on high-Q WGMs in single liquid microdrops coupled with tapered fibers of sub-micron waist sizes. Liquid microdrops are WGMs-supporting microcavities with naturally very smooth surfaces. Unlike solid microcavities, these cavities are time-dependent as physical processes, like evaporation and thermally induced oscillations, continuously take place in them. Their dynamic geometry makes evanescent coupling, and the detection and analysis of the WGMs challenging. However, novel studies and applications open up with well coupled microdrop-tapered fiber systems, such as for better understanding the complex dynamics of a liquid drop (like its capillary and acoustic modes), dynamics of evaporation at atomic scales, investigating micro-optomechanics and exchange of energy between the optical and acoustical WGMs, efficient trace sensing of dissolved targets, etc. I will first describe our setups for the fabrication and preparation of microcavities and low loss tapered fibers, and the entire measurement setup. Then, I will present and discuss our observations of WGM resonances of Q ∼ 7e7 in microdrops of silicone oils and of Q ∼ 7e6 in microdrops of glycerol-water mixture. I will explain the hindrance caused by the thermally induced shape fluctuations in measuring the high Q values of the microdrops, and the experimental technique used to overcome the hindrance. I will conclude with our analysis of the contribution of material-dependent loss in the total Q values.