Time-Domain Modelling of Pulsed Photoconducting Sources - Part I: The Norton Equivalent Circuit

In the circuit theory, the Norton and Thevenin equivalent generators are tools that simplify the solutions of networks involving passive or active components. They have been extensively used in the frequency domain to describe time-harmonic sources. A time-stepped evolution is instead typically used to include transient sources. As a particular case of the latter, the Norton equivalent circuit is extended here to investigate pulsed photoconducting sources, where a dc bias voltage and a pulsed optical laser are combined to generate terahertz (THz) bursts. The proposed derivation relies on the application of the electromagnetic (EM) equivalence theorem. The main conclusion of this derivation is the understanding that, from the three different spectral regions (dc, THz, and optics), only the THz radiation is to be explicitly included in the equivalent circuit. The theory is validated by a campaign of measurements reported in a connected paper.