SEMICONDUCTOR PHOTODETECTOR

A semiconductor photodetector (1) for detecting short duration laser light pulses of predetermined wavelength in a light signal (2) comprises a micro-resonator (3) of vertical Fabry-Perot construction having a Bragg mirror pair, namely, a front mirror (5) and a rear mirror (6) with an active region (8) located between the front and rear mirrors (5,6). An N-type substrate (11) supports the rear mirror (6). The light signal (2) is directed into the active region (8) through the front mirror (5) while a pump beam (17) is directed into the active region (8) at an end (18) thereof. The spacing between the front and rear mirrors (5,6) is such as to cause light of the predetermined wavelength to resonate between the mirrors (5,6). The semiconductor material of the active region (8) is selected so that one photon from each of the light signal (2) and the pump beam (17) are required to transfer one electron from a valence band (21) of the active region (8) across a bandgap (22) to a conduction band (20) so that the active region operates on the principle of Two-Photon Absorption. On the active region (8) being simultaneously subjected to the pump beam and light of the predetermined wavelength in the light signal (2) electrons are transferred by Two-Photon Absorption from the valence band (21) to the conduction band (20), thus causing a change in the voltage developed across the active region (8). The change in voltage is detected between an electrode (15) on the substrate (11) and electrodes (8) on the front mirror (5) thereby indicating the presence of light of the predetermined wavelength in the light signal (2).