Photoconductive Antenna Research Articles

Detection properties of photoconductive antennas fabricated on low-temperature-grown GaAs and ErAs:GaAs at subterahertz band

Terahertz (THz) spectroscopy with high sensitivity is essential for biological application considering the strong absorption and scattering effects therein. As the most commonly used THz detector, the photoconductive antenna’s (PCA) response greatly relies on the properties of the substrate’s material. THz detection properties of the PCAs fabricated on low-temperature-grown GaAs (LT-GaAs) and ErAs:GaAs superlattices were compared at the sub-THz band. The detection efficiency of the PCAs with regard to incident laser power was characterized. In addition, using the PCAs as detectors, the signal-to-noise ratio (SNR) and dynamic range (DR) of a terahertz time-domain spectroscopy were quantified. The result indicates that the PCA detector with LT-GaAs has higher efficiency than the one with ErAs:GaAs. Consequently, the corresponding THz spectrometer has better SNR and DR. This result is contrary to the previous report, in which enhanced detection efficiency was observed with ErAs:GaAs-based PCA, which is probably due to the different structures of ErAs:GaAs superlattices used in the experiment.

Излучательная эффективность терагерцовых антенн с традиционной топологией и металлической метаповерхностью: сравнительный анализ

We report on experimental study of the photoconductive antennas (PCA)-emitters featuring conventional topology as well as metallic metasurface with plasmonic grating, based on the InGaAs/InAlAs superlattices. We measure and determine the photocurrents and the emission spectra of the PCAs, as well as the energy characteristics of the terahertz (THz) radiation, and the efficiency of the optical-to-THz conversion for different bias voltages and average laser excitation power. The integral THz emission power of 10 μW as well as conversion efficiency of 0.2% are demonstrated for the PCA with the metasurface, which are not reachable for the conventional due to thermal breakdown of the antenna. Therefore, the PCAs with the metasurface can be considered to be the attractive THz emitters that can potentially become among the elemental base for modern THz spectroscopic systems for biomedical applications.

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.

Terahertz radiation enhancement in photoconductive antennas with embedded split-ring resonators

This Letter proposes a novel method for enhancing terahertz (THz) radiation from microstructure photoconductive antennas (MSPCA). We present two types of MSPCA, which contain split-ring resonators (SRRs) and dipole photoconductive antennas (D-PCAs). The experimental results reveal that when the femtosecond laser is pumping onto the split position of the SRR, the maximum THz radiation power is enhanced by 92 times compared to pumping at the electrode edge of the D-PCA. Two π phase shifts occur as the pumping laser propagates from the negative electrode to the positive electrode. Analysis shows that photoinduced carrier charges move within the split position of the SRR.

Terahertz emitters based on ultrafast spin-to-charge conversion

Terahertz technology shows great potential applications in imaging, sensing and security. As is well known, the conventional solid-state broadband terahertz sources rely primarily on the nonlinear optical crystals and photoconductive antennas. Therefore, one major challenge for the next generation of terahertz technology is to develop the high-efficient, ultra-broadband and low-cost terahertz sources. In recent years, much attention has been paid to the spintronic terahertz emitters made of the metallic magnetic heterostructures on a nanometer scale. In this paper, the underlying physical mechanisms associated with this type of terahertz emitter is discussed. They mainly include the ultrafast demagnetization and the spin-charge interconversion processes. In order to further improve the terahertz emission efficiency, three main aspects are considered: appropriate choice of the materials (including conditions of the sample growing), film thickness, and new structure design. In the end, a short conclusion and future perspective for this research direction are given briefly.

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.

Определение влияния времени жизни носителей заряда арсенида галлия на эффективность непрерывной и импульсной генерации и детектирования излучения терагерцового диапазона частот

A configuration and test samples of photoconductive dipole antennas based on SI-GaAs: Cr and LT-GaAs for terahertz radiation generation and detection were developed. Their operating characteristics in pulsed mode and in the mode of operation as photomixers was experimentally investigated.

