Fig. 1: Schematic illustration of the experimental setup.

After passing through an aperture, the excitation laser beam propagating along the positive z-axis is focused onto the USAF-1951 resolution target by the lens under investigation. The photoacoustic signal is detected by the ultrasound transducer (UT). The insets show detailed views of a metalens schematic, including a diffraction grating phase function a and the nanostructures b.