Photoconductive Antenna Research Articles

Generation of Terahertz Pulsed Radiation with Photoconductive Antennas Based of Low-Temperature-Grown Gallium Arsenide and Its Applications

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Norton Equivalent Circuit for Pulsed Photoconductive Antennas—Part II: Experimental Validation

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.

Norton Equivalent Circuit for Pulsed Photoconductive Antennas–Part I: Theoretical Model

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.

Vertical versus planar pulsed photoconductive antennas that emit in the terahertz regime

The design process of a photoconductive antenna (PCA), which emits efficiently in the electromagnetic terahertz range, demands some considerations that are discussed through this work. In this work, several essential characteristics of a photoconductive antenna made with LT-GaAS are studied by means of well established commercial software (COMSOL 5.3 [1]). An approach to the efficiency is also made through the study of geometry, the laser illumination position, the substrate doping distribution, the direction of the bias applied to the semiconductor, the matching impedance at the laser operating frequency and, finally, the plasmonics effects or penetration laser enhancement due to the use of nano antennas. We study and compare two kinds of structures, one which is quasi-bidimensional or planar and the other which is vertical. Additionally, the photoconductive antennas are also modeled by using a simplified equivalent circuit which helps to understand the antennas’ performance. Therefore, some fundamental parameters, like the transient capacitance between the metal contacts are also studied. Furthermore, we introduce an optimized vertical design which achieves the best results.

Enhanced optical-to-THz conversion efficiency of photoconductive antenna using dielectric nano-layer encapsulation

Photoconductive antennas (PCAs) are among the most conventional devices used for emission as well as detection of terahertz (THz) radiation. However, due to their low optical-to-THz conversion efficiencies, applications of these devices in out-of-laboratory conditions are limited. In this paper, we report several factors of enhancement in THz emission efficiency from conventional PCAs by coating a nano-layer of dielectric (TiO2) on the active area between the electrodes of a semi-insulating GaAs-based device. Extensive experiments were done to show the effect of thicknesses of the TiO2 layer on the THz power enhancement with different applied optical power and bias voltages. Multiphysics simulations were performed to elucidate the underlying physics behind the enhancement of efficiency of the PCA. Additionally, this layer increases the robustness of the electrode gaps of the PCAs with high electrical insulation as well as protect it from external dust particles.

An Experimental Study on LT-GaAs Photoconductive Antenna Breakdown Mechanism

In this paper, an experimental study on the low-temperature-grown gallium arsenide (LT-GaAs) photoconductive antenna (PCA) breakdown mechanism is presented. A terahertz (THz) PCA is an important device for the THz wave generation. Due to that the typical LT-GaAs PCA breakdown scenarios, different trigger positions and different bias voltages, have been studied. Breakdown mechanism analysis has shown that when the trigger position was on the antenna electrode edge, the PCA breakdown was more probable to happen, because the electrode edge electric field was nearly 3 times greater than the electric field of the electrode center. Moreover, it has been shown that the thermal effect was the main cause of the LT-GaAs PCA breakdown. Last, the PCA lifetime has been calculated using the current waveform and the thermal effect analysis.

Analysis and Design of Photoconductive Antenna Using Spatially Dispersive Graphene Strips with Parallel-Plate Configuration

In this paper, a photoconductive antenna (PCA) is designed using spatially dispersive graphene strips (GSs) with parallel-plate configuration. This configuration maintains the properties of a single GS and at the same time provides more tunability for designing a graphene-based PCA (GPCA). When a GS is surrounded by a high-index media, propagating wave vector along the structure becomes spatially dispersive, because the group velocity of the propagating wave is greatly reduced and it becomes comparable to Fermi velocity in graphene. In this situation, it is necessary to use a nonlocal conductivity model for graphene. In this paper, a nonlocal per unit length circuit model is employed to study wave propagation in the GPCA. In the circuit model, the nonlocal behavior will be modeled via a per unit length quantum capacitance that under certain conditions it is simplified to quantum capacitance in graphene. In deep subwavelength regime, due to strong coupling between GSs in the the double stacked configuration, it can be replaced by a single GS that its conductivity is two times greater than the former case. Finally, the GPCA is fed by a wide-band photocurrent in order to terahertz radiation and detection are investigated.

