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. A 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 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 low noise-equivalent pressure below 0.64 mPa Hz-1/2 over a frequency range of 2-30 MHz, which is two orders of magnitude better than piezoelectric devices of the same size. Furthermore, the device enables photoacoustic signal detection at a 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. With its exceptional sensitivity, broad bandwidth, and inherent compatibility with chip-scale multiplexing, this platform offers a promising solution for minimally invasive photoacoustic endoscopy, high-resolution imaging systems, and highly dense ultrasound sensor networks.