EVANESCENT-WAVE SCATTERING BY ELECTROPHORETIC MICROPARTICLES - A MECHANISM FOR OPTICAL SWITCHING

The total internal reflection of light occurring at the interface betw een glass and a low-index liquid containing suspended microparticles c an be electrically controlled. The particles are charged and the glass is coated with a thin, transparent conductor. When the conductor is b iased to attract the particles, they scatter and absorb light from the evanescent optical field near the interface, thus reducing the reflec tivity.; When the conductor is biased to repel the particles, total in ternal reflection is achieved. Experimental results are given for the time, voltage, and angle-of-incidence dependence of the reflectivity a t the interface between an In-Sn-oxide-coated glass surface and a susp ension of 0.47-mu m-diameter silica particles in acetonitrile. The swi tching is found to be fast (similar to 100 ms) and reproducible. In ce rtain conditions the on/off ratio for a single reflection can be as la rge 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 parti cle density in the evanescent-wave region.