  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.
