Photoacoustic manipulation of microparticles by using a continuous-wave laser

Abstract

The manipulation of matter at the micro- and nanoscale has seen significant advancements, driven by continual progress in tools and techniques. Among emerging technologies, photoacoustic tweezers could integrate optical and acoustic principles. Traditionally, the photoacoustic effect relies on shortpulsed lasers to ensure thermal and stress confinement, irradiating an absorbing material to induce localized heating, rapid expansion, and the generation of acoustic pressure waves. Recent studies, however, suggest that acoustic waves can also be generated by using longer pulse durations. Here, we present, to the best of our knowledge, the first demonstration of silicon microparticle manipulation in water using a continuous-wave (CW), low-power laser with µs-long rectangular pulses (20 µs). The laser is focused through a 60x objective onto titanium nanofilms deposited on the cell wall. Despite surpassing thermal (~140 ns) and stress confinement (~0.3 ns) time for titanium, our results reveal the effective generation of acoustic waves, capable of displacing 2.5-µm diameter particles by up to 4µm in average under 3.3 ms or average velocities of up ≥900 µm/s. Using digital holography, a light ring was also used to manipulate microparticles. These findings underscore the feasibility of photoacoustic manipulation of microparticles expanding the horizons for applications of photoacoustic technology.