A novel pulsed photoconductive THz source is presented that is able to radiate mW-level average powers, 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 micro-lenses array splits the laser beam into N × N spots that host the active excitation of the antenna arrays. An 'ad hoc' network has been adopted 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 slot array prototype has been designed, fabricated, and measured. The proposed solutions achieve excellent power radiation levels by making use of an accurate electromagnetic design. This solution can offer enhancements to any active system relying on pulsed photoconductive antennas.

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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.

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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.

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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.

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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.

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In recent years, Terahertz technology has attracted the interest of researchers for its potential applications in a variety of domains. In particular, THz sensing has found application in security screening, medical imaging, spectroscopy, and non-destructive testing. The emergence of all these applications has been driven by the availability of photoconductive antennas, which have made available bandwidth in the THz spectrum at relatively low cost, thanks to several breakthroughs in photonics, and semiconductor technology. Photoconductive antennas are optoelectronic electromagnetic sources that resort to optically pumped semiconductor materials. Such devices exploit the photoconductivity phenomenon to generate and radiate power over a broadband up to the THz frequencies. However, nowadays the use of photoconductive antennas are confined to niche short-range applications, because of the bottleneck of the low power emitted. Early in this research work, it was understood that such bottleneck came from the fact that there was not a clear description about the coupling between the photocondcutive source and the antenna. For this reason, this work has been focused to develop a Thévenin or Norton equivalent circuit for the photoconductor generators of photoconductive antennas. A Norton equivalent circuit for pulsed photoconductive antennas has been derived, starting by the electrodynamic model of the photogeneration of free carriers in laser pumped semiconductor material. Such equivalent circuit allows to maximize the radiated power as function of the geometry of the gap, the properties of the semiconductor material, and the features of the laser pump, providing a clear description of the coupling between the photoconductor generator and the antenna over the operative bandwidth. An electromagnetic model of the quasi-optical (source-to-detector) channel, typically used for measuring power and spectrum radiated by photoconductive antennas, has been proposed. Such model jointly with the developed Norton equivalent circuit allows a complete characterization of the power budget from the source to the detector. Providing for the first time a complete description about the dispersion introduced by the quasi-optical channel on the energy spectrum radiated by photoconductive antennas. The entire proposed model (equivalent circuit and channel) has been validated by spectrum and power measurements of photoconductive antenna prototypes. The proposed equivalent circuit and the electromagnetic model of the quasi-optical channel provide a powerful engineering tool to design photoconductive antennas, opening the way for more standard engineering optimization of wide band laser pumped sources, resorting to the vast heritage of wide band microwave engineering tools that have been developed mostly for analyzing detectors in radiometric domains. The radiation performances of logarithmic spiral antennas as feed of dense dielectric lenses has been intensively analyzed. The results of the investigation have demonstrated the presence of the leaky wave radiation, when the spiral antenna are printed at the air dielectric interface, leading to a design of a logarithmic spiral antenna lens antenna, which provides an high aperture efficiency over a decade frequency bandwidth. However, only using extremely thin substrate allows to feed this design with a planar feeding system without limiting the bandwidth. A new design of a logarithmic spiral lens antenna has been proposed for relaxing such limitation, introducing a small air gap between the spiral feed and the bottom lens interface, which enhances the leaky wave radiation. Such new design, coupled with a synthesized elliptical lens, achieves directive patterns without sidelobes over a decade frequency bandwidth. Moreover, the new spiral design can be used also as feed of a hemispherical lens with low extension height, when the dispersion of the radiated pulses has to be minimized. A novel design for photoconductive sources has been proposed, aiming to increase dramatically the radiated power with respect to the current photoconductive antennas. The new source is based on the well established concept in the microwave community of connected array. Thanks to the intrinsic wide band behavior of the connected array, the proposed solution is able to radiate efficiently the wide band energy spectrum generated by the photoconductive source. Such design is suitable to be employed also as receiver of ultra-wide bandwidth radiation, increasing the sensitivity with respect to the current photoconductive receivers. In order to implement the design of the photoconductive connected array, an ad-hoc biasing network has been proposed, in order to properly bias all the array cells, preserving the connected structure of the elements. Moreover, a design of an optical system has been proposed, in order to optically excite all the elements of the photoconductive array coherently. Using the proposed Norton equivalent circuit for photoconductive generator, a photoconductive connected array generating an average power of 2.35mW over a bandwidth from 0.1THz − 0.4THz has been designed. A demonstrator of the proposed photoconductive source design is going to be realized, and a complete characterization of the prototype will be performed by means of power and spectrum measurements, proving the validity of the concept.@en

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.

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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.

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In this paper, the performance of logarithmic spiral antennas as feeds of dense dielectric lens are investigated in detail. The performances are evaluated in terms of clean symmetric radiation patterns, high polarization purity, antenna efficiency, and radiation dispersivity. A logarithmic spiral antenna placed in the dielectric-air interface can provide high aperture efficiencies over large bandwidths if coupled to a synthesized elliptical lens. The use of an air gap increases the directivity of the spiral radiation inside the dielectric allowing for lens directive patterns without sidelobes and reducing the dispersivity of the radiated pulse. The directivity enhancement of the fields inside the dielectric is validated by the measurement of a prototype. The highest frequency at which these antennas can be fed by a planar microstrip line is limited by the thickness of the microstrip substrate.@en

In this article, we present a freely accessible software tool that allows for fast characterization of dynamic phenomena in a wide variety of transmission lines that include characteristic impedance, effective dielectric constant, and losses, such as radiation into space and surface waves. For printed transmission lines, the radiation effects are of particular importance when the transverse dimensions of the transmission lines become significant in terms of wavelength. Generally, dispersion and losses of the line due to these dynamic phenomena are predicted by full-wave simulations as quasi-static formula do not suffice. The presented software tool, freely downloaded from http://terahertz.tudelft.nl, is capable of accurately analyzing the most widely used transmission lines at high frequencies.@en

A novel design of an optically pumped THz photoconductive source in array configuration is presented. The proposed solution is based on the exploitation of the ultrawide band properties of connected antenna arrays. The photoconductive elements of the array are excited by an array of microlenses which splits and focuses the laser beam onto the active areas of the connected array. An ad-hoc DC biasing solution is introduced in order to properly bias the array elements; thus, realizing a connected array of photoconductive sources. If needed, the array can be used to illuminate a dielectric lens, which collimates the radiated THz electromagnetic field in the boresight direction. The proposed solution allows to achieve a high radiated power on a large bandwidth in the terahertz regime when the photoconductive material is illuminated by a laser operating in the pulsed mode. The proposed solution is therefore suitable and effective to be employed in terahertz time-domain systems.@en