Infrared modulation using the field-effect properties of germanium.

The absorption of infrared radiation, by free carriers in germanium, has been used to amplitude modulate infrared radiation. In the experiment, a space charge layer consisting of free carriers is established at the inner surface of a germanium sample by using the field-effect technique. Infrared radiation of a wavelength to which the germanium is transparent is then Internally reflected from this surface. The infrared radiation passes twice through the space charge layer for each internal reflection. With each pass through the space charge layer, the amplitude of the transmitted infrared is decreased. Amplitude modulation of the transmitted infrared is accomplished by varying the density of the free carriers in the space charge layer. The observed infrared modulation followed the field effect theory very well for frequencies below 1,000 cps. As the frequency was increased beyond 1,000 cps, a decreasing magnitude and a lagging phase shift characterized the modulated infrared signal. Modulation of the infrared radiation was not observed for frequencies above 10,000 cps. However, with modification of the equipment, it is felt that the frequency could be extended to at least 108 cps --Abstract p. i