Optical trapping of Rayleigh particles using a Gaussian standing wave

We suggest a modification of a single beam optical trap which enables more effective axial trapping of nanoparticles. We employed interference of an incident wave and the wave which is reflected by the bottom of the trapping cell to create a standing wave trap. The scattering force is strongly suppressed for a highly reflective surface in this configuration and consequently the axial force is represented only by the axial gradient force. The main advantage of the standing wave set-up is that it produces a much stronger axial gradient force than the single beam trap, even without high N.A. focusing optics. The trap is less than four times deeper than the single beam one produced by a laser of the same power so that smaller particles could be trapped in the vicinity of an array of stable positions separated by lambda/2 along the beam axis. Even the axial trap stiffness is several orders higher than in the single beam trap. (C) 1998 Elsevier Science B.V. All rights reserved.