Ring-shaped photoacoustic tweezers for single particle manipulation.

This Letter reports ring-shaped photoacoustic (PA) tweezers that are capable of manipulating single or multiple micron-sized particles. By illuminating a thin layer of an optically absorptive liquid medium with a focused annular pulsed laser beam and a higher pulse repetition rate (e.g., 800 Hz), both acoustic radiation force and instantaneous vaporization repulsion are generated within a certain distance of the illumination region. This makes it possible to conduct continuous and versatile locomotion of single or multiple microparticles. In this Letter, interactions between two or more particles are demonstrated, such as separation, attachment, and grouping of microparticles. The PA tweezers combine some of the advantages of conventional optical and acoustic tweezers and are expected to be a useful alternative approach for the manipulation of microscale objects.