Plasmonic Nanodisk Thin-Film Terahertz Photoconductive Antenna

This paper presents the design, fabrication, and measurement of a plasmonic thin-film terahertz photoconductive antenna. Conventional terahertz photoconductive antennas suffer from poor optical-to-terahertz conversion efficiency, often on the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> . This is due to the low quantum efficiency of the device. The goal of this work is to demonstrate enhanced terahertz emission from a plasmonic thin-film device architecture. The combination of plasmonic nanodisks, a 120-nm low-temperature-grown gallium arsenide thin-film, and a bottom-located bowtie antenna has demonstrated the feasibility of producing such devices. Fabrication attempts and failure analysis is discussed in this work. Experimental characterization measuring the peak-to-peak electric field values of the terahertz pulses emitted from the device prototypes showed approximately five times improvement in plasmonic thin-film devices compared to conventional devices. The plasmonic thin-film devices had a measureable terahertz bandwidth of ~5 THz. This indicates that the plasmonic thin-film architecture has a potential for producing high optical-to-terahertz conversion efficiencies across a wide frequency range.

Photoelectric Properties of InAs/GaAs Quantum Dot Photoconductive Antenna Wafers

In this paper, the study of the photoconductivity in self-assembled InAs/GaAs quantum dot photoconductive antenna in the wavelength region between 1140 nm and 1250 nm at temperatures ranging from 13 to 400 K is reported. These antennas are aimed to work in conjunction with quantum dot semiconductor lasers to effectively generate pulsed and continuous wave terahertz radiation. For the efficient operation, laser wavelengths providing the highest photocurrent should be determined. To study the interband photoconductivity of quantum dot photoconductive antennas, at room and cryogenic temperatures, we employed a broadly-tunable InAs/GaAs quantum dot based laser providing a coherent pump with power exceeding 20 mW over a 100 nm tunability range. The quantum dot antenna structure revealed sharp temperature-dependent photoconductivity peaks in the vicinity of wavelengths, corresponding to the ground and excited states of InAs/GaAs quantum dots. The ground state photoconductivity peak vanishes with a temperature drop, whereas the excited state peak persists. We associate this effect with different mechanisms of photoexcited carriers extraction from quantum dots.

Terahertz antenna technology for imaging applications: a technical review

AbstractThe terahertz (THz) regime of the electromagnetic spectrum is rich with the emerging possibilities in imaging applications with unique characteristics to screening for weapons, explosives and bio-hazards, imaging of concealed objects, water content, and skin, and these advantages can be harnessed by using the effective THz sources and detectors. In THz imaging systems, the pulsed THz sources and detectors find unique applications and thus we have emphasized on re-visiting these kinds of systems. Several novel imaging techniques which exploit the distinctive properties of the THz systems have been presented. Moreover, the THz antenna is one of the most important components of a THz imaging system as it plays a significant role in both impedance matching and power source. Therefore, the recent developments in THz antenna design for imaging applications are reviewed and the potential challenges of such THz systems are investigated. The photoconductive antennas form the basis of many THz imaging and spectroscopy systems and finds promising applications in various scientific fields. However, for the imaging applications, there is a requirement of planar and compact THz antenna sources with on-chip fabrication and high directivity in order to achieve large depth-of-field for better image resolution. Therefore, the key modalities of improving photoconductive dipole antennas performance are identified for imaging applications. Also, the ways to improve the directivity of the photoconductive dipole antenna are discussed. The main purpose of this review is to provide an assortment of all relevant literature to bring researchers up-to-date on the current state-of-the-art and potential challenges of THz antenna technology for imaging applications.

Magnetic Field-Assisted Radiation Enhancement From a Large Aperture Photoconductive Antenna

The generation of terahertz radiation using photoconductive antennas is becoming very popular. Several experimental and simulation studies have been performed to study the characteristics of the photoconductive antenna (PCA). Although various methods have been proposed to increase the radiated power from it, the radiated power remains very low. In this paper, we present an analytical study of improving the radiated power from a large aperture PCA using an external magnetic field source. The transit time behavior of the carriers is computed using the basic semiconductor carrier dynamics model, including the transient mobilities with the dependencies on the electric field and carrier’s concentration. Analytical studies show that substantial enhancement in the radiated field can be achieved when such external magnetic field is applied. Furthermore, the polarity of the radiated field depends on the orientation of the applied magnetic field. The results obtained from analytical calculations exhibit similar behavior as reported in some experimental results.

