Scattering of an evanescent surface wave by a microscopic dielectric sphere

A ray-optics scattering model has been developed to determine if multiple reflections between the sphere and the plate could alter the exponential relationship between the scattering intensity and the separation distance as contact is approached. Results indicate that the effect of multiple reflections is dependent on sphere size, refractive indices, and the penetration depth of the evanescent wave. An experimental validation of the model was performed with polystyrene spheres (diameters 7–30 μm) immersed in an alcohol mixture and resting on an MgF2 film that had the same refractive index. Film thicknesses varied between 0 and 300 nm. No significant effect of multiple reflections was measured at an incident angle approximately 2° above the critical angle, which was in agreement with the predictions of the ray-optics model. By contrast, the scattering intensity from a 300-μm sphere was predicted to be much more sensitive to the separation distance at separations below one penetration depth when the incident angle was increased to over 6° above the critical angle.