OPTICAL AND TERAHERTZ POWER LIMITS IN THE LOW-TEMPERATURE-GROWN GAAS PHOTOMIXERS

Optical heterodyne conversion, or photomixing, occurs in an epitaxial low-temperature-grown GaAs layer with voltage-biased metal electrodes on which two laser beams are focused with their frequencies offset by a desired difference frequency. Difference-frequency power couples out of the photomixer through a log-spiral antenna at THz frequencies. Pu mping such a device with the maximum optical power of similar to 90 mW at 77 K led to a measured output power of 0.2 mu W at 2.5 THz, approx imately twice the maximum output power of a photomixer operated near 3 00 K. Photomixers that were operated above the maximum optical power w ere destroyed, often because of a thermally induced fracture in the Ga As substrate. The fracture seemed to occur at high pump power when the temperature of the photomixer active area was elevated by roughly 110 K, independent of the bath temperature. (C) 1997 American Institute o f Physics. [S0003-6951(97)03745-5].