Low-temperature-grown (LTG) GaAs is used as an optical-heterodyne converter or photomixer, to generate coherent continuous-wave output radiation from microwave frequencies up to 3.8 THz. The photomixer consists of an epitaxial layer of LTG GaAs with interdigitated electrodes fabricated on the top surface. Terahertz photocurrents are generated in the gaps between the electrodes, and power is radiated into free space through a three-turn self-complementary spiral antenna. In a photomixer having a 0.27-ps electron-hole lifetime and small electrode capacitance, the output power is practically flat up to about 300 GHz and then rolls off at a rate of approximately 12 dB/oct.

Recent optical heterodyne measurements with distributed-Bragg-reflector diode-laser pumps demonstrate that low-temperature-grown ͑LTG͒ GaAs photomixers will be useful in a compact all-solid-state terahertz source. Electrical 3 dB bandwidths as large as 650 GHz are measured in mixers with low electrode capacitance. These bandwidths appear to be independent of pump-laser wavelength over the range 780-850 nm. Shorter wavelength pumping results in a significant reduction of the bandwidth. The best LTG-GaAs photomixers are used to generate coherent continuous-wave output radiation at frequencies up to 5 THz.

A cw output power up to 0.8 mW is obtained from a low-temperature-grown (LTG) GaAs, 0.3 μm gap, interdigitated-electrode photomixer operating at room temperature and pumped by two modes of a Ti:Al2O3 laser separated in frequency by 0.2 GHz. The output power and associated optical-to-electrical conversion efficiency of 1% represent more than a sixfold increase over previous LTG-GaAs photomixer results obtained at room temperature. A separate LTG-GaAs photomixer having 0.6 μm gaps generated up to 0.1 mW at room temperature and up to 4 mW at 77 K. Low-temperature operation is beneficial because it reduces the possibility of thermal burnout and it accentuates a nearly quartic dependence of output power on bias voltage at high bias. The quartic dependence is explained by space-charge effects which result from the application of a very high electric field in the presence of recombination-limited transport. These conditions yield a photocurrent-voltage characteristic that is very similar in form to the well-known Mott–Gurney square-law current in trap-free solids.