Terahertz Photoconductive Antenna Research Articles

An overview of terahertz antennas

The terahertz (THz) antennas, which have features of small size, wide frequency bandwidth and high data rate, are important devices for transmitting and receiving THz electromagnetic waves in the emerging THz systems. However, most of THz antennas suffer from relatively high loss and low fabrication precision due to their small sizes in high frequency bands of THz waves. Therefore, this paper presents a detailed overview of the most recent research on the performance improvement of THz antennas. Firstly, the development of THz antennas is briefly reviewed and the basic design ideas of THz antennas are introduced. Then, THz antennas are categorized as metallic antennas, dielectric antennas and new material antennas. After that, the latest research progress in THz photoconductive antennas, THz horn antennas, THz lens antennas, THz microstrip antennas and THz on-chip antennas are discussed. In particular, the practical difficulties for the development of THz antennas are discussed with promising approaches. In addition, this paper also presents a short review of the process technology of THz antennas. Finally, the vital challenges and the future research directions for THz antennas are presented.

Theoretical analysis of compact cylindrical microlenses for terahertz photoconductive antennas in the photomixer regime

A terahertz photoconductive antenna placed on the back side of a semiconductor slab with and without a compact cylindrical semiconductor microlens on the front side is studied theoretically. The antenna is operated as a photomixer giving narrowband radiation at 1 THz. Radiation patterns and emitted powers are found to oscillate with slab thickness as a consequence of multiple-reflection interference. It is further shown that an antireflection layer on the lens may eliminate these oscillations to a large extent. In the absence of a lens, most of the radiation is trapped inside the semiconductor slab, and the radiation pattern is far from that of a pencil-beam. Both light trapping and radiation patterns are shown to be significantly improved by a very compact lens with a size smaller than a cubic wavelength. The improvements on outcoupling of radiation in a predominantly forward direction versus lens radius and height are mapped out. The calculated outcoupling efficiency of the antenna-lens system takes into account the Purcell effect and radiation trapped in the semiconductor slab. The antenna-lens system is modeled rigorously by using the Green’s function volume integral equation method in a form that exploits cylindrical symmetry.

Терагерцовое излучение фотопроводящих антенн на основе сверхрешеточных структур \LT-GaAs/GaAs:Si\

A multilayer structure based on low-temperature LT-GaAs with crystallographic orientation (111) A is proposed for the fabrication of terahertz (THz) photoconductive antennas. The structures contain active layers of undoped LT-GaAs and high-temperature GaAs:Si layers doped with Si acceptors. At an optical pump power of 19 mW and a bias voltage of 30 V, a photoconductive antenna on an optimized {LT-GaAs / GaAs: Si} (111) A structure emitted THz pulses with an average power of 2.3 μW at a pulse repetition rate of 80 MHz, the conversion efficiency was 1.2 ∙ 10-4. It is shown that the dependence of the integrated power of THz antenna pulses based on {LT-GaAs / GaAs: Si} (111) A structures on the applied voltage is superlinear, and on the optical pump power it has the form of a saturation curve. The possibility of practical application of the obtained antennas for the tasks of terahertz spectroscopy of biological solutions is shown.

Comparison of full-wave models of terahertz photoconductive antenna based on ordinary differential equation and Monte Carlo method

The investigation and comparison of two full-wave models of photoconductive terahertz antenna, made from low-temperature grown gallium arsenide, is performed. One model solves simple approximation of drift-diffusion equations another uses Monte Carlo simulation for estimation of the electrical current in the active region of antenna. Simulation results revealed that the simple model can be useful in the cases when the duration of photoexcitation is relatively long (FWHM $$\ge $$ 250 fs). In a case of shorter laser pulses (FWHM $$\ge $$ 130 fs), electron recombination (or trapping) time should be in the order of 0.1 ps, which is characteristic of highly compensated semiconductors. In other cases, transient dynamics of electron drift velocity at sub-picosecond timescales makes significant impact to the growth speed of the photocurrent. This leads to the overestimation of electric field amplitude in the high-frequency range when the simple model is used. Full-wave simulation shows good agreement with experimental results when the detectors’ response is included in the calculation. The calculated results were confirmed experimentally which increases the reliability of the full-wave model presented in the paper.

