Optical Traps As Force Transducers: The Effects Of Focusing The Trapping Beam Through A Dielectric Interface

The use of optical forces generated by a focused laser beam for manipulating single polymer chains and colloidal particles has begun to receive increasing attention in polymer and colloid science and in biophysics. Devices based on such optical traps are increasingly used for probing the elasticity of single polymer chains and for studying the structure and mechanical behavior of biopolymers and their relation to biological activity. Colloidal forces and viscoelastic behavior of the suspending fluids can also be examined through the use of optical traps as force transducers and from the dynamics of optically bound probe particles. Such applications require a precise understanding of the trap potential and its dependence on the distance between the trap center and the glass/liquid interfaces that exist in experimental arrangements. This paper addresses some of the issues relevant to the above class of applications of optical traps using wave optics calculations. It is shown that the use of simple geometric optics corrections for the change in focus caused by a dielectric interface could lead to errors in the estimation of the measured forces. The occurrence of secondary traps and corresponding deviations from the commonly assumed harmonicity of trap potentials are also identified