Toward Ultrasensitive, Broadband, Reflection‐Mode In Vivo Photoacoustic Microscopy Using a Bare Glass

Abstract Accurate volumetric photoacoustic image reconstruction requires ultrasonic detection with sufficient sensitivity to pressure transients over a broad spectral bandwidth. By using the ultrafast temporal dynamics and high‐sensitivity responses to refractive index changes of optical surface waves, a structureless ultrasonic sensor is proposed for photoacoustic impulse measurement that is manufactured simply by covering a ubiquitous bare glass with several drops of water. Ultrasensitive photoacoustic detection with a broadband frequency response is achieved by converting the photoacoustically modulated phase shift in the optical reflection into a time‐varying polarization. This sensor achieves noise‐equivalent pressure sensitivity at ≈93 Pa while maintaining an acoustic bandwidth of ≈174 MHz. Incorporation of the transparent sensor into an optical‐resolution photoacoustic microscope enables reflection‐mode 3D imaging with isometric spatial resolution on the micrometer scale. Label‐free volumetric visualizations of adult zebrafish are obtained in vivo. The favorable performance of the proposed sensor offers considerable potential for advancement of biomedical photoacoustic investigations.