This article investigates the validity of the physical optics/geometrical optics (PO/GO) approximation in modeling low-permittivity integrated lens antennas when there is significant illumination of the shadow region as a function of the feed and lens geometry. The comparative analysis reveals that for certain extended hemispherical lenses, feed illumination of the lens surface beyond the critical angle leads to significant discrepancies in radiation pattern and antenna gain between PO/GO predictions and full-wave simulations. These discrepancies are traced to the constructive far-field contribution of the shadow region currents relative to those originating from the top lens region below the critical angle. The measurements of fabricated extended hemispherical lenses confirm these findings, showing up to 2-dB gain enhancement over PO/GO predictions and showcasing the limitations of traditional high-frequency modeling techniques in capturing this phenomenon. The significant gain enhancement, along with the clean measured radiation patterns, highlights the potential of using lens antenna designs with feeds that provide strong shadow region illumination. These findings provide new insights into the development of high-performance integrated lens antennas for advanced communication and sensing applications.

Sub-THz wireless links targeting Tbps throughput face a fundamental tension between bandwidth and energy efficiency. Zero-Crossing Modulation (ZXM) is a promising approach to reduce receiver power consumption by combining 1-bit quantization with temporal oversampling. However, the resulting 1-bit receive chain introduces a strong nonlinearity, which limits the applicability of conventional linear multipleinput multiple-output (MIMO) spatial equalization techniques and makes the impact of coupling and crosstalk a primary system design constraint. This motivates the use of ZXM in combination with 1-bit quantization for a quasi-optical (QO) MIMO system, where the spatial inter-stream interference is very small. Hence, we present a system-level link-budget analysis for an 8×8 QO MIMO architecture at sub-THz, based on a signal-to-interference-plus-noise ratio (SINR) analysis that treats interstream interference as additional noise after 1-bit quantization. The end-to-end coupling matrix is modeled as the cascade of antenna coupling and RF front-end crosstalk contributions. Using this framework, we quantify when the link transitions from noise-limited to interference-limited operation, show SINR saturation with increasing transmit power, and derive feasible operating regimes over distance and spectral-efficiency targets for wideband 50 GHz operation. The results substantiate the feasibility of energy-efficient Tbps-class wireless connectivity.

Radiative near-field links have gained noticeable interests recently for high-data-rate wireless communication. Unlike far-field links, near-field links can have negligible path loss within hundreds of meters for electrically large antennas at high frequencies. In this work, we propose a multi-lens quasi-optical (QO) system for 100-m near-field backhaul communication at H-band. The QO system is designed with compact size (aspect ratio of 1.3:1) and high coupling efficiency of 82%. Moreover, the rotation of an auxiliary lens realizes beam scanning for the link alignment. The scan range is in the order of 1 m with less than 2 dB scanning coupling loss and scanning magnification of 14.5:1.

Photoconductive antennas (PCAs) are promising candidates for sensing and imaging applications. In recent years, our group has investigated their properties under pulsed laser illumination in transmission using a time-domain (TD) Norton equivalent circuit. Here, we extend this analysis to the link between a photoconductive source and a receiver introducing for the latter a second TD Norton equivalent circuit. We also evaluate the transfer function of a dispersive quasi-optical (QO) link. Specifically, a field correlation approach based on the high-frequency techniques is used to evaluate the spectral transfer function between two bow-tie-based PCAs, including the QO link. The detected currents in the receiving circuit are reconstructed using stroboscopic sampling of the modeled THz pulses, equivalent to what is actually performed by THz TD systems. Both the amplitude and the waveforms of these currents are evaluated. The QO link is then experimentally characterized to validate the proposed methodology. The comparison between the simulations and the measurements is excellent.

Large format focal plane arrays (FPAs) of dielectric lenses are promising candidates for wide field-of-view submillimeter imagers. In this work, we optimize the scanning gain of such imagers via shaping lens surfaces. We develop an optimization procedure using a field correlation technique between the fields generated by a reflector on the top of the lenses and those generated by the lens feeds. Based on this procedure, an FPA of quartz lens antennas combined with leaky-wave feeds is designed to efficiently illuminate the reflector, achieving a directivity of 50.5 dBi up to scanning 20.3°. The obtained scanning gain loss of 2.6 dB is much lower than that associated with the direct fields coming from the reflector (about 6 dB). The proposed FPA is validated by full-wave simulations with excellent agreement. We have fabricated and measured an example shaped quartz lens optimized for the scanning angle of 20.3° at 180 GHz. The comparison between the simulations and the measurements also shows excellent agreement.

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.

We present a freely accessible graphical user interface (GUI) for analyzing antenna-fed quasi-optical (QO) systems in reception (Rx). This analysis is presented here for four widely used canonical QO components: parabolic reflectors and elliptical, extended hemispherical, and hyperbolic lenses. The employed methods are geometrical optics (GO) and Fourier optics (FO). Specifically, QO components are illuminated by incident plane waves. By using a GO-based propagation code, the scattered fields are evaluated at an equivalent sphere centered on the primary focus of the component. The FO methodology is then used to represent the scattered fields over the focal plane as plane wave spectrum. A field correlation between this spectrum and the antenna feed radiating without the QO component is implemented to evaluate the induced open-circuit voltage on the feed in Rx. By performing a field matching between these two spectral fields, feed designers can optimize the broadside and/or steering aperture efficiencies of QO systems in a fast manner. The tool is packaged into a MATLAB GUI, which reports the efficiency terms, directivity, and gain patterns of antenna-coupled QO systems. The described tool is validated via full-wave simulations with excellent agreement.

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.