Photoacoustic tweezers generated by multiple superposed laser pulses in air

We propose a novel method for generating photoacoustic tweezers via multi-ultrasonic resonance in air. In this study, a focused ultrashort laser pulse with a 50 ns pulse width is used to generate ultrasonic resonance by transferring thermal energy to atmospheric H₂O in air. Traveling photoacoustic tweezers are generated by modulating the superposition of multiple ultrasonic waves. Through numerical simulations, we obtained the acoustic pressure and temperature fields of the photoacoustic waves. Experimentally, 10 μm microspherical polystyrene particles were placed messily in a 200 μm wide square microfluidic tube. The resulting shapes of the microparticles after manipulation by the photoacoustic tweezers proved that our experimental results align well with theoretical predictions. We demonstrate that the interaction of laser pulses with water vapor can generate both acoustic waves and photoacoustic tweezers.