Interaction between spoof localized surface plasmon and terahertz vortex beam

We theoretically and experimentally investigate a method of exciting multipole plasmons, including terahertz dark spoof localized surface plasmon (Spoof-LSP) modes, by using normally incident terahertz vortex beam. The vortex beam with angular intensity profile and phase singularities, has well-defined angular momentum which can be decomposed into the polarization-state-related spin angular momentum (SAM) for characterizing the spin feature of photon, and the helical-wavefront-related orbital angular momentum (OAM) that is characterized by an integer <inline-formula><tex-math id="Z-20200915102424-1">\begin{document}$ (l) $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20200695_Z-20200915102424-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20200695_Z-20200915102424-1.png"/></alternatives></inline-formula>, called the topological charge. By illuminating terahertz vortex beam on the metallic disk with periodic subwavelength grooves normally, we find that the terahertz dark multipole plasmons can be excited by the terahertz vortex beam carrying different OAM and SAM. We analyze the correspondence between the spin and orbital angular momentum of vortex beam and the excited dark multipolar plasmon modes. In the experiment, a terahertz stepped spiral phase plate (SPP) with high transmission and low dispersion based on the Tsurupica olefin polymer is developed and the stepped SPP can generate a terahertz vortex beam having a topological charge of 1. Then, we further study the excitation of dark multipolar Spoof-LSPs by utilizing the stepped SPP in combination with the near-field scanning terahertz microscopy. The collimated terahertz wave, which is radiated from a 100 fs (<i>λ</i> = 780 nm) laser pulse pumped photoconductive antenna emitter, is converted into terahertz circular polarized light (CPL) which can carry SAM by the combination of the quarter wave plate and the polarizer, and then terahertz CPL impinges on the stepped SPP, producing the terahertz vortex beam which can carry OAM. The spatial two-dimensional electric field distribution is collected in steps of 0.02 mm along the <i>x</i>-direction and <i>y</i>-direction by a commercial terahertz near-field probe which is located close (≈ 10 μm) to the one side of polyimide film by three-dimensional electric translation stage and a microscope (FORTUNE TECHPLOGY FT-FH1080). The experimental results are in good agreement with simulations. We believe that our method will open the way for detailed research on the terahertz physics, plasma and imaging fields.

Фотопроводящий THz-детектор на основе сверхрешеточной гетероструктуры с плазмонным усилением

We report on a high-sensitive THz plasmonic-enhanced photoconductive antenna (PCA)-detector based on an InGaAs/InAlAs superlattice heterostructure. A noticeable increase in the sensitivity and signal-to-noise ratio of the proposed PCA-detector is shown compared to a similar detector without plasmonic electrodes. The efficiency of plasmonic electrodes has been experimentally confirmed via pulsed THz time-domain spectroscopy by measuring the dependence of the THz amplitude of the detector’s signal on the polarization of laser excitation.

Ultrafast carrier dynamics and THz conductivity in epitaxial-grown LT-GaAs on silicon for development of THz photoconductive antenna detectors

Carrier dynamics and photoconductivity in epitaxial-grown low-temperature GaAs on nominal and vicinal Si(1 0 0) substrates (‘LT-GaAs/Si’) were studied to predict their actual performance as THz photoconductive antenna (PCA) detectors. An optical-pump terahertz-probe technique was used to obtain the transmittance, carrier lifetime and photoconductivity of two LT-GaAs/Si samples, grown using different substrates and different growth protocols. The LT-GaAs grown on Si(1 0 0) substrate with a 4° tilt to 〈1 1 0〉 has better crystallinity, in agreement with other reports; while the LT-GaAs layer grown on nominal Si(1 0 0) substrate, though more structurally defective, has a much faster electron trapping time. Fabricated test PCAs with either dipole or bowtie geometries confirm the characterization results. The photoconductivity and carrier lifetime results manifest in the PCA performance, in responsivity, and in detection bandwidth. The prototypes’ sensitivities, bandwidths and dynamic ranges show that with some growth optimization, LT-GaAs/Si can be tailored to create economical, broadband THz detectors.

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.

