Terahertz Photoconductive Antenna Research Articles

Lateral photoconductivity of InAs/GaAs quantum dots for 1.5 μm-wavelength excitation photoconductive terahertz antenna devices

We report lateral photoconductive properties of multilayer-stacked undoped InAs/GaAs quantum dots (QDs) for the application of photoconductive terahertz (THz) antenna devices that operate in a 1.5 μm-telecom-wavelength band. The excitation power-dependent photocurrent showed a high value without saturation under high excitation power for the excitation wavelength of 1460 nm. From the reflection pump-probe signal, a fast photocarrier lifetime was derived. These results, together with the low dark current characteristic, support the applicability of the multilayer-stacked undoped InAs/GaAs QDs to photoconductive THz antennas operating in a 1.5 μm-wavelength band.

High Efficiency Interdigitated Terahertz Photoconductive Antenna Integrated With Cylindrical Lens Array

High Efficiency Interdigitated Terahertz Photoconductive Antenna Integrated With Cylindrical Lens Array

Terahertz photoconductive antennas based on silicon-doped GaAs (111)A

In this study we investigate a relatively new material for terahertz (THz) photoconductive antennas (PCAs): GaAs epitaxial films grown on (111)A-oriented semi-insulated GaAs substrates and doped with Si. GaAs:Si (111)A films with the required high resistance without postgrowth annealing have been grown by molecular-beam epitaxy. The Si doping of (111)A-oriented GaAs was expected to lead to the formation of acceptors that facilitate the activation of As[Formula: see text] traps. The relaxation times of nonequilibrium charge carriers in the films were measured in pump-probe experiment, and, finally, the electron mobility was estimated from the carrier lifetime and current–voltage characteristics measured under pulsed pumping by Ti:sapphire femtosecond laser. Dipole and bow-tie PCAs were fabricated, and the THz emission spectra and the total THz emission power were measured with varying applied voltage and optical excitation power. The properties of GaAs:Si (111)A films and the PCAs based on them are compared with LTG-GaAs (100) and (111)A-oriented films.

High Resistivity and High Mobility in Localized Beryllium-Doped InAlAs/InGaAs Superlattices Grown at Low Temperature

InAlAs:Be/InGaAs superlattices grown at low temperatures were investigated in this study. To obtain the highest resistivity and mobility simultaneously, a growth temperature above 200 °C was applied. The electrical properties were conducted via Hall effect measurement and a photoresponse test. The experimental results demonstrate that the sample grown at 257.5~260 °C exhibits the highest resistivity (1290 Ω × cm) and lowest carrier concentration (3.18 × 1014 cm−3), along with the highest mobility (187.2 cm2/Vs). Furthermore, the highest photoresponse (1.21) relative to dark resistivity was obtained under 1500 nm excitation. The optimized growth parameter of InGaAs/InAlAs multilayered structures is of great significance for fabricating high-performance terahertz photoconductive semiconductor antennas.

An Investigation of the Effect of Embedded Gold Nanoparticles in Different Geometric Shapes on the Directivity of THz Photoconductive Antennas

This study aims to show how Terahertz (THz) Photoconductive Antennas (PCAs) affect the radiation directivity when gold nanoparticles of various geometric shapes are embedded in the gap region between the electrodes of THz PCAs. Three different PCAs, in the frequency range of 0.1 to 2 THz, conventional and after the addition of cylindrical, triangular, square, and hexagonal geometric gold nanoparticles to the antenna gap region between the electrodes, were built and simulated. The antenna directivity increased from 4.71 dBi to 4.92 dBi when square nanoparticles were added to the bowtie PCA, from 4.33 dBi to 4.45 dBi when triangular nanoparticles were added to the dipole PCA, and from 7.18 dBi to 7.52 dBi when square nanoparticles were added to the Vivaldi PCA.

