Exploring a photo-acousto-optic effect for noncontacting photoacoustic sensing

To avoid the use of ultrasound transducers and coupling gel in photoacoustic microscopy (PAM), we propose photo-acousto-optic tomography (PAOT) for noncontact photoacoustic (PA) sensing. The process consists of two parts. The first portion is the same as typical PAM, which employs a pulsed laser to induce acoustic waves. The difference from typical methods lies in the second part of the process, which applies a DC beam, rather than a conventional transducer, to sense the PA signal. A two-beam optical microscope system was designed to verify the PAOT effect, whereby an AC spot acted as the source to induce a PA signal, while a DC beam is applied to induce the acousto-optic effect for detection of the acoustic wave. We demonstrated the preliminary result that 5-100 Hz AC radiation could derive PA waves in a water-like medium along with detection sensitivity as high as 4.9%-10.0%; besides, the signal waveform could be detected by a DC spot 10-100 μm away for noncontact sensing with detection sensitivity of about 3.7%-10.4%. Without the need for a transducer or coupling gel, PAOT has the potential to modify conventional PAM into a pure optical system, which could make PA imaging more promising in practical applications.