Refractive-Index-based ultrasound sensing with photonic crystal slabs

We demonstrate ultrasound detection with 500-$\mu\mathrm{{m}}$-diameter photonic-crystal slab (PCS) sensors fabricated from CMOS-compatible technology. An ultrasound signal impinging a PCS sensor causes a local modulation of the refractive index (RI) of the medium (water) in which the PCS is immersed, resulting in a periodic spectral shift of the optical resonance of the PCS. The acoustic sensitivity is found to scale with the index sensitivity $S$ and quality factor $Q$. A noise equivalent pressure (NEP) of 650 Pa with averaging (7.4 Pa$/\sqrt{\mathrm{Hz}}$) and relative wavelength shifts of up to 4.3$\times10^{-5}$ MPa$^{-1}$ are measured. The frequency response of the sensors is observed to be flat from 1-20 MHz, with the range limited only by our measurement apparatus.