Coupling the thermal acoustic modes of a bubble to an optomechanical sensor.

Optomechanical sensors provide a platform for probing acoustic/vibrational properties at the micro-scale. Here, we used cavity optomechanical sensors to interrogate the acoustic environment of adjacent air bubbles in water. We report experimental observations of the volume acoustic modes of these bubbles, including both the fundamental Minnaert breathing mode and a family of higher-order modes extending into the megahertz frequency range. Bubbles were placed on or near optomechanical sensors having a noise floor substantially determined by ambient medium fluctuations, and which are thus able to detect thermal motions of proximate objects. Bubble motions could be coupled to the sensor through both air (i.e., with the sensor inside the bubble) and water, verifying that sound is radiated by the high-order modes. We also present evidence for elastic-Purcell-effect modifications of the sensor's vibrational spectrum when encapsulated by a bubble, in the form of cavity-modified linewidths and line shifts. Our results could increase the understanding of bubble acoustics relevant to a variety of fields such as lab-on-a-chip microfluidics and biosensing, and could also inform future efforts to optimize the properties of micro-mechanical oscillators.