Cavity Opto-Mechanics in Liquid Droplets

Abstract

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 [1]. 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 [2]. 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 [5]. 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. Their optical quality factor is around 107 and is limited by optical absorption by the liquid material [7]. Cavity-enhanced Brillouin scattering stimulate surface-acoustic waves circumferentially propagating in a single droplet that thus turns into a high-quality hypersound-laser micro-mechanical oscillator, in the 60–70 MHz range. Analysing the beat note between the pump and Stokes-scattered light fields, we investigate its mechanical finesse and acoustic amplitude as a function of the pump optical power. Then, its short-term and long-term frequency stability is characterized with a time-domain and Allan variance analysis [8]. The stimulated generation of acoustic waves on microlitre-nanolitre droplets gives momentum to new optical schemes for characterization of material viscous-elastic properties, laboratory investigation of atmospheric phenomena and mass sensing for analysis of biological fluids based on ultrasound-hypersound coherent generation and detection. New experiments and simulations are currently underway in order to study nonlinear properties and sensing capabilities of other liquid materials using different optical wavelengths.