Enhanced terahertz radiation of photoconductive antenna fabricated on GaAs-on-sapphire

The terahertz (THz) radiation properties of a photoconductive antenna (PCA) fabricated on a GaAs-on-sapphire (GoS) substrate are reported at sub-THz band. The GaAs layer with a thickness of approximately 1 µm was directly deposited on a sapphire wafer by means of molecular beam epitaxy. A butterfly-shaped antenna structure was then fabricated on the GoS substrate by photolithography, and the device was tested as the emitter of an in-house built THz time-domain spectrometer. The performance of this antenna was compared with a commercial one, which had an identical antenna structure but was fabricated on low-temperature-grown GaAs (LT-GaAs). The results showed that the GoS-based PCA radiated an enhanced THz field, which could be as much as 1.9 times that of the LT-GaAs-based PCA, indicating that GoS could be a promising photoconductive material. In addition, the optical transparency of the sapphire substrate allows the device to be illuminated from the backside, which is crucial for THz near-field imaging applications where the sample is usually in close proximity to the front surface of the PCA device.

Quantitative Detection of Interfacial Air Gap in Insulation Equipment Based on Terahertz Wave Contrast Method

The nondestructive testing of internal defects in power grid equipment has always been a spotlight of research. Since terahertz wave has good application in the nondestructive detection fields, we apply terahertz wave to detect the interfacial air gap of insulation equipment. At present, terahertz wave is difficult to measure the thickness of the internal air gap. Hence, this paper presents a method for quantitative measurement of internal air gap based on the waveform database. In addition, the propagation characteristics of the terahertz wave in medium are analyzed. Furthermore, based on the value of $n(\omega)$ and $\kappa (\omega)$ , the calculation formula of the transmission and reflection coefficients is simplified. After obtaining the reflected wave of the single outer layer material, we quantitatively calculate the time-domain reflection wave of interface. According to the principle of terahertz detection based on photoconductive antenna, the detection of multiple reflection waves is analyzed. To verify the superposition characteristics of multiple waves, the terahertz reflection waves of samples with interfacial air gap are tested. By comparing the measured waves and the artificially calculated waves, we prove that the superposition of the waveform is approximately linear. Based on the good coincidence between artificially generated waveform and measured waveform, we propose a method of quantitatively diagnosing the interface air-gap defect. Indeed, the deviation between the test results and the actual air-gap size is small.

Enhancement of photocurrent in THz photoconductive antenna by a gold nanorod array

In this article, a novel THz photoconductive antenna (THz-PCA) is proposed which is composed of a conventional THz-PCA integrated with a plasmonic nanorod array to enhance surface absorption and boost the THz wave generation. It is shown that by carefully matching the plasmon resonance wavelength of the designed plasmonic structure to the incoming laser light wavelength (λ=780 nm), it is possible to obtain a nearly perfect absorption. Specifically, light absorption inside the LT-GaAs substrate at is increased to more than 96 % which is considerably improved relative to approximately 60 % of light absorption in the conventional structure. Moreover, the designed plasmonic structure provides light absorption and photo-carrier generation mostly at the surface of the GaAs substrate which is highly beneficial due to the fact that surface charge carriers have the minimum travel distance to the antenna terminals resulting in lowest recombination probabilities. The proposed structure is numerically investigated by solving Maxwell’s equations for electromagnetic response along with the charge carrier continuity and Poisson’s equations for electrical response. For the Au nanorod array with a pitch of Λ=0.8 μm and nanorod radius of r=120 nm, photocurrent shows an improvement of nearly 250 % due to the surface absorption enhancement discussed above.

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.

Measurement and Analysis of Noise Spectra in Terahertz Wave Detection Utilizing Low-Temperature-Grown GaAs Photoconductive Antenna

Noise power spectral density (NPSD) in time domain terahertz (THz) wave detection systems utilizing GaAs-based photoconductive antennas (PCAs) was investigated quantitatively. The contributions of the PCA noise and the amplifier noises at the amplifier output depend strongly on the resistance of the PCA, the circuit parameters, and the frequency. The PCA has two types of noise: one can be modeled by the Johnson-Nyquist (thermal) noise for the PCA resistance, while the other has an NPSD inversely proportional to the frequency with its intensity dependent on the properties of the GaAs and the metallization. At a high frequency range ~ 100 kHz, voltage-type amplifier noise could appear if the cable capacitance between the PCA and the amplifier is large. As a result, a low-noise range tends to appear in the intermediate frequency range. In comparison with the PCAs with Ti/Au metallization, the PCAs with Pd/Ge/Ti/Au having lower contact resistance lead to lager influence of the Johnson-Nyquist noise at the output.