Enhanced terahertz emission from Bi incorporated GaSb

Broadband THz radiation emission from photoexcited GaSb1−xBix (0 < x < 1.65%) was investigated. We observed an enhancement in the THz emission with Bi incorporation in GaSb. GaSb1−xBix alloy with showed an emission amplitude comparable to that from LT-GaAs, which is over an order of magnitude higher compared to GaSb. Transient photocurrents due to p-type surface band bending was found to be the dominant emission mechanism in these alloys. Strengthening of the surface field due to Bi induced p-doping is responsible for the observed enhancement. The NIR pump incident angle and THz detection angle study suggests that the photocarrier momentum is preferentially aligned along the refracted excitation beam direction inside the semiconductor alloy. GaSb1−xBix emitter showed better spectral performance compared to a commercially available photoconductive antenna in a THz time domain spectroscopy system. This demonstrates the application potential of this material in THz spectroscopy.

Optimized Spintronic Terahertz Emitters Based on Epitaxial Grown Fe/Pt Layer Structures

We report on generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer. Our model takes into account generation of spin polarization, spin diffusion and accumulation in Fe and Pt and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 75 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on optical rectification in nonlinear crystals.

Atomic Force Microscopy and EDX Analysis for Investigation Photoconductive LT-GaAs Terahertz Antennas

We investigate the influence of the surface properties of a low-temperature-grown GaAs photoconductive antenna on the terahertz (THz) response power. A comparison to the surface roughness which is extracted from an atomic force microscope is given. We used energy dispersive x-ray spectroscopy (EDX) measurements to determine the Ga/As compositional ratio in the LT-GaAs.

The Influence of the Annealing Regime on the Properties of Terahertz Antennas Based on Low-Temperature-Grown Gallium Arsenide

Low-temperature gallium arsenide (LT-GaAs) films were grown by the method of molecularbeam epitaxy (MBE) at a reduced temperature (230°C) on GaAs(100) substrates and subjected to postgrowth annealing in various regimes. Photoconductive antennas (PCAs) with flag geometry formed on the film surface were characterized by terahertz (THz) response power at various bias voltages. The method of THz spectroscopy was used to study the characteristics of PCAs based on LT-GaAs films annealed in various regimes and the optimum interval of postgrowth annealing temperatures (670–720°C) was established.

Ultrafast terahertz detectors

Terahertz (THz) technologies have broad application prospects in ultrafast space communication, heterodyne detection, biological detection, non-destructive testing and national security. Ultrafast THz detectors, which can respond to the THz light with modulation rate larger than 1 GHz, are the key component of fast imaging, space communication, ultrafast spectroscopy and THz heterodyne applications. Theoretically, the traditional THz detectors based on heat effects are difficult to meet the requirements for fast detections, while the semiconductor based THz detectors can work under the condition of ultrafast detection. Photoconductive antennas with ultrafast response time are suitable for room-temperature broad-spectrum THz detections. Schottky barrier diodes, superconductor-insulator-superconductor mixers and hot electron bolometers are promising candidates for high-speed THz spatial heterodyne and direct detections attributable to their high conversion efficiency and low noise. High-mobility field effect transistors based on two-dimensional graphene material have the advantages of high sensitivity and low impedance, which make this kind of device have great potential applications in room-temperature high-speed detections. THz quantum well detectors (THz QWPs) based on inter-subband transitions are very suitable for the applications in high-frequency and high-speed detections because of the advantages of high responsivity, small value and integrated packaging. Recently, we have demonstrated 6.2 GHz bandwidth modulation by using THz QWPs, the fast THz receiving device. On the other hand, low working temperature and low coupling efficiency are the main factors that restrict the applications of THz QWPs. From the Brewster angle, 45 polished facet coupling structure, to one-or two-dimensional metal grating and surface Plasmon polariton coupling configuration, researchers often explore the appropriate coupling mechanism which can not only couple the normal incidence THz light, but also improve the coupling efficiency substantially. The sub-wavelength double-metal micro-cavity array coupling structure has two advantages which make THz QWPs a key candidate for fast imaging and detection in THz band:firstly, the patch antennas on the device surface can effectively increase the light absorption region, and the periodic structure can make the normal incidence THz light fulfill the rule of intersubband transition. Secondly, the sub-wavelength size double metal structure can restrict the light within a very small volume, and the electric current will be enhanced by the resonance effect when the cavity mode is equal to the peak response frequency, which can suppress the dark current and improve the optical coupling efficiency of the device. In this paper, several ultrafast THz detectors are reviewed and the advantages and disadvantages of various detectors are also analyzed.

