Broadband Ultrasound Detection Using Silicon Micro-Ring Resonators

High sensitivity, broadband bandwidth, small size and scalability to a fine-pitch matrix of ultrasound (US) sensors are essential for high-resolution ultrasonography and photoacoustic tomography. In this work, we demonstrated a method to build micro-ring resonators (MRR) as US sensors on the Silicon-On-Insulator (SOI) platform. To improve the sensitivity, transverse magnetic (TM) mode is utilized to increase the mode interaction with the cladding as well as reduce the scattering loss caused by sidewall roughness. Besides, the SU-8 elastomer is introduced as cladding to enhance US response. The fabricated 10 μm-radius MRR with only one standard lithography process shows a high quality (Q) of 7.4 × 10 ⁴ and a steep slope of 351 dB/nm. The US sensing bandwidth reaches 165 MHz at −6 dB with a high sensitivity of 14.5 mPa Hz ^−1/2 . The high refractive index of the silicon waveguide contributes to an order of magnitude smaller sensing area than the polymer-based MRR. The proposed sensitive MRRs on the SOI platform makes it very promising for mass manufacturing integrated dense US arrays.