Intense sub-terahertz radiation from wide-bandgap semiconductor based large-aperture photoconductive antennas pumped by UV lasers

The characteristics of terahertz (THz) radiation generated from large-aperture photoconductive antennas (LAPCAs) were investigated. The antennas were fabricated using different wide-bandgap semiconductor crystals (ZnSe, GaN, 6H–SiC, 4H–SiC and β–Ga2O3) as the substrate. We used an amplified sub-picosecond KrF excimer laser for illumination of the LAPCAs. THz emission scaling was studied as a function of the bias field and the pump laser energy. It was found that the radiated THz energy scales quadratically as a function of the bias field and sub-linearly as a function of the optical fluence for most of the substrates. Further, we demonstrate that SiC, and especially 4H–SiC LAPCAs offer the best THz generation performances. In order to generate intense THz radiation, we fabricated both 6H- and 4H–SiC LAPCAs with an interdigitated structure. From the field autocorrelation trace, it was found that the spectra lie in the sub-THz regime, extending up to 400 GHz, with a peak frequency at 50 GHz, making the bridge between the microwaves band and the THz band. The maximum generated THz energy was 11 μJ, which is to date the highest THz energy measured from LAPCA sources, with a corresponding peak electric field of 115 kV cm−1 and a corresponding ponderomotive potential of 60 eV. Nonlinear THz experiments were performed using these energetic THz pulses, and open aperture Z-scan experiments in an n-doped InGaAs layer revealed a transmission enhancement of 1.7. It was also shown that in order to have efficient THz generation, the energy contrast of the laser must be kept high.

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.

Enhanced terahertz emission bandwidth from photoconductive antenna by manipulating carrier dynamics of semiconducting substrate with embedded plasmonic metasurface.

In this article, we demonstrate a technique to enhance the Terahertz (THz) emission bandwidth from photo-conductive antenna (PCA) based on semiconducting substrates by manipulating the surface carrier dynamics of the semiconductor. Bandwidths in PCAs are limited by the decay of the photogenerated charge carriers, which in case of SI-GaAs is in the orders of 50 picoseconds. We show, with an embedded design of plasmonic meta-surface in the photoconductive gap of a PCA, it is possible to enhance the emission bandwidths by more than 50 percent. This is due to the fact that these nano-structures act as local recombination sites for the photogenerated carriers, effectively reducing the carriers' lifetime. Additionally, the defect sites reduce the terminal current, thereby reducing the Joule heating in the device. Furthermore, the meta-surface also facilitates higher in-coupling of the exciting infrared light on to the PCA, thereby increasing the optical-to-THz conversion efficiency of the device.

Terahertz pulse time-domain holography with balance detection: complex-domain sparse imaging.

We investigated the peculiarities of the terahertz pulse time-domain holography principle in the case of raster scanning with the balance detection system. The noise in this system represents a Skellam distribution model, which differentiates it from systems based on a photoconductive antenna. We analyzed this Skellam model and provided both numerical and experimental investigations. We found that the variance of the noise in the balance detection system does not depend on the true signal. Complex-domain images obtained in this model are filtered by block-matching algorithms adapted for spatio-temporal and spatiospectral volumetric data. We presented a new cube complex-domain filter algorithm that uses block matching in all 3D data sets simultaneously in spatial and frequency coordinates. A combination of temporal and complex-domain filters allows us to expand the dynamic range of terahertz frequencies for which we can obtain amplitude/phase information. Experimental data demonstrate an improvement in the quality of the resultant images both in the time domain and complex-spectral domain. The simulation and experimental results are in good agreement.