Infrared modulator utilizing field-induced free carrier absorption

This research describes an ir modulator that utilizes field-induced free carrier absorption. The intensity of a light beam can be varied by a voltage-controlled absorption process within the modulator. The basic absorption process is free carrier absorption. A modulation voltage is appropriately applied to a semiconductor crystal within the modulator-inducing carriers (holes or electrons) on the surface of the crystal. The light beam is reflected internally from the semiconductor surface where a fraction of the energy in the beam is absorbed by the surface space charge. The density of the space charge on the semiconductor surface is controlled by the applied voltage, which permits the amount of absorption to be varied. Theoretical relationships are derived that describe the modulation process. The theory is compared with the experiment with good agreement.