Multi-Energy Valley Scattering Characteristics for a SI-GaAs-Based Terahertz Photoconductive Antenna in Linear Mode

In this paper, the relationship between the terahertz radiation and the spatial distribution of photogenerated carriers under different bias electric field is studied. Terahertz pulses and the photocurrent of SI-GaAs photoconductive antenna are measured by the terahertz time-domain spectroscopy system. The occupancy rate for photogenerated carriers for different energy valleys is obtained by comparing the photocurrent of terahertz field integrating with respect to time with the photocurrent measured by oscilloscope. Results indicate that 93.04% of all photogenerated carriers are located in the Γ valley when the bias electric field is 3.33 kV/cm, and 68.6% of all photogenerated carriers are transferred to the satellite valley when the bias electric field is 20.00 kV/cm. With the bias electric field increasing, the carrier occupancy rate for the satellite valley tends to saturate at 72.16%. In order to obtain the carrier occupancy rate for the satellite valley and saturate value at the high bias electric field, an ensemble Monte Carlo simulation based on the theory of photo-activated charge domain is developed.

An advanced approach to control the electro-optical properties of LT-GaAs-based terahertz photoconductive antenna

This work reports on an advanced approach to the design of THz photoconductive. antenna (PCA). The LT-GaAs thin films used for the PCA fabrication were synthesized by MBE method on GaAs (100) substrate by adjusting the As pressure, As/Ga fluxes ratio, growth/annealing temperatures and annealing time. These parameters crucially affect electro-optical properties of the PCA samples as evidenced by the THz radiation power and time-domain spectroscopy measurements. The annealing temperature of 670 °C was found to be optimal for constructing a PCA possessing high amplitude of the THz radiation over the spectral range up to 1 THz at the resonance of 0.1 THz. The comparison of this PCA with the reference ZnTe crystal reveals a 2-fold increase in THz power. Furthermore, this antenna attains a 1.5-, 3-, and 2-fold increase in THz power, photocurrent efficiency, and actuating dc BV, as compared with the commercial ZOMEGA antenna. These results pave the way towards the creation of highly efficient LT-GaAs-based PCAs.

Enhancement of the frequency peak of terahertz photoconductive antennas using metamaterial (MTM) superstrate structures

A new design based on a metamaterial superstrate structure for THz photoconductive antennas is reported herein. To enhance the THz output frequency peaks, the use of metamaterials on THz photoconductive antennas is proposed, and simulated by using the CST Microwave Studio software. Use of such a metamaterial superstrate structure over the electrodes of an antenna leads to an enhancement of the frequency peak from 0.8 to 1.3 THz.

Surface plasmons in metamaterial heterostructures

ABSTRACT The extensive research of layered materials has revealed that it is possible to obtain a new degree of freedom for tuning the dispersion properties. Based on this concept, researchers have investigated the hyperbolic metamaterials. Recently, several kinds of photonic crystals have been proposed as classical counterparts of the layered materials. However, observation of nanostructured metamaterials from the perspective of the metamaterial structural design properties has remained difficult until now. Thus, we consider the propagation of surface plasmon polaritons (SPPs) at the boundary of geometrically different metamaterials. Herein, we consider two-layered and multilayered composites. All the heterostructures under consideration are made of the alternating graphene and dielectric sheets. We analytically estimate dispersion properties, absorption and propagation lengths of SPPs. The variation of graphene dielectric functions is described by the Drude model. On top of all, we analytically identify the key factors influencing propagation length along with the absorption enhancement highly desirable in terahertz photoconductive antennas and consider analytical variations aiming to tune the dispersion relations.

