Drude's description of the response of low-temperature gallium arsenide to optical pulse excitation is used to evaluate the components of a time-domain Norton equivalent circuit of a photoconductive antenna (PCA) source. The saturation of the terahertz (THz) radiated power occurring at large optical excitation levels was previously associated by the scientific community to radiation and charge screening of the bias. With the present circuit, we are able to model accurately the measured saturation as only due to the EM feedback from the antenna to the bias. The predicted THz radiated power is shown to match very accurately the measurements when the circuit is combined with an accurate description of the experimental conditions and the modeling of the THz quasi-optical (QO) channel.

State-of-the-art THz pulsed commercial systems operating over large bandwidth suffer from high dispersion or low radiation efficiency due to the poor coupling between the transmitter and receiver photoconductive antennas (PCAs). In this work, we present the fabrication and characterization of a leaky-lens PCA that has the potential to solve this problem. The presented PCA is based on a low-temperature grown gallium arsenide (LT-GaAs) membrane with a 1:15 bandwidth coverage (0.1-1.5 THz), where the frequency response is constant. In order to fabricate the PCA on an LT-GaAs membrane, a novel fabrication process is developed. This process is dramatically faster than previously used processes (∼1.5 h instead of ∼20 h). Furthermore, an experimental validation of the radiated power together with the comparison to a standard bow-tie-based PCA fabricated on the same LT-GaAs wafer is shown in this article. We show that the PCA source on the LT-GaAs membrane is more efficient due to the enhanced leaky wave radiation. The leaky-lens PCA stands out as a great candidate to improve the coupling efficiency in THz pulsed commercial systems, where the maximum laser power that can be used is limited by the dispersion in the optic fiber.

The time evolution of voltages and currents in a pulsed photo conductive antenna (PCA) source is evaluated resorting to a rigorous procedure that stems from semiconductor physics first, to define the phenomena involved in the generation of the photocurrent, and then relies on an equivalent circuit in time domain, providing a direct estimation of the power generated by the PCA as well as its spectral distribution. The circuit model is validated via a campaign of measurements of standard PC antenna sources. The saturation phenomena in the THz radiated power occurring at large optical excitation levels, previously observed by the scientific community and associated to different phenomena, are accurately predicted by the present method, which ascribe their main cause to the feedback from the antenna: indeed, the electromagnetic field generated by the device tend to reduce the strength of the forcing field used to accelerate the photo-carriers.

The observable field is defined as the portion of the incident field that can contribute to the power received by an antenna. Recently, the observable field was estimated for a plane wave incidence. Here, the procedure is extended to a general incident field expressed as a superposition of homogeneous plane waves. The observable field concept provides a methodology to evaluate the maximum power that could be received by an ideal terminal antenna. In particular, it emerges that to maximize the received power, the pattern in transmission of the antenna should be synthesized to reproduce the angular pattern of the observable field. This is specifically relevant in cases of non-line of sight (NLOS) at high frequencies, where the power received can drop by orders of magnitude. As a case study, we consider a communication scenario which involves a base station and distributed receivers embedded in a complex scattering environment.

Pulsed terahertz time-domain systems rely on antennas printed on photoconductive substrates (PCA), which show extremely fast conductivity transients when illuminated by femto-seconds laser pulses. This work introduces a time domain representation of the PCA transmitter that accounts for time evolving voltages at the terminals of the photoconductive gap; such model is able to explain the saturation phenomena observed in measurements performed under high power laser excitations that previous models could not account for.

Recently, powerful, reliable and cost-effective THz radiation photoconductive-emitters have been developed, providing up to 1mW of pulsed power in the range of frequencies between 0.1 and 0.7THz. In this paper we study the potential use of such sources for future pulsed radar systems aiming at stand-off imaging applications. Specifically, we investigate the image frame rate and Signal to Noise Ratio budget that can be obtained using these photoconductive sources. It emerges that adopting a receiving array of 14 times 14 simeq 200 elements, a 3D image with a Field of View (FoV) of 6cm times 6cm times 100cm and resolution of 4mm times 4mm times 1mm can be generated in 80Hz with a SNR { min}= 30dB, including realistic quasi-optical channel efficiencies.