Abstract: If a semiconductor device is prepared so that it contains a photoconductive region in electrical series with a photovoltaic region, (i.e., a Schottky barrier or p-n junction) it can function as an optical amplifier and detector. When weak ac light plus an intense dc light are focused on this sample in an appropriate manner, the detected ac electric current will correspond to the intensity of the dc light but have the phase of the ac light; thus a weak ac light signal is effectively amplified, or a dc light signal is converted into an ac electrical signal capable of synchronous detection.