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.

An analysis has been carried out of optical heterodyne conversion with an interdigitated-electrode photomixer made from low-temperature-grown (LTG) GaAs and pumped by two continuous-wave, frequency-offset pump lasers. The analytic prediction is in excellent agreement with the experimental results obtained recently on a photomixer having 1.0~pm-wide electrodes and gaps. The analysis predicts that a superior photomixer having 0.2~pm-wide electrodes and gaps would have a temperature-limited conversion efficiency of 2.0% at a low difference frequency, 1.6% at 94 GHz, and 0.5% at 300 GHz when connected to a broadband 100 fi load resistance and pumped at hv=2.0 eV by a total optical power of 50 mW. The predicted 3-dB bandwidth (193 GHz) of this photomixer is limited by both the electron-hole recombination time (0.6 ps) of the LTG-GaAs material and the RC time constant (0.5 ps) of the photomixer circuit.