The schematic of the UV-PAM system is shown in Fig. 2 . A nanosecond UV pulsed laser (266 nm wavelength, WEDGE HF 266 nm, Bright Solutions Srl., operating at 16 kHz) is first expanded by a pair of plano-convex lenses (LA-4600-UV and LA4663-UV, Thorlabs Inc.) and spatially filtered by a pinhole (50 µm in diameter, #59–257, Edmund Optics Inc.). The beam is then reflected upward by a mirror and focused on a specimen (with and without clearing) by an objective lens (LMU-5X-NUV, numerical aperture (NA) = 0.12, Thorlabs Inc.) to excite PA signals. The specimen is placed on a quartz coverslip, then the specimen and coverslip are sandwiched in the water tank with two membranes. The water tank is filled with water for acoustic wave propagation so that the excited PA waves can be detected by an ultrasonic transducer (V324-SU, 50 MHz central frequency, 6 mm focal length, Olympus DNT Inc.). The ultrasonic signals are then amplified by amplifiers (56 dB, two ZFL-500LN-BNC+, Mini-circuit Inc) and digitized by a data acquisition card (ATS9350, Alazar Technologies Inc.). Two motorized stages (L-509.10SD00, PI (Physik Instrumente) Singapore LLP) translate the sample in the - and -axis for raster scanning.Fig. 2 ![]()
Schematic of the UV-PAM system. DAQ, data acquisition card.