Improvement of Sub-Wavelength Grating Electrodes for Efficient Terahertz Photoconductive Antenna Based on Extraordinary Optical Transmission

Low efficiency of Terahertz (THz) radiation and radiation power have limited the development of THz Science and Technology. Thus, with the aims of achieving greater photoconductive current and improving radiation characteristics of THz photoconductive antenna (PCA), this study utilized Extraordinary Optical Transmission (EOT) of light passing through various subwavelength metal structures to control and restrain the light wave in subwavelength scale. Furthermore, by grooving the grating electrode structure, the influence of metal grating’s EOT on the transmission field of PCA were investigated and analyzed. Simulation results show that the effect of local electric field enhancement is significant. When the incident power is 0.1[Formula: see text]W, the peak value of the local electric field reaches [Formula: see text][Formula: see text]V/m. In addition, comparing to the grating electrode with no groove structure in which the field intensity was less than [Formula: see text][Formula: see text]V/m, the local electric field increased by 16.4 times, respectively. Correspondingly, the photocurrent intensity of the improved photoconductive plasmonic structure is increased by 72.3 times. In conclusion, the improved plasma photoconductive structure was shown to obviously enhance the transmission field strength of semiconductor materials and the current of the PCA, and accordingly, to improve the THz radiation capability.

Design and performance enhancement of terahertz photoconductive antenna based on nano-crossline contacts

Photoconductive antennas (PCAs) of high conversion efficiency is an essential device for generating or detecting terahertz (THz) waves. This paper presents an enhanced THz-PCA by incorporating buried nano-crosslines (BN-CL) contacts to offer high optical-to-terahertz power conversion efficiency. The BN-CL contact makes that most photocarriers are created at nanoscale distances from the PCA electrodes and, hence, carries drift in a sub-picosecond duration to generate THz radiation. The full-wave multiphysics model is used for the hybrid simulation of THz PCAs and investigated the impact of geometrical variables on a proposed PCA's transient photocurrent. The designed THz antenna has a bandwidth of more than 4 THz and high efficiency of 94%. Both designs of BN-CL and N-CL are implemented with and without double dielectric layers of anti-reflection coating (ARC) and distributed Bragg reflector (DBR) layers. The obtained results of the transient photocurrent are 891 μA and 1089 μA for 10 elements of implemented BN-CL and N-CL contacts, respectively. The photocurrent of BN-CL contacts is enhanced to 1464 μA when 50 element contacts are used. Also, the conversion efficiency of this PCA reaches a maximum value of 1.032% at a pump power of 1.5 mW.

Experimental and Computational Analysis of Broadband THz Photoconductive Antennas

The work presents the fabrication and measurements of four LT-GaAs photoconductive terahertz (THz) antennas with different geometries of metallic electrodes. The goal is to analyze the overall bandwidth of the antennas through a comparison between the spectra of the generated photocurrent in the antenna gap, the radiated electric field THz pulse, and the S11 parameter of the metallic electrodes. The photocurrent density and the S11 parameters are computed using COMSOL multiphysics, while the generated THz pulse was experimentally measured using a time-domain spectroscopy system. The polarizations of the photoconductive antennas are experimentally measured, using x-cut quartz crystal halfwave plates, showing polarization in the direction of the electrode’s long axis. Pinholes are used to verify system alignment and quality of the radiated signal spectra. The results show that the spectra of the radiated THz pulses in all four antennas considered in this work are dominated by the behavior of the S11 parameter at the lower part of the frequency band, but with the decreasing photocurrent dominating the spectra at higher frequencies.

Full-wave multiphysics model for simulation and investigation of terahertz photoconductive antenna using LUMERICAL and CST softwares

In this work, the physical mechanism of the full-wave multiphysics model is described underlying the simulation of terahertz photoconductive antenna (PCA) using hybrid softwares incorporating LUMERICAL and CST. The feature of this simulation is that the multiphysical phenomena that occur in the PCA, such as light-matter interaction, photoexcited carriers, and full-wave propagation, are considered and embodied in the simulation. The accuracy of the presented simulation framework is verified by comparing its results with some commercial softwares such as COMSOL, SILVACO, and the numerical of three-dimensional finite difference time domain (3-D FDTD) method.Themethod. The maximum discrepancy of the simulation results between this hybrid software framework and others is less than 2%, which indicates the accuracy of these simulation results. In addition, this hybrid simulation framework can be used to characterize the parameter-dependent performance of a PCA. The obtained results are used as a guideline to present modified PCAs where a nanostructure is added inside the antenna gap to enhance the PCA performance. The two modified PCAs are defined as nano-line and nano-crossline PCAs, where nano-lines are connected to one or both contacts. The conversion efficiencies of the nano-line and nano-crossline PCAs are enhanced by 28.5 and 64 times, respectively, with respect to conventional PCA.

