Droplets are very simple physical systems whereby surface tension shapes liquids into ideal optical and mechanical cavities. Stimulated Brillouin scattering (SBS) is the strongest optical nonlinearity in dielectrics and can generate acoustic waves with unrivalled efficiency, typically at GHz rates . Basically, SBS in dielectrics is driven by the refractive-index modulation induced in the material by the light field itself. Forward Brillouin scattering allows much lower vibrations rates than in the backward direction (< 100 MHz), thereby reducing material viscosity losses . Until recently, forward SBS opto-mechanics has been demonstrated only with solid whispering-gallery-mode (WGM) micro-resonators [3, 4]. In liquids, we observed such effects using oil droplets . In our work, we excite optical WGMs in vertically-suspended liquid-polymer droplet microresonators using free-space laser beams (Fig. 1) [5, 6]. Droplets are made from a low-viscosity liquid polymer with radii ranging between 140 and 150 μm.
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