Improving unbiased terahertz photoconductive antenna based on dissimilar Schottky barriers using plasmonic mode excitation

A new design for an unbiased antennaless terahertz (THz) photoconductive antenna (PCA) using an array of bimetallic nanostructures located underneath the low temperature gallium arsenide (LT-GaAs) layer is designed and proposed which is highly under focus for THz imaging applications. Each element of array consists an asymmetric metal-semiconductor-metal (MSM) structure with dissimilar Schottky barriers. Simulation results demonstrated that by irradiating this structure using 800 nm femtosecond laser, THz field as high as a conventional biased PCA can be generated. Finite element method was used to optimize the PCA’s geometrical parameters through solving the Maxwell’s equation in combination with drift-diffusion/Poisson’s equations. According to the simulation results, by optimizing the array’s periodicity, width and height of the proposed bimetallic nanostructures, plasmonic resonant cavity mode between two adjacent nanostructures enhances the optical absorption and so, the transient photocurrent. Transient photocurrent of 45 μA was obtained that is 29% higher than the one from the conventional biased PCA.

Time-Domain Characterization of the Radiation Pattern of the Terahertz Photoconductive Antennas

We present a novel method to characterize the radiation pattern of the terahertz photoconductive antennas in time-domain. The method uses a modified version of the terahertz time-domain spectroscopy system, where we use the basics of physical optics and the optical path compensation principle using the delay line in the system in order to detect the spatial distribution of the terahertz radiation of the photoconductive antennas in two major planes. Using this method, we investigate two terahertz photoconductive antennas, namely dipole and spiral antennas, and compare the measurement results with simulations in time-domain. The comparison of the calculation and measurement results shows good agreement, where we measure an average and maximum optical path difference error of 4.7 and 8%, which results in an average and maximum half-power beam width error of 1.87 and 3.2°, respectively. We believe that the proposed method will be useful for predicting the system level performance of the terahertz time-domain spectroscopy systems in real-life applications.

Efficacy of proposed 2DEG-based photoconductive antenna using magnetic bias-controlled carrier transport

An externally applied magnetic field was used to induce increased photocarrier transport along the high mobility channel in GaAs/AlGaAs modulation-doped heterostructures (MDH). The terahertz (THz) emission from GaAs/AlGaAs MDH increases with increasing magnetic field, applied parallel to the heterojunction. The THz emission enhancement factors due to the magnetic field in MDH are higher than in undoped GaAs/AlGaAs heterojunction and in bulk SI-GaAs. This demonstrates that properly utilizing the high-mobility channel for carrier transport promises to be a viable design consideration for efficient THz photoconductive antenna (PCA) devices. Moreover, it was observed that for MDH, as well as for an undoped GaAs/AlGaAs heterojunction, the enhancement for one magnetic field direction is greater than the enhancement for the opposite direction. This is in contrast to the symmetric enhancement with magnetic field direction observed in a bulk SI-GaAs. An analysis of photocarrier trajectories under an external magnetic field supports the explanation that the enhancement asymmetry with magnetic field direction in MDH is due to the cycloid motion of electrons as affected by the GaAs/AlGaAs interface.

Pump dependent carrier lifetimes in InAs/GaAs quantum dot photoconductive terahertz antenna structures

In this paper, we study the behavior of photoexcited carriers in GaAs wafers containing self-assembled InAs quantum dots (QDs). These samples were designed and grown to be employed as photoconductive antennae for terahertz (THz) generation. The implanted QDs serve to shorten the overall carrier lifetime and enable pumping with lasers operating at wavelengths between 1μm and 1.3μm. Optical pump-THz probe measurements reveal carrier lifetime shortening for higher pump powers in the unbiased antennae due to increased carrier capture via Auger relaxation. This phenomenon significantly broadens the THz spectra from QD-based PCAs and explains the increased THz intensity at higher frequencies presented earlier.

