Trapping of dielectric microsphere by radiation force

Abstract

We report the trapping of a 5 μm-dia polysterene sphere (refractive index n = 1.61 at 486 nm) by the radiation (light) force that is generated when the sphere interacts with a tightly-focused laser beam. The resulting refraction and reflection alter the momenta of the photons in the beam, and give rise to a force that acts on the center of mass of the microsphere. For an aberration-free focused beam, the direction of the force is along the optical axis. If the refractive index of the sphere is greater than its surrounding medium then the force pulls it towards the most intense part of the beam where stable equilibrium is achieved. The generated force is large enough to counteract the effect of convection, gravitational force, and buoyancy, and particle is trapped at this particular location of the object space.
The trapping beam which is produced by a multimode Argon Ion laser (total output power = 200 mW), is focused towards object space using an infinity conjugate objective lens (N.A. = 0.70). The optical system is built around a research-grade inverted optical microscope. The object space is illuminated by white light and tens of microspheres can be viewed simultaneously from a video monitor. The microspheres are suspended in water and move freely either individually or in aggregates when not in interaction with the laser beam.