Contents I. Introduction
Photoconductive antennas (PCA) are photomixers that generate and detect terahertz (THz) radiation. They are employed in both time-and frequency-domain THz systems [1] –[3]. In the former case, femtosecond optical pulses are used to excite the photoconductor. In the latter case, an optical beating of two continuous wave (cw) lasers generates a cw excitation at the beating frequency. To achieve high dynamic ranges over bandwidths of several THz in such systems, a strong and fast response of the photoconductor to the optical excitation is required. This photoresponse is mainly determined by the properties of the photoconductive material, namely the charge carrier mobility and lifetime as well as the resistivity [3]. Therefore, the development of photoconductive materials with high carrier mobility, low carrier lifetime and a high resistivity is crucial to improve the performance of PCAs. Indium gallium arsenide (InGaAs) is a common choice for the photoconductor since it can be grown lattice-matched on indium phosphide (InP) and absorbs light at wavelengths in the optical c-band (1530 nm to 1570 nm) where low cost off-the-shelf photonic components are available. The aforementioned properties of InGaAs can be modified by introducing dopants into the crystal that serve as defects where photo-excited charge carriers can be trapped and recombine. In this way, the carrier lifetime can be reduced to the order of picoseconds or even femtoseconds. The doping also increases the resistivity of the material by reducing the free carrier density. The presence of such defects, however, also reduces the carrier mobility so that a trade-off has to be found according to the desired application for the PCA.
