Multiple reflection: A Route to enhanced sensitivity in beam-deflection optical ultrasound sensing

All-optical ultrasound sensors have recently emerged as promising candidates to replace conventional piezoelectric transducers in photoacoustic imaging, owing to their broad bandwidth, compact active area, and good compatibility with light excitation. While non‑interferometric designs offer advantages in robustness, ease of fabrication, and multiplexing capability, their sensitivity is often limited by short acousto‑optic interaction lengths. Here, we demonstrate that multiple reflection provides an effective strategy to overcome this constraint. The core of our approach is a carefully designed optical cavity that enables the probe beam to undergo multiple round‑trips within a confined region, thereby substantially extending the effective acousto‑optic interaction path. When applied to beam‑deflection ultrasound sensing, a direct side‑by‑side comparison reveals that this design achieves a 3.5‑fold enhancement in sensitivity over conventional single‑pass detection. We further illustrate the utility of this sensitivity‑enhanced all‑optical ultrasound sensor through photoacoustic microscopy of a leaf‑skeleton phantom and in vivo imaging of the vascular network in a mouse ear.