A. Ohlinger et al., Phys. Rev. Lett. (2012)
An optically trapped nanoparticle can act as an ultrasensitive detector of sound. Pressure waves from a nearby sound source are detected by tracking the small displacements of a trapped gold nanoparticle from its equilibrium position in the neck of a focused laser beam (red region). The sound waves are produced by an aggregate of nanoparticles that vibrate when heated by a laser (green region).