A photoconductive model for superior GaAs THz photomixers

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Abstract
Theoretical methods are used to evaluate the THz output from photomixer structures consisting of interdigitated electrodes and planar antennas on top of a low-temperature-grown GaAs layer. Consistent with experiment, the THz power from a standard photomixer is found to be limited by low external quantum efficiency (∼1%). This arises primarily from low photoconductive gain, which is attributed to a long transit time (between electrodes) for the majority of photocarriers generated in the structure. The modeling is then applied to an improved structure containing a thinner absorbing layer (≈0.34 μm for λ=0.85 μm pump) with a dielectric mirror below it to induce resonant-cavity absorption near the surface where the gain is higher. Through increased gain and absorptivity, the model predicts ≈7× greater THz output for the same optical pump power.