Georgiy Tkachenko, Daan Stellinga, Andrei Ruskuc, Mingzhou Chen, Kishan Dholakia, and Thomas Krauss
Optics Letters 43(14), 3224-3227 (2018)
Contactless manipulation of micron-scale objects in a microfluidic environment is a key ingredient for a range of applications in the biosciences, including sorting, guiding and analysis of cells and bacteria. Optical forces are powerful for this purpose but typically require bulky focusing elements to achieve the appropriate optical field gradients. To this end, realizing the focusing optics in a planar format would be very attractive and conducive to integration of such microscale devices either individually or as arrays. Here, we report on the first experimental demonstration of optical trapping in water using planar silicon metalenses illuminated with a collimated laser beam. The structures consist of high-contrast gratings with a locally varying period and duty-cycle. They are designed to mimic parabolic reflectors with a numerical aperture of 0.56 at a vacuum wavelength of 1064 nm. We achieve both two and three-dimensional trapping, with the latter realised by omitting the central Fresnel zones. The study highlights the versatility of such lithographically defined metastructures for exerting optical forces without the need for traditional optical elements.