Analysis of dual band emission in SRR integrated terahertz photoconductive antenna

Analysis of dual band emission in SRR integrated terahertz photoconductive antenna

Plasmonic grating assisted photo‐absorption enhancement in terahertz photoconductive antenna

AbstractA large aperture photoconductive antenna (PCA) with a plasmonic grating structure was fabricated and characterized. The intensity of the terahertz radiation from the plasmonic PCA was enhanced two‐fold compared with that from the conventional PCA without the plasmonic grating structure. By using an electrically isolated nano‐grating structure, the performance enhancement was found to be due to the plasmonic effect other than bias field enhancement. The photo‐absorption enhancement near the GaAs surface and the plasmonic grating interface was investigated to elucidate the terahertz radiation enhancement mechanism in the plasmonic PCA. The highly localized electric field near the nanoscale grating shows optical absorption at the GaAs surface was enhanced. The enhanced optical absorption increased the photo‐generated carrier density at the device surface, leading to enhanced intensity and bandwidth of the terahertz radiation from the plasmonic PCA.

Modeling and Characterizing of an Enhanced Practical Terahertz Photoconductive Antenna

Optical generation can be divided into photoconductive generation and optical rectification. Moreover, the generation of photoconductive is more efficient than optical rectification when standard laser oscillator systems are used. At photoconductive antenna (PCA), the efficiency and terahertz (THz) power generated are affected by different parameters (e.g. incident power of laser pump, dielectric properties and the applied DC bias voltage difference). In this research, four proposed designs of PCAs are presented and compared. Furthermore, microfabrication aspects and concerns have been considered in this study, which are rarely presented in the literature. Therefore, effects of different parameters are shown in this work such as adding adhesion layer in between electrodes and the substrate, varying of gap properties and dipole length. Output frequency, photocurrent and the total effective energy of different antenna models at different values of laser power have been studied and compared.

Thermal evaporated group IV Ge(Sn)-on-Si terahertz photoconductive antenna.

We have experimentally demonstrated thermal evaporated group IV Ge1-xSnx-on-Si terahertz (THz) photoconductive antennas (PCA) pumped by an Er-doped femtosecond laser for broadband THz generation. The Ge1-xSnx THz PCAs, free from material epitaxial growth methods, can offer comparable material properties in photocarrier generation, transportation, recombination, and the collection as group III-V THz PCAs. At the optical pumping power of 90 mW and a bias voltage of 40V, the Ge1-xSnx THz PCAs have achieved a broadband spectrum over 1.5 THz with a 40 dB signal-to-noise ratio (SNR). This CMOS-compatible group IV THz source can be monolithically integrated on the Si photonic platform, paving the way toward THz system-on-chip (SoC) for many on-site applications in the non-destructive evaluation, biomedical imaging, and industrial inspections.

Enhancing directivity of terahertz photoconductive antennas using spoof surface plasmon structure

Terahertz photoconductive antenna (PCA) is an important device for generating ultrabroadband terahertz radiations, being applicable in various scenarios. However, the metallic electrodes in PCAs, a pair of coplanar strip lines (CSL), always produce horizontal electrode modes in a broad THz band, thus resulting in low directivity in the vertical direction. Here, we introduce spoof surface plasmon polariton (SSPP) structures to suppress horizontal electrode modes in a broad band. The suppression principles are accounted to both the forbidden band of the fundamental SSPP mode and the orthogonality between source and higher-order SSPP modes. In the SSPP-modified PCA, we achieve around 2 dBi higher directivity in the vertical direction compared to a typical CSL PCA. Unlike the narrow bands inheriting from conventional metamaterial resonators, the relative operational band of the SSPP-modified PCA is as broad as 48%. This planar SSPP structure is compatible with the well-developed micro fabrication technologies. Thus, our scheme can be combined with the semiconductor material engineering and plasmonic nanoscale structures for further increasing THz output power.

Theoretical investigations on the influence of photonic crystal shapes on the performance of graphene-based terahertz photoconductive bowtie dipole antenna

Graphene-based photoconductive bowtie dipole antenna for terahertz radiation is investigated under different geometrical shapes of photonic crystal employed in the semi-conductor substrate. The antenna is designed using graphene radiator and Gallium Arsenide substrate to operate at 1.4 THz, 1.9 THz and 2.1 THz frequencies. Cylindrical, square, triangular and hexagonal shaped photonic crystal substrates are investigated. The analysis is carried out for various antenna parameters such as reflection coefficient, VSWR, directivity and radiation efficiency. Further, polarisation aspects such as axial ratio and cross-polarisation level are also discussed. Opto-electronic simulation is performed to determine the influence of photonic crystal shapes on the emission characteristics of photoconductive antenna. Simulation results indicate that hexagonal-shaped photonic crystals provide directivity of 15 dBi, axial ratio of 29 dB, efficiency of 98% and emission spectrum bandwidth of 220 GHz, which ascertain its suitability for terahertz photoconductive antenna.

