Ray optics calculation of the radiation forces exerted on a dielectric sphere in an evanescent field.

A ray optics approach was used to calculate the forces and the torque exerted on a dielectric sphere in the evanescent field produced by a linearly polarized Gaussian beam. The particle was assumed to be immersed in a dielectric fluid next to a solid dielectric plate with the evanescent field produced at the solid-fluid interface. Comparisons with calculations performed by use of more rigorous electromagnetic wave theory show that the ray optics results agree to within a factor of 2 even for particle radii as small as twice the incident wavelength. Calculation of the forces for conditions typical of a total internal reflection microscopy experiment show that the evanescent field has a negligible effect on either the net forces exerted on the particle or the particle motion (i.e., rotation or translation parallel to the interface). By our modifying the parameters of the experiment, however--namely, the incident beam power, radius of incident beam, and evanescent wave penetration depth--forces that are comparable with the net particle weight and capable of translating the particle several micrometers per second, as well as rotating the particle several revolutions per second, can be produced. The ability to micromanipulate a particle in this fashion could offer useful applications for studying particle and surface interactions.