Sagnac-witnessed laser deflection is an ultra-sensitive acoustic detector

Laser-deflection-based acoustic sensing is known for high bandwidth but low sensitivity. By embedding the sensing laser within a Sagnac interferometer and incorporating split-beam detection-originally developed for optical trapping microscopy-we demonstrate sensitive acoustic detection in air with a 2 MHz bandwidth. In a direct comparison, our method far-exceeds performance metrics of a state-of-the-art, commercially-available, high-bandwidth microphone. In upcoming large-volume-bubble-chamber searches for dark matter, our method could replace traditional acoustic sensors confined to the chamber's exterior where signals are weakest.