Terahertz photoconductive antenna based on antireflection dielectric metasurface with embedded plasmonic nanodisks: erratum.

This erratum reports corrections in Figs. 5 and 8 of our previous paper [Appl. Opt.60, 7921 (2021)APOPAI0003-693510.1364/AO.431678].

Integrated optics spiral photoconductive antennas coupled with 1D and 2D micron-size terahertz-wavelength plasmonic metal arrays

Terahertz (THz) photoconductive antenna (PCA) emitters having one-dimensional (1D) and two-dimensional (2D) micron-size metal line arrays (MLA's) at the transmission side of the semi-insulating GaAs substrate were fabricated via UV photolithography and electron beam deposition. At a fluence of ∼1.2 mJ/cm2 and 20 VPP bias, the enhancement in the THz signal peak-to-peak amplitudes are ∼6 times for 1D MLA and ∼11 times for 2D MLA, compared to the reference PCA, respectively. An all-optical effect via THz extraordinary transmission is conjectured for the enhancement mechanism. This metamaterial PCA design presents a feasible, yet more cost-effective alternative to photo-conducting gap nanostructure fabrication using e-beam lithography.

Terahertz photoconductive antennas based on arrays of metal nanoparticle structures

In this study, in order to amplify the radiation at Terahertz photoconductive antennas, metal nanoparticles are used in semiconductor layers based on plasmonic principles. The use of nanoparticles between antenna electrodes and semiconductor layers not only enhances the THz radiation intensity but also changes the radiation frequency peak. The changes of electric charge carriers versus the incidence of laser pulses and the production of an electric current on the antenna surface are simulated with the COMSOL Multiphysics software through the FEM method. The changes of the electric current at the semiconductive surface generate electric field radiation. This has been simulated using the CST STUDIO software through the FDTD method.

Performance characterization of a self-made terahertz photoconductive antenna.

A terahertz (THz) photoconductive antenna is prepared using, to the best of our knowledge, a novel method, which has high yield and strong stability. It eliminates the stripping process of a thin-film THz antenna and effectively prevents the toxicity of the corrosion solution, the easy damage in the transfer process, and the weak bonding with the substrate. First, a 200 µm copper wire is bundled on a low-temperature GaAs epitaxial wafer, and then the electrode of the photoconductive antenna is fabricated using the vacuum evaporation method. Finally, the THz time-domain signal with a high signal-to-noise ratio and good repeatability is obtained using an 800 nm laser. Additionally, the influence of pump light and detection light power on THz signal intensity is studied when the total optical power is unchanged. Results show that when the total power of the laser is greater than a certain value, there is an optimal ratio between the pump power and the detection power, which can maximize the signal-to-noise ratio of the THz wave. This provides a basis for the effective application of a THz antenna and lays a foundation for improving the detection sensitivity of samples.

Operation of quantum dot based terahertz photoconductive antennas under extreme pumping conditions

Photoconductive antennas deposited onto GaAs substrates that incorporate InAs quantum dots have been recently shown to efficiently generate both pulsed and CW terahertz radiation. In this Letter, we determine the operational limits of these antennas and demonstrate their extreme thermal breakdown tolerance. Implanted quantum dots serve as free carrier capture sites, thus acting as lifetime shorteners, similar to defects in low-temperature grown substrates. However, unlike the latter, defect-free quantum-dot structures possess perfect lattice quality, thus not compromising high carrier mobility and pump intensity stealth. Single gap design quantum dot based photoconductive antennas are shown to operate under up to 1 W of average pump power (∼1.6 mJ cm−2 energy density), which is more than 20 times higher than the pumping limit of low-temperature grown GaAs based substrates. Conversion efficiency of the quantum dot based photoconductive antennas does not saturate up to 0.75 W of pump power (∼1.1 mJ cm−2 energy density). Such a thermal tolerance suggests a glowy prospect for the proposed antennas as a perspective candidate for intracavity optical-to-terahertz converters.