  Integrated photonic sensors can provide large scale, flexible detection
schemes. Photonic crystal slabs (PCS) offer a miniaturized platform for
wideband, sensitive ultrasound detection by exploiting the photoelastic effect
in water. However, poor modal overlap with the sensing medium and
non-negligible absorption loss of the aqueous medium have previously limited
PCS sensor performance. In this study, we detail the development and
optimization of a PCS-based acoustic sensor, by adding to it a low-loss
high-index polymer cladding layer. Exploiting a mode-optimized TM-like optical
resonance present in a PCS, with high bulk index sensitivity (>600 nm/RIU) and
quality factor Q (>8000), we demonstrate real-time ultrasound-sensing at a
noise equivalent pressure (NEP) of 170 Pa (1.9 Pa/rt Hz). The PCS sensor is
backside-coupled to optical fiber which, along with its intensity-based
ultrasound-sensing architecture, will allow us to scale up easily to a 2D
array. This work paves the way to a sensitive compact ultrasound detector for
photoacoustic-based diagnostics and monitoring of tissue.
