Cellular and subcellular ultrasound at the quantum limit with nano-optomechanics

Ultrasound is a noninvasive, real-time, and therefore widely used imaging modality; yet its application in cellular and sub-cellular biology is significantly limited by rapidly increasing acoustic losses in aqueous solutions with decreasing wavelength. Here we introduce a nano-optomechanical cavity transducer platform to generate and detect ultrasound in aqueous solutions with a sub-micron acoustic wavelength. We analyze the transduction process through a combination of finite element method modeling and the coupled differential equations that describe the dispersive optomechanical interaction. Our findings project a signal-to-noise ratio in the thousands, limited by diffraction losses and thermal noise. Ultimately, we establish a viable path towards ultrasound imaging systems operating with a high resolution at the cellular and sub-cellular level. The work presented here also is broadly applicable to biochemical sensing, medical diagnostics, underwater acoustic sensing, and nanoscale imaging, which may benefit from the comprehensive analysis of optomechanical crystal devices operating in aqueous solutions.