A Polymeric Optical Waveguide Resonator for Reflection‐Mode Isometric‐Resolution Photoacoustic Microscopy In Vivo

Faithful reconstruction of a 3D photoacoustic image requires broadband detection to ultrasonic pressure impulses with sufficient sensitivity. An optical waveguide resonator (OWR) is developed by coating the polymeric elastomer polydimethylsiloxane (≈350 nm in thickness) on a coverslip, in which a tightly‐localized waveguide electric field formed within the polymeric elastomer is accompanied with an enhanced resonance intensity that allows extremely sensitive responses to the photoacoustic transients with ultrafast temporal dynamics. A noise‐equivalent‐pressure sensitivity of ≈17 mPa/Hz −1/2 over a spectral bandwidth of ≈201 MHz is achieved in the OWR, enabling a significantly improved depth resolution of ≈7.0 µm and excellent signal‐to‐noise ratio in photoacoustic imaging. With the combined capabilities of sensitive broadband detection, superior optical transparency, and miniature footprint of the OWR, photoacoustic microscopy is implemented for reflection‐mode volumetric imaging with isometrically micrometer‐scale spatial resolution, providing label‐free microstructural visualizations of zebrafish in vivo. The proposed sensing technology offers new opportunities to advance biomedical photoacoustic investigations.