Evanescent-wave scattering by electrophoretic microparticles: a mechanism for optical switching

The total internal reflection of light occurring at the interface between glass and a low-index liquid containing suspended microparticles can be electrically controlled. The particles are charged and the glass is coated with a thin, transparent conductor. When the conductor is biased to attract the particles, they scatter and absorb light from the evanescent optical field near the interface, thus reducing the reflectivity. When the conductor is biased to repel the particles, total internal reflection is achieved. Experimental results are given for the time, voltage, and angle-of-incidence dependence of the reflectivity at the interface between an In-Sn-oxide-coated glass surface and a suspension of 0.47-µm-diameter silica particles in acetonitrile. The switching is found to be fast (~ 100 ms) and reproducible. In certain conditions the on/off ratio for a single reflection can be as large as 2:1. A simple theoretical model is developed to interpret these experiments. The model gives a reasonable fit to the data and allows one to extract information such as the particle mobility and the particle density in the evanescent-wave region.