Pulsed terahertz time domain spectroscopy of vertically structured photoconductive antennas

We present a terahertz (THz) photoconductive emitter structure, which employs a n-doped layer underneath a low-temperature-grown GaAs region to enable the THz transient to couple vertically through a defined mesa. A nonlinear bias dependence is observed, yielding an order in magnitude improvement in power for a mesa device with a 100 mu m(2) area over a conventional planar control reference device at 32 V and 5 mW illumination power. We relate the bias dependence of the THz signal to the breakdown voltage observed in the current-voltage characteristic. Reducing the antenna gap size through reducing the thickness of the low temperature-GaAs region below 1 mu m shows a large improvement in the bandwidth of the device, with an enhancement of the normalized intensity between 0.2 to 2 THz for a bow-tie antenna geometry.