New Design for Increased THz Power From LTG GaAs Photomixers

Photoconductive mixing in ultrafast photoconductors [e.g., low-temperature-grown (LTG)-GaAs] represents an optoelectronic means of generating coherent radiation at THz frequencies from optical pump lasers. To date the power from such photomixers has been limited to microwatt levels by thermal burn-out. In this paper, we analyze a new design in which the LTG-GaAs is fabricated both in a resonant planar dipole (to efficiently couple out the THz power) and in a vertical optical cavity (to efficiently couple in the optical pump power). This is accomplished with an electrically-floating metal layer lying below the thin LTG-GaAs layer by approximately 0.5 micron. Optical analysis shows that the metal layer can increase the external quantum efficiency of the photomixer approximately 3 times through creation of a vertical optical cavity in the LTG-GaAs layer. Heat-transfer analysis shows that the metal layer reduces the photomixer thermal resistance approximately 2.5 times, allowing for an increase in the optical pump power by roughly the same factor. Finally, full-wave electromagnetic analysis shows that the metal layer has a small impact on the radiation pattern of the planar dipole above it, presumably because the layer is much smaller laterally than a wavelength and is electrically floating. In total, these beneficial effects are predicted to increase the output THz power by approximately a factor often compared to any results reported to date.