Intensity Distribution and Trapping Potential of a Defocussed Optical Tweezer

Optical trapping technique has been used for various purposes since its advent [1]. One of the interesting applications of optical trapping has been to manipulate large assemblies of colloidal particles. In this paper we investigate a defocussed optical tweezer created by a low numerical aperture objective whose focal plane is controlled by an external telescope assembly [2]. We observe the intensity profile of this trapping beam away from the focal plane and model this intensity distribution using Fresnel Kirchoff integral. We show a close match between our observations and theoretical distribution. We correlate this intensity profile with the potential of the optical trap for a small particle in Rayleigh regime. By comparing this potential with the thermal fluctuations we define a trapping width for small particles and show that it matches with our experimentally observed dimensions of assemblies of small colloids. This study shows that intensity distribution using Fresnel Kirchoff integral can be used to understand trapping of small particles.