Isometrically Resolved Reflection-Mode <i>In Vivo</i> Photoacoustic Microscopy Enabled by an Integrated Ultrasonic-Optical Sensor

Photoacoustic microscopy often faces challenges in direct measurement of broadband photoacoustic waves due to the use of narrow-bandwidth and optical opaque ultrasonic transducers, which compromises the depth localization accuracy in photoacoustic volumetric imaging. To overcome these challenges, we propose an Integrated Ultrasonic-Optical Sensor (IUOS) that seamlessly integrates excitation laser transmission and ultrasonic detection within a single device. Employing a nearly common-path Michelson interferometer configuration, the IUOS transduces photoacoustic pressure transients into time-varying polarization states of the reflected beam, achieving an impressive sensitivity of approximately 127 Pa over a broad bandwidth of 158 MHz at -6 dB. Furthermore, the sensor allows nearly lossless transmission for the excitation laser at a wavelength of 532 nm. When integrated into an optical-resolution photoacoustic microscope (OR-PAM), it allows for reflection-mode volumetric imaging with enhanced axial resolution. Notably, the sensor has successfully achieved label-free volumetric visualization of zebrafish <i>in vivo</i> with greatly improved depth localization accuracy, highlighting its considerable potential to advance biomedical photoacoustic research.