Selective photoacoustic handling of microparticles via annular laser beams.

Precise manipulation of bioparticles in micro- and nano-fluidic environments is crucial for applications in cancer diagnostics, drug delivery, and single‑cell analysis. Despite the optical and acoustic tweezers provide high accuracy and stability, their flexibility and selectivity are often constrained by factors such as laser‑induced heating, reliance on the optical properties of target particles, and the labor‑ and time‑intensive fabrication of interdigital transducers (IDTs). In this study, we develop an annular beam-driven photoacoustic tweezer (PAT) to implement high-precision, selective handling of microparticles via annular beam-generated transient photoacoustic waves (T-PAWs). Realized via an axicon-enabled optical setup, a nanosecond laser beam is shaped into a ring pattern and focused on a polycrystalline silicon substrate coated with chromium and aurum layers, which supports a confined liquid layer. The annular laser beam photoacoustically generates water-borne T-PAWs and establishes an annular acoustic potential well (APW), which in turn establishes a radially inward acoustic radiation force (ARF), to continuously guide target microparticles, and simultaneously a concurrent outward‑propagating pressure along the ring periphery to exclude non-target microparticles. To elucidate the mechanism of laser‑induced T-AWs, we develop a multiphysical finite element model integrating photo‑thermo‑acoustic coupling, and experimentally validate the capability of the developed PAT in selectively capturing, assembling, and isolating single and multiple silicon microparticles and hydrogel microspheres. This annular PAT bridges selective, label-free trapping with fabrication simplicity by eliminating the need for IDTs, offering a rapidly reconfigurable method and non-contact approach for versatile bioparticle manipulation.