An Ultrasensitive and Broadband Micro-Ring based Optomechanical Ultrasound Sensor for Photoacoustic Imaging

Recent advances in ultrasonography and photoacoustic tomography call for ultrasound sensors with high sensitivity, broad bandwidth, miniaturized footprint, and scalability. Conventional piezoelectric transducer is fundamentally constrained by limited sensitivity, element density, and system integration complexity. In this work, we introduce a micro-ring based optomechanical ultrasound sensor using a novel SiN/Si hybrid waveguide structure with a 30 nm air gap engineered within the sensing region. The micro-ring resonator exhibits a high-quality factor of 4.8×10⁴ and a free spectral range (FSR) of 18 nm at the wavelength of 1580 nm. With a compact diameter of 18 µm, the sensor achieves a record-low noise-equivalent pressure below 0.64 mPa Hz⁻¹/² over a frequency range of 2–30 MHz, which is three orders of magnitude better than piezoelectric devices of the same size. Furthermore, the device enables photoacoustic signal detection at the depth up to 10 cm and offers a wide acceptance angle of ± 20° for frequencies up to 20 MHz. High-resolution photoacoustic tomography of a graphite pencil lead is demonstrated, revealing sub-millimetric structural details. We also show an 8-element sensor array employing wavelength-division multiplexing that allows parallel readout through a single optical fiber, effectively overcoming the wiring-density bottleneck inherent in conventional piezoelectric arrays. With its exceptional sensitivity, broad bandwidth, and inherent compatibility with chip-scale multiplexing, this platform offers a transformative solution for minimally invasive photoacoustic endoscopy, high-resolution imaging systems, and highly dense ultrasound sensor networks.