Harnessing evanescent photoacoustic waves for multi-domain imaging.

Photoacoustic microscopy (PAM) offers a non-invasive imaging method that overcomes the limitations of light scattering in biological tissues by visualizing optical contrast through the detection of photo-generated acoustic signals. While optical microscopy has significantly advanced through the exploration of optical evanescent waves, the potential of evanescent photoacoustic (PA) waves in PAM remains largely unexplored. In this work, we demonstrate the generation and detection of evanescent PA waves in PAM by positioning the sample near an interface, which directs these waves into the far-field beyond the supercritical angle (SA). These SA-PA signals exhibit distinct characteristics, including dependence of intensity on the source depths and symmetry in far-field angular patterns. Leveraging these features, we develop a proof-of-concept for supercritical angle photoacoustic microscopy (SA-PAM), which utilizes evanescent PA waves to enable new PAM functionalities, such as surface topography reconstruction and edge detection. This approach highlights the role of acoustic near-field exploration in advancing PA technology.