Dielectric Constant Decrease upon Illumination in a Barium Titanate Crystal

A photodielectric effect has been observed in the c-axis dielectric constant of a barium titanate crystal in which illumination produces a decrease in the dielectric constant. When the crystal is exposed to illumination, the dielectric constant decreases in proportion to the time interval of the exposure, until a saturation level decrease of about 0.5 percent is reached. The rate of decrease is wavelength dependent. When illuminated with 403-nm light, the decrease has a value of -0.11 percent per incident millijoule per centimeter squared. The new constant level reached after saturation relaxes back toward the initial higher level when the illumination is removed. There is a partially irreversible component. Measurements made as a function of wavelength indicate that the decrease, before saturation, is proportional to total photo-carrier density. It is suggested that this photodielectric phenomenon could find applications in devices involving the optical control of dielectric properties.