Femtosecond-laser-induced optical confinement with ping-pong motion

We introduce a novel method using a kilohertz (kHz) amplified 800 nm laser for the first experimental confinement of microparticles within a single beam. This study demonstrates that high-energy kHz pulses can confine 1-μm-radius polystyrene beads in water within ∼26 μm. This approach utilizes the unique properties of high-energy pulsed lasers, distinct from continuous-wave and megahertz pulsed lasers traditionally used in optical trapping. The pulsing nature of the kHz laser generates strong instantaneous forces that attract and confine particles within a specific region, inducing a “ping-pong” motion within the confined space. When the laser pulses strike the microparticles, the strong gradient forces pull the particles toward the laser focus, while the scattering force from the laser pushes them away. This interaction creates a dynamic equilibrium, causing the particles to oscillate continuously in a back-and-forth motion until the laser is blocked. This phenomenon differs from conventional optical trapping, which offers unique particle confinement possibilities. When combined with optical trapping, especially at femtomolar concentrations or single-particle conditions, this novel development shows that the kHz laser draws particles from significant distances toward the focal point, enhancing its trapping efficiency. All experiments were conducted on a single setup, varying only laser characteristics, ensuring high credibility in the results.