# Refractive-Index-based ultrasound sensing with photonic crystal slabs

## TL;DR

This paper presents a novel ultrasound sensor using photonic crystal slabs that detect refractive index changes in water caused by ultrasound, achieving high sensitivity and a flat frequency response from 1-20 MHz.

## Contribution

The work introduces CMOS-compatible photonic crystal slab sensors for ultrasound detection based on refractive index modulation, with demonstrated high sensitivity and broad frequency response.

## Key findings

- Noise equivalent pressure of 650 Pa with averaging
- Relative wavelength shifts up to 4.3×10^{-5} MPa^{-1}
- Flat frequency response from 1-20 MHz

## Abstract

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.