Donut-shaped beam could trap particles

Donut-like laser beams have recently attracted much interest for use in trapping micro- and nanoscale particles. Scientists at National Cheng Kung University (Tainan City, Taiwan) and Tokai University (Kanagawa, Japan) have proposed a three-lens configuration for generating a stable donut-like vortex laser beam with controlled Ince-Gaussian-mode operation of laser-diode-pumped solid-state lasers. Controlling the lateral off-axis angle of the pump beam onto the laser crystal generates a donut-like beam after passing through the three-lens configuration of a simple astigmatic mode converter in a concave-convex laser cavity.

Despite a decenter error between the concave mirror and the spherical-cylindrical lens of ±5 μm, the resonator is still able to generate a beam with a high mode purity of 99%. Moreover, simply adding a Q switch inside the resonator allows the proposed structure to produce a pulsed donut-shaped beam. The donut-like laser could be useful as an optical tweezer for confining and manipulating microparticles, biological cells, and molecules. With further modification, the proposed structure can generate a novel coherent white-light donut-shaped supercontinuum. Contact Shu-Chun Chu at [email protected].