An omnidirectional and ultra-low frequency acoustic sensor using optically levitated microparticles in a dual-beam trap

Omnidirectional and low-frequency acoustic sensors are crucial for comprehensive full-wave acquisition, sound source localization, and seismic imaging. Conventional micro-electro-mechanical system (MEMS)-based acoustic sensors rely on complex multi-layer or array structures to achieve omnidirectional detection, yet they cannot operate below 20 Hz due to dimensional limitations imposed by MEMS fabrication processes. Optically based acoustic sensors also face challenges in realizing multi-dimensional and omnidirectional detection, especially in the ultra-low-frequency regime. Here, we introduce an optical-tweezer-based omnidirectional acoustic sensor capable of detecting frequencies as low as 1 Hz. In this system, acoustic waves perturb the force balance of the optically levitated particle, which, in turn, acts as the sensing element by converting incoming sound into detectable vectorial motion within the optical potential. In ambient air, the trapped particle exhibits overdamped motion, enabling detection down to 1 Hz. The acoustic detection bandwidth of the particle can also be flexibly tuned by varying the trapping power and particle diameter, with a measured high-frequency response reaching ∼20 kHz. Using a 2 μm diameter levitated fused-silica particle, the calibrated minimum detectable pressure reaches 162 μPa/√Hz at a frequency of 3.4 kHz. Experimental results confirm that the particle responds uniformly to acoustic waves from all incident directions. Moreover, the particle displacement along the sound propagation direction is over an order of magnitude greater than that in the orthogonal direction, demonstrating the sensor’s superior directional discrimination capability. The proposed sensor bridges a key gap in omnidirectional ultra-low-frequency acoustic sensing and offers a promising solution for a wide range of applications, including seismic monitoring, oil and gas exploration, sonar detection, and biomedical diagnostics.