Optical Forces Arising from Phase Gradients

Jesse Amato-Grill, NYU Department of Physics Yohai Roichman, NYU Department of Physics Yael Roichman, NYU Department of Physics Bo Sun, NYU Deparment of Chemistry David Grier, NYU Department of Physics; Center for Soft Matter Research, New York University,

Abstract Intensity gradients in a beam of light exert forces upon illuminated objects--a principle which is the basis of the technique known as optical trapping. This study demonstrates, both theoretically and experimentally, that phase gradients can redirect radiation pressure to create optical force fields transverse to the optical axis. We derive the momentum flux due to phase gradients from the vector potential describing a beam of light, and we show that the resulting forces are generally non-conservative. To realize such “phaseforces,” we use a phase-only spatial light modulator to imprint computer-generated holograms on the wavefronts of a slightly converging laser beam, which is then focused into the intended three-dimensional optical trapping pattern. We observe the effects of phase gradients by subjecting micrometer-sized silica spheres to phase gradients in uniform-intensity optical traps of various geometries. By studying the circulation of such particles in “ring traps,” we find that orbital angular momentum, a familiar phenomenon arising from light with a helical phase profile, is a particular manifestation of phase gradient forces. Further, we show that arbitrary phase profiles can be used to create a new category of optical traps complementary to the more familiar intensity-gradient traps known as optical tweezers. The azimuthal forces resulting from phase gradients thus promise to provide novel avenues for controlling microscopic systems.

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