Observation of thermal acoustic modes of a droplet coupled to an optomechanical sensor

The bulk acoustic modes of liquid droplets, well understood from a theoretical perspective, have rarely been observed experimentally. Here, we report the indirect observation of acoustic vibrational modes in a picoliter-scale droplet, extending up to similar to 40 MHz. This was achieved by coupling the droplet to an ultra-sensitive optomechanical sensor, which operates in a thermal-noise limited regime and with a substantial contribution from acoustic noise in the ambient medium. The droplet vibrational modes manifest as Fano resonances in the thermal noise spectrum of the sensor. This is among the few reported observations of droplet acoustic modes and of Fano interactions in a coupled mechanical oscillator system driven only by thermal Brownian motion.

Automated summary

We have observed the acoustic vibrational modes of liquid droplets up to 40 MHz by coupling them to an ultra-sensitive optomechanical sensor. These modes manifest as Fano resonances in the thermal noise spectrum of the sensor, driven only by thermal Brownian motion.