A new mode of operation of photoconductive detectors is described in which a non-linear current-voltage characteristic is induced by illumination. Outputs at harmonic frequencies can be obtained from devices biased with alternating current or at the modulation frequency of a modulated, alternating bias. Several different ways of achieving the non-linearity are described, one of which uses conventional detector structures. The advantages of this mode of operation are that the large bias-voltage pedestal, normally associated with long-wavelength photoconductive detectors, can be separated from the photo-signal by simple electrical filters and that the problem of flicker noise, in both the detector and the following electronics, is considerably reduced. Photoconductive devices thus become competitive with photodiodes, which may be more difficult to produce with ideal performance and stable characteristics, in detecting radiation with very low modulation frequencies. Possible applications for the device may be in slow, parallel-scanned linear arrays; in "staring" arrays; in ultra-sensitive detectors employ-ing long integration times, or in position-sensitive, point source detectors.