Optical forces arising from phase gradients

Optical traps use forces exerted by specially structured beams of light to localize microscopic objects in three dimensions. In the case of single-beam optical traps, such as optical tweezers, trapping is due entirely to gradients in the light's intensity. Gradients in the light field's phase also control optical forces, however, and their quite general influence on trapped particles' dynamics has only recently been explored in detail. We demonstrate both theoretically and experimentally how phase gradients give rise to forces in optical traps and explore the sometimes surprising influence of phase-gradient forces on trapped objects' motions.