Optically excited nano-mechanical modes of a droplet

We show that a surging radiation pressure excites well-defined nanoscale optomechanical discrete modes of a sessile droplet. Using a pump-probe approach, we isolate growth and damped oscillations of droplet modes from background evaporation and steady-state bulge. For a microlitre droplet, we find that the amplitude of a mode increases linearly with the rate of pump power surge, reaching ∼20 nm for 2.7 kW/s. The mode frequency is characteristic of droplet geometry and physical properties but independent of the optical excitation. A Navier-Stokes model of droplet for realistic parameters reproduces the key experimental results. Furthermore, we show phenomenon of frequency splitting for the n = 3 mode of non-spherical droplets. This work opens a route to probe nanoscale shape oscillations of complex droplets subjected to spatiotemporally structured stimulus.