Trapping and levitation of a dielectric sphere with off-centred Gaussian beams. I. Experimental

The authors describe an experiment in which a dielectric sphere, a few mu m in size, is trapped and levitated by two vertical laser beams, whose separation can be varied continuously. Each beam is itself the superposition of two counterpropagating (up and down) Gaussian beams. This four-beam configuration has applications as a double optical levitation trap or as an all-optical dynamometer. The report is dedicated to the experimental study of forces exerted on a dielectric sphere in water in a non-symmetrical situation, i.e. with off-centred beams. The authors derive the shapes of the longitudinal and of the transverse forces as a function of the distance between the sphere centre and the beam axis in the linear regime, i.e. in conditions where these forces are found proportional to the beam intensity.