Generation of THz radiation by photoconductive antennas on based thin films InGaAs and InGaAs/InAlAs.

Generation of THz radiation by photoconductive antennas on based thin films InGaAs and InGaAs/InAlAs.

Влияние режима отжига на свойства терагерцевой фотопроводящей антенны на основе LT-GaAs

AbstractLow-temperature gallium arsenide (LT-GaAs) films were grown by the method of molecularbeam epitaxy (MBE) at a reduced temperature (230°C) on GaAs(100) substrates and subjected to postgrowth annealing in various regimes. Photoconductive antennas (PCAs) with flag geometry formed on the film surface were characterized by terahertz (THz) response power at various bias voltages. The method of THz spectroscopy was used to study the characteristics of PCAs based on LT-GaAs films annealed in various regimes and the optimum interval of postgrowth annealing temperatures (670–720°C) was established.

Transmitting and detecting THz pulses using graphene and metals-based photoconductive antennas

In this paper, photoconductive antennas (PCAs) are designed using graphene ribbons (GRs) and metals at THz frequencies such that both transmitting and detecting modes for PCAs are investigated. A graphene-based PCA (GPCA) can support surface plasmon polaritons (SPPs) at either THz or optical frequencies, whereas a metal-based PCA (MPCA) only supports such waves at optical frequencies. Due to the 2D nature of graphene, its electronic modeling and electromagnetic modeling significantly differ from 3D bulk metals; consequently, some challenges appear when graphene is modeled in electronic and electromagnetic solvers. Hence, in this paper, the electronic modeling and electromagnetic modeling of graphene are comprehensively examined. We show that, through application of an electrostatic bias to a GR, its Fermi energy level can be shifted to an arbitrary value. This feature provides many striking advantages for a GPCA compared to a MPCA: supporting slow waves, reconfigurability, supporting SPPs at THz frequencies, mitigation of screening effects, and more radiated THz power. Although the supporting slow waves in a GR at THz frequencies, through excitation of a SPP propagation mode, result in a miniaturized structure for a GPCA, they cause spatial dispersion for the GR’s conductivity because the photoconductor used for a PCA usually has a high refractive index. Consequently, a more complicated analysis is required for designing a GPCA. Moreover, by studying the fabrication challenges relating to a GR, some of them are considered during its modeling. Finally, the radiated THz pulses are detected through a coherent detection scheme.

Improving the efficiency and directivity of THz photoconductive antennas by using a defective photonic crystal substrate

One of the shortcomings of photoconductive (PC) antennas in terahertz (THz) generation is low effective radiated power in the desirable direction. In this paper, we propose a defective photonic crystal (DPC) substrate consisting of a customized 2D array of air holes drilled into a solid substrate in order to improve the radiation characteristics of THz PC antennas. The effect of the proposed structure on the performance of a conventional THz PC antenna has been examined from several aspects including radiation efficiency, directivity, and field distribution. By comparing the radiation performance of the THz antenna on the proposed DPC substrate to that of the conventional solid substrate, it is shown that the proposed technique can significantly improve the efficiency and directivity of the THz PC antenna over a wide frequency range. It is achieved by reducing the amount of power coupled to the substrate surface waves and limiting the radiation in undesirable directions. In addition, it is found that the sensitivity of directivity to the substrate thickness is considerably decreased and the adverse Fabry–Perot effects of the thick substrate are reduced by the application of the proposed DPC substrate.