Iso-index coupled-wave electrooptic filter

A new optical filter, first demonstrated more than a decade ago and recently extended to other frequencies and materials, is based on the accidental isotropy of refractive indexes that occurs in certain uniaxial semiconductors near the band edge. This paper shows how coupling of light energy can take place between ordinary and extraordinary polarizations at the isotropic point by application of a dc electric field. When placed between crossed polarizers, these materials can thus act as narrow-band filters. The field-of-view characteristics have been analyzed; and it is concluded that this type of filter can, in principle, accommodate a 2π field with less than 20 percent increase in passband over that of the narrow-field condition. It is noted, in particular, that AgGaS2exhibits the required change of sign of its birefringence at a wavelength of 4970 Å (blue-green), with a rate of change that could provide a passband of only 0.2 Å in a 1 cm sample.