Norton Equivalent Circuit for Pulsed Photoconductive Antennas–Part I: Theoretical Model

Abstract

A novel equivalent circuit for pulsed photoconductive sources is introduced for describing the coupling between the photoconductive gap and the antenna. The proposed circuit effectively describes the mechanism of feeding the antenna by the semiconductor when this latter is illuminated by a laser operating in a pulsed mode. Starting from the classical continuity equation, which models the free carriers density with respect to the laser power pump and the semiconductor features, a Norton equivalent circuit in the frequency-domain is derived. According to the Norton's theorem, the equivalent source representation is decoupled from the antenna. In particular, for photoconductive antennas, the Norton circuit takes into account of the electrical and optical properties of the semiconductor material, the features of the laser excitation, as well as the geometrical dimensions of the gap. The presence of the electrodes around the gap is part of the antenna and, therefore, it is taken into account in the antenna impedance. The proposed circuit allows the analysis of the coupling between the photoconductive source and the antenna, providing a tool to analyze and design photoconductive antennas.

Norton Equivalent Circuit for Pulsed Photoconductive Antennas—Part II: Experimental Validation

“…Three different photoconductive antenna geometries have been manufactured, in order to measure their radiated average power and compare it with the results derived by the equivalent circuit model discussed in [10]. The chosen antenna geometries are a H-dipole, a bow-tie, and a logarithmic spiral antenna, shown in Fig.…”

“…The radiated energy spectral density E s radiated by each PCA prototypes are calculated by using the Norton equivalent circuit described in [10]:…”

“…are the equivalent Norton generator current spectrum and impedance, respectively, which model the photoconductive source, and Z a (ω) is the antenna impedance. In (6)-(7), the following parameters are defined as in [10]: the laser absorption efficiency η; the gap dimensions (W x , W y , W z ); the area of the spatial distribution of the Poynting vector of the laser beam A laser ; the electron charge e; the carriers transient mobility µ; the frequency response to the laser excitation of the photoconductor material H (ω); the spectrum of the timevarying envelope of the laser pulseS (ω); the bias voltage V bias ; and the time duration of the pulsed photoconductivity τ σ . For the experiment we consider a bias voltage V bias = 40 V and a laser average powerP laser = 30 mW.…”

“…The lack of such quasi-optical characterization for the channel typically renders impossible to calculate how much power was actually generated by the source, giving rise to the wide spread use of arbitrary units. In Part I of this two papers sequence [10], some of the authors have presented a model to estimate the power radiated by a pulsed PCA source, based on its optical, electrical, and geometrical operating parameters. In this second part of the sequence a measurement setup is discussed, prototypes of standard PCAs are presented, and the average powers generated by the PCAs and their pertinent spectra have been measured.…”

Contribution assessment of antenna structure and in-gap photocurrent in terahertz radiation of photoconductive antenna

“…5,21,23 To address this issue, a second type of model was developed with the hypothesis that the THz radiation of the PCA is solely from the antenna structure, which works as a common electric antenna fed by a photocurrent source. A representative is the equivalent circuit model, 12,13,[35][36][37] in which the PCA is described as a lumped-element circuit. To ensure a closed circuit, this model assumes that the gap region is fully illuminated by a laser beam, which, unfortunately, is not true for many cases.…”