A Polarization-Resolved Study of Nanopatterned Photoconductive Antenna for Enhanced Terahertz Emission

Terahertz (THz) frequencies, despite having the potential for several important applications, have been relatively underexplored in the past owing to the unavailability of proper sources and detectors. The scenario has been changing over the past few decades due to the advent of convenient THz sources and detectors. THz photoconductive antennas (PCA), due to their attractive features, such as cost effectiveness and room temperature operation, are playing a key role in current and future research prospect in the field of THz spectroscopy, both as sources and detectors. Complex PCA designs have been proposed and studied to boost the THz emission efficiencies. Elucidating the underlying physics in such devices requires a thorough investigation of a few physical parameters. This requires the integration of several experimental techniques under identical conditions. In this paper, we show such a study, including a parametric variation of pump polarization, conducted on a PCA with a nanopatterned active region, which boosts the emitted THz radiation. Through the set of measurements, we unravel the subtle interplay of the various physical processes responsible for the emission of THz radiation from the device.

Shaping the spectrum of terahertz photoconductive antenna by frequency-dependent impedance modulation

In this paper, we report on an approach for shaping the spectra of THz pulse generation in photoconductive antennas (PCAs) by frequency-dependent impedance modulation. We introduce a theoretical model describing the THz pulse generation in PCAs, and accounting for impedances of the photoconductor and of the antenna. Additionally we account a detector response for comparative analysis with experimental data. In order to showcase an impact of frequency-dependent impedance modulation on the spectra of THz pulse generation, we applied this model to simulating broadband PCAs with log-spiral topology. Finally, we fabricated two different log-spiral PCAs and characterized them experimentally using the THz pulsed spectroscopy. The observed results demonstrate agreement between the theoretical model and experiment, justifying the potential of shaping the spectra of THz pulse generation in PCAs by modulation of frequency-dependent impedances. This approach makes it possible to optimize PCA performance and thus accommodes the needs of THz pulsed spectroscopy and imaging in fundamental and applied branches of THz science and technology.

Improvement of Terahertz Photoconductive Antenna using Optical Antenna Array of ZnO Nanorods

An efficient terahertz (THz) photoconductive antenna (PCA), as a major constituent for the generation or detection of THz waves, plays an essential role in bridging microwave-to-photonic gaps. Here, we propose an impressive approach comprising the use of arrayed zinc oxide nanorods (ZnO NRs) as an optical nanoantenna over an anti-reflective layer (silicon nitride) in the antenna gap to boost the photocurrent and consequently the THz signal. The numerical approach applied in investigating the optical behavior of the structure, demonstrates a significant field enhancement within the LT-GaAs layer due to the optical antenna performing simultaneously as a concentrator and an antireflector which behaves as a graded-refractive index layer. ZnO NRs have been fabricated on the PCA gap using the hydrothermal method as a simple, low cost and production compatible fabrication method compared to other complex methods used for the optical nanoantennas. Compared to the conventional PCA with a traditional antireflection coating, the measured THz power by time domain spectroscopy (TDS) is increased more than 4 times on average over the 0.1–1.2 THz range.

Design of terahertz photoconductive antenna arrays based on defective photonic crystal substrates

In this paper, arrays of two different terahertz (THz) antennas based on defective photonic crystal (DPC) substrates are proposed and their radiation characteristics are investigated by using full-wave simulation. We show that employing DPC substrates in THz photoconductive antenna (PCA) arrays reduces the amount of power coupled to the substrate surface waves and, as a result, decreases the mutual coupling between the elements of the antenna array and increases the directivity in the boresight direction. By studying 1 × 2 arrays of bowtie and four-leaf-clover-shaped antennas, we show that either the improvement of directivity in a broad bandwidth or the enhancement of photomixer antenna impedance at a resonant frequency can be considered in the design of the DPC substrate. Therefore, the proposed approach can significantly improve the efficiency and radiation performance of THz photomixer antennas resulting in an improved effective THz radiated power in the desirable direction. The proposed DPC substrate can be considered as a way to realize sub-millimeter-sized dielectric resonator antennas at terahertz frequencies. In addition, this cost-effective technique can reduce the necessity of exploiting hyper-hemispherical Si lenses by overcoming the adverse effects of the thick substrate which is of prime importance in PCA arrays from a practical viewpoint.

