PARAMETRIC STUDY OF THE FORCES ON MICROSPHERES HELD BY OPTICAL TWEEZERS

Optical-trapping forces exerted on polystyrene microspheres are predic ted and measured as a function of sphere size, laser spot size, and la ser beam polarization. Axial and transverse forces are in good and exc ellent agreement, respectively, with a ray-optics model when the spher e diameter is greater-than-or-equal-to 10 mum. Results are compared wi th results from an electromagnetic model when the sphere size is less- than-or-equal-to 1 mum. Axial trapping performance is found to be opti mum when the numerical aperture of the objective lens is as large as p ossible, and when the trapped sphere is located just below the chamber cover slip. Forces in the transverse direction are not as sensitive t o parametric variations as are the axial forces. These results are imp ortant as a first-order approximation to the forces that can be applie d either directly to biological objects or by means of microsphere han dles attached to the biological specimen.