Two- and three-beam interferometric optical tweezers

We present an experimental demonstration of multiple optical tweezers based on interference of two co-propagating beams that intersect at a given angle and form interference fringes (asymmetric optical traps) at the focal plane of a focusing lens. Since this arrangement provides only two-dimensional trapping when the objects are pushed against the coverglass, we added the third counter-propagating beam. This beam did not interfere with the previous two but compensated their radiation pressure. Therefore, stable three-dimensional confinement into multiple fringes is achieved. We quantified experimentally the maximal optical forces exerted on I pm polystyrene bead in both configurations and compared them with theoretical predictions. Reasonable good coincidence was found especially for two-dimensional trapping. (c) 2005 Elsevier B.V. All rights reserved.