Terahertz photoconductive emitter with dielectric-embedded high-aspect-ratio plasmonic grating for operation with low-power optical pumps

We report on the design, optimization and fabrication of a plasmon-assisted terahertz (THz) photoconductive antenna (PCA) for THz pulse generation at low-power optical pumps. The PCA features a high aspect ratio dielectric-embedded plasmonic Au grating placed into the photoconductive gap. Additionally, Si3N4-passivation of the photoconductor and the Al2O3-antireflection coating are used to further enhance antenna performance. For comparative analysis of the THz photocurrents, THz waveforms and THz power spectra we introduced the THz photocurrent δi and the THz power enhancement δTHz factors, which are defined as ratios between the THz photocurrents and the THz power spectra for the plasmon-assisted and conventional PCAs. We demonstrated superior performance of the plasmon-assisted PCA δi=30 and δTHz=3 ⋅ 103 at the lowest optical pump power of P=0.1 mW. Nevertheless the increase to P=10 mW lead to monotonically decrease in the both values to δi=2 and δTHz=102 due to screening effects. These results demonstrate a strong potential of the plasmonic PCA for operation with low-power lasers, thus, opening opportunities for the development of portable and cost-effective THz spectrometers and imaging systems.

Comparative Efficiency and Power Assessment of Optical Photoconductive Material-Based Terahertz Sources for Wireless Communication Systems

Terahertz band has recently attracted the attention of the communication society due to its huge bandwidth and very high-speed wireless communications capability. It has been utilized in a variety of disciplines including physics, biology and astronomy for years; and the main concerns have always been obtaining highly efficient and high-power terahertz sources. Today, these problems are still the most important issues in establishing an operable wireless terahertz communication link. In this paper, recent studies in the field of terahertz source design are investigated based on the terahertz output power and efficiency. Solid-state sources and optical sources were comparatively reviewed with optical photoconductive material (OPM)-based methods which are combined with the terahertz antennas in the design phase generally. For wireless communication, the most suitable frequencies are between 0.3[Formula: see text]THz and 1[Formula: see text]THz due to the attenuation profile of the atmosphere. For this reason, based on the recently published studies, it has been observed that OPM and resonant tunneling diode-based sources are the most promising terahertz sources in terms of efficiency and power. Key issues and the main problems of terahertz photoconductive antennas which are the base of OPM method were also discussed in this paper.

Design of highly directive terahertz photoconductive dipole antenna using frequency-selective surface for sensing and imaging applications

The prospects for improving the gain and directivity of a photoconductive dipole antenna (PCA) using a bandpass frequency-selective surface (FSS) as a superstrate at terahertz frequencies for imaging and sensing applications are presented. The physical parameters of the proposed bandpass FSS for the PCA are determined using a simple synthesis technique based on a single square loop bandpass FSS. Furthermore, the effect of the unit-cell periodicity of the FSS on the resonance condition of the antenna structure is studied, and its effect on the gain, directivity, and front-to-back lobe ratio of the proposed antenna is also presented. Moreover, control over the transmission characteristics of the FSS in a particular frequency band is also discussed.

Tunable Plasmonic Properties and Absorption Enhancement in Terahertz Photoconductive Antenna Based on Optimized Plasmonic Nanostructures

Herein, we numerically investigate terahertz photoconductive antennas (PCAs) based on optimized plasmonic nanostructures and absorption enhancement in nanocylinders. Plasmonic behavior in the visible to near-infrared light spectrum is achievable due to the metallic nanostructure employment. Herein, we study the absorption enhancement of silver and transparent-conducting oxides (TCO) nanocylinders with different diameters by means of effective medium approximation. This study also reports on the stronger enhancement in the case of TCO nanocylinders. The results show that resonant absorption amplitude and wavelength are dramatically affected by the thickness of the nanostructure as well as by the distances between nanocylinders. The outputs reported here provide a fertile ground for precise control of the nanowire structures for sensing and other enhanced optical applications. It is worthwhile noting that in case of TCO nanocylinders, absorption enhancement for NIR wavelengths, being relevant for present terahertz generation setup, reaches up to fivefold leading to 25-fold increase in terahertz radiation.