A pulsed photoconductive terahertz (THz) source is presented that is able to radiate milliwatt (mW) level average power over a large bandwidth, by exploiting both the optical and electrical properties of photoconductive sources and the ultrawideband properties of connected antenna arrays. An optical system composed of a microlenses array splits the laser beam into N×N  spots that host the active excitation of the antenna arrays. An “ad hoc” network is introduced to bias the array active spots in order to implement a connected antenna array configuration. The array feeds a silicon lens to increase the directivity of the radiated THz beam. A dipole and a slot array are designed. Prototypes have been fabricated and measured. Power and spectrum measurements of the prototypes are in excellent agreement with the expected results. The proposed solutions achieve excellent power radiation levels by exploiting accurate electromagnetic design. Thus, they can offer enhancements to any active system relying on pulsed photoconductive antennas.

DESHIMA is a spectrometer for astronomical applications targeting sources at sub-mm wavelengths from 240GHz to 720GHz that will operate in the ASTE telescope in Atacama Desert, Chile. In this work, a quasi-optical system based on a hyper-hemispherical leaky lens antenna and a series of Dragonian reflectors is presented as the coupling chain for the EM radiation captured by the telescope into the detector. The design procedure is based on a field matching technique in reception. The achieved average illumination efficiency over the band is approximately 70%. The directivity patterns in the sky are also estimated. The side lobe, and cross-polarization levels, over the whole frequency band, are below-16dB, and-18dB, respectively. The measurement of the system is on-going, and will be presented at the conference.

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 theorem, the equivalent source representation is decoupled from the antenna. In particular, for photoconductive antennas (PCAs), 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 PCAs.

This second part of two papers' sequence presents the experimental validation of the Norton equivalent circuit model for pulsed photoconductive antennas (PCAs) provided in the first paper of the sequence. To this goal, different prototypes of photoconductive antenna sources have been manufactured and assembled. The average powers radiated and their pertinent energy spectral densities have been measured. In order to obtain a validation of the original equivalent circuit proposed, an auxiliary electromagnetic analysis of the complete setup, including the quasi-optical (QO) link for the signals from the antenna feeds to the detectors had to be developed. By using the combined theoretical model (circuit and quasi-optics), an excellent agreement is achieved between the measured power and the power estimated. This agreement fully validates the circuit model, which can now be used to design new PCAs, including optical and electrical features of the semiconductor materials, as well as the details of the antenna gaps and the purely QO components.

Optically pumped pulsed THz emitters exploit the transient motion of photo-generated charge carriers in semiconductors, to produce, coupled to micro-antenna, radiated power over a wide bandwidth up to the THz frequencies. The radiation performance of the antenna greatly affects dispersion of the energy spectrum generated by the photoconductive source and if not properly designed it causes low radiated power. This work presents the design, the fabrication process, the electromagnetic and the thermal analyses of a pulsed photoconductive micro-antenna based on the leaky lens antenna concept. This device shows high radiation efficiency over a band ranging from 0.1 to 1.5 THz, thus being a suitable emitter for THz time-domain sensing system.

The exponentially increasing demand for highspeed wireless links can be only efficiently satisfied with the development of future XG wireless communication networks, based on higher carrier signal frequencies, starting from 100 GHz. In this contribution, a circularly polarized G-band leaky-wave fed lens antenna with an integrated dielectric grid polarizer is presented, which can fulfill the challenging requirements for these future XG networks. A design is proposed in low dielectric permittivity material with a feed matching better than -10dB over a 44 % of relative bandwidth. The circularly polarized lens aperture efficiency is higher than 75% over a 35 % relative bandwidth, with an axial ratio lower than 3dB. Analytical tools have been applied to optimize the lens aperture efficiency, validating the results via full wave simulations. A lens prototype has been now fabricated and is currently being measured.

In this paper a rigorous electromagnetic characterization of the setup for measuring the THz power radiated by pulsed photoconductive antenna is discussed. Such characterization is expressed in terms of efficiencies which quantify how much power is lost in the coupling between the various components involved in the measurement setup. The conducted analysis highlights how such efficiencies affect the energy spectrum of the measured pulsed signal. Measurement results with two different detectors will be shown during the conference and will be compared against the power estimation obtained by a recently developed equivalent circuit model for photoconductive antennas. The proposed electromagnetic modeling allows us to effectively improve the design of THz time domain systems.

A novel solution for the manufacturing of a reflectarray antenna with cosecant-squared radiation pattern is presented. The unit cell simply consists of a 3-D printed nylon grid inserted in between two thick metallic plates. To introduce the required phase delay, C and reverse C slots are laser cut into the plate illuminated by the feeder. This solution allows obtaining an efficient, robust, and compact antenna with low-cost manufacturing process, even for no mass production. Measurements confirm the feasibility of the proposed solution and show its performances.

In this paper, a fast iterative physical optics (FIPO) algorithm is proposed for analysis of scattering from electrically large objects involving multiple reflections. When the scenario to be analyzed is electrically very large, a Fast Far Field Approximation (FaFFA) algorithm, based on a domain decomposition of the scatterer surface, can be conveniently used to greatly speed-up the calculation of the induced currents at each step of the iterative procedure. In this work, an efficient and accurate interpolation scheme has been combined to the standard FaFFA algorithm implementation further reducing the complexity of the computation.

A novel Norton equivalent circuit model for characterizing the photoconductive feed of photoconductive antennas is introduced. It incorporates the physics of the photoconductive antenna's excitation by accounting for: (i) the electrical properties of the photoconductive material; (ii) the features of the optical power excitation; (iii) the geometrical dimensions of the gap between the electrodes that couple the antenna to the photoconductive material. The model is applicable to describing the antenna feeding mechanism for the photoconductors being illuminated by means of lasers operating in both pulsed and continuous modes. The proposed model is validated by comparing the power estimated by it with power measurements of different photoconductive antennas. The advocated solution is conducive to analyzing and designing photoconductive antennas. In particular, it is expected to be at the core of antenna optimization tools for maximizing the Terahertz (THz) power radiation, this making it an important enabler for designing THz time-domain systems.

A very flexible and efficient iterative physical optics (IPO) algorithm is presented for analyzing the electromagnetic (EM) scattering of complex and electrically large problems. The algorithm accounts for multiple interactions between the objects comprised in the scenarios under the physical optics (PO) approximation. Various techniques for accelerating and parallelizing the algorithm were used, thus obtaining an efficient tool that can be used in novel high-frequency solvers.