Time Domain Modeling of Photoconductive Antennas

Abstract

Photoconductive antennas (PCAs) have been extensively utilized for the generation of broadband pulses over very large bandwidths. PCAs rely on a semiconductor (e.g. LT- GaAs) gap pumped by a laser and coupled to a passive structure biased at a certain voltage level. When the laser impinges on the semiconductor gap with an appropriate carrier frequency, enough energy is provided such that free electron-hole pairs are generated from the electrons that move from the valence band to the conduction band. As a result, the resistivity of the material decreases to a few ohms which in turns allows a time-varying current to flow across the gap. In recent year different hybrid equivalent circuits [1], [2], [3] have been developed in order to take into account all these complex phenomena although none of these models account for the frequency dependence of the impedance of the antenna, being formulated in the time domain. This approximation works for non-dispersive antennas such as the bow-tie, but fails in the characterization of more diverse and complex structures. The Norton equivalent circuit’s aim proposed in [4] was to fill the aforementioned gap by introducing an analytical model completely in frequency domain, although the difficulty in the characterization of the generator impedance obstructed the way for a wide acceptance in the community. In this thesis a novel approach based on a commercially available electromagnetic simulator [5] to characterise the biasing of the passive structure, the optical laser excitation and the impulse response of the photoconductor is proposed. The accuracy of the model is verified by calculating the average power radiated by a bow-tie and the results are compared to the measurements in [6]. Moreover, a revised version of the Norton equivalent circuit [4] which describes more accurately the effective generator impedance is presented. While the computer-aided model offers great introspection in the characterisation of voltages and currents and thus in the maximisation of the power radiated, the revised Norton equivalent circuit offers an even better accuracy and reduces significantly the computational time.