Trapping and levitation of a dielectric sphere with off-centred Gaussian beams. II. GLMT analysis

Abstract

For pt. I see ibid., vol. 2, p.261, (1993). We study the forces exerted by a Gaussian laser beam on a small dielectric sphere in water as a function of beam-off centring, i.e. the distance between the beam axis and the sphere centre. Our experimental data are from the first part work of Angelova and Pouligny who studied the levitation of polystyrene latex spheres, a few micrometres in radius, by a couple of vertical laser beams in water. We also report on newly acquired results, particularly the observation of sustained oscillations of the levitated sphere in water. These data are analysed from the viewpoint of radiation pressure forces, which we calculate using the generalized Lorenz-Mie theory (GLMT). We find that calculated equilibria fit those experimentally recorded with beams of unequal intensities. Experiments with equally intense beams lead to instabilities in the levitated particle position. This is also well explained by GLMT, but the instability threshold is found to be very sensitive to uncontrolled water convection around the particle.