The electro-optic effect in. crystals of class Vd such as ADP can be used for modulating or shuttering light beam, but birefringent nature of the crystals limits the angular aperture. If crystals of class Td are available, this difficulty is overcome, and the construction of a light modulator with a large useful angular aperture becomes possible. ZnS or CuCl are crystals of Td class with a large electro-optic effect, but it is very difficult to grow large crystals by artificial means.
The electro-optic coefficient of zincblende, which is cubic-ZnS in nature, is measured by using Senarmont compensator method for various wavelength, and the application to light modulator of wide aperture is experimented. The electro-optic coefficient of ZnS crystal is 5.9×10-8cm/statvolt at room temperature and wavelength of 546mμ. The voltage for obtaining half-wave retardation As about 10 kV.
