Large-aperture Photoconductive Antenna Research Articles

A Diamond Terahertz Large Aperture Photoconductive Antenna Biased by a Longitudinal Field

The novel design of a terahertz large aperture photoconductive antenna (LAPCA) is reported. It features a longitudinal orientation of the bias electric field within the photoconductive substrate, and has the advantage of a small interelectrode gap, resulting in a higher field for the same applied voltage. The proposed LAPCA configuration has been tested with a nitrogen-doped (∼10 ppm) synthetic monocrystalline diamond, which is a promising material for high-intensity and high-power terahertz sources. Two antennas with different high-voltage electrode realizations were assembled, pumped by a 400 nm femtosecond laser, and tested for THz emitter function. The experimental data are found to be in good correlation with the numerical simulation results. The performance of antennas with the conventional transverse E-field configuration and the novel longitudinal configuration is compared and discussed.

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.

Generation of intense sub-cycle terahertz pulses with variable elliptical polarization

We demonstrate the generation of intense, sub-cycle terahertz (THz) pulses with variable elliptical polarization and peak fields above 80 kV/cm from large aperture photoconductive antennas using a specific interdigitated structure. The latter is composed of horizontal and vertical electrodes, which allow the generation of two quasi-half-cycle THz pulses with orthogonal polarization. A time delay between the two THz pulses is introduced by a phase delay mask covering only the parts of the antenna with horizontal electrodes. By changing the mask thicknesses, we can control, on demand, the polarization state of the THz pulses from linear polarization through elliptical to circular polarization over a quarter of a THz cycle.

Antenna design harnesses intense terahertz waves by controlling polarization at the source

Interdigitated large aperture photoconductive antenna emits two THz pulses with crossed polarization to overcome challenges in working with intense waves.

Efficiency of Photoconductive Terahertz Generation in Nitrogen-Doped Diamonds

The efficiency of the generation of terahertz radiation from nitrogen-doped (∼0.1–100 ppm) diamonds was investigated. The synthetic polycrystalline and monocrystalline diamond substrates were pumped by a 400 nm femtosecond laser and tested for the photoconductive emitter operation. The dependency of the emitted THz power on the intensity of the optical excitation was measured. The nitrogen concentrations of the diamonds involved were measured from the optical absorbance, which was found to crucially depend on the synthesis technique. The observed correlation between the doping level and the level of the performance of diamond-based antennas demonstrates the prospects of doped diamond as a material for highly efficient large-aperture photoconductive antennas.

Photoconductive terahertz generation in nitrogen-doped single-crystal diamond.

The generation of terahertz radiation in a photoconductive emitter based on nitrogen-doped single-crystal diamond was realized for the first time. Under 400 nm femtosecond laser pumping, the performance of diamond antennas with different dopant levels was investigated and compared with a reference ZnSe antenna. Terahertz waveforms and corresponding spectra were measured. A low saturation level for high-nitrogen-containing diamond substrate was revealed. The results indicate the prospects of doped diamond as a material for high-efficiency large-aperture photoconductive antennas.

Time-domain measurement of coherent transition radiation using a photoconductive antenna with micro-structured electrodes

The longitudinal electric field profile of coherent transition radiation (CTR), which is a radially polarized terahertz (THz) pulse from an electron bunch, was measured using a large-aperture photoconductive antenna (PCA) with a diameter of 8.8 mm and micro-structured concentric electrodes to detect THz light pulses. Photo-induced charge carriers were generated on the PCA by the irradiation of femtosecond laser pulses on the electrode plane. Pulsed light irradiation enabled time-resolved measurement of the THz light pulses, and time-domain measurement of CTR was conducted by measuring the electric-field-induced current output from the PCA by sweeping the timing of the laser irradiation. The frequency spectra obtained from the detected longitudinal electric field profiles of the CTR showed a reasonable response signal from the low-frequency region as low as 0.1 THz. In the present demonstration, the large-aperture PCA-detected THz wave from the CTR was from 0.1 to at least 1 THz. THz generation using the PCA was also performed and measured using a Michelson interferometer with a helium-cooled bolometer. Comparison of the THz pulse of the CTR detected using the PCA and that generated using the PCA revealed that time-domain measurement of the THz pulse using the PCA was limited by the response time of the charge carriers in the semiconductor. Enlargement of the aperture of the PCA enhanced the detectable frequency region toward a lower frequency.

Terahertz generation by means of ZnGeP2 large aperture photoconductive antenna

The generation of terahertz (THz) radiation using a ZnGeP2 (ZGP) large-aperture photoconductive antenna (PCA) was demonstrated. The semiconductors were excited above and below the bandgap (400 and 800 nm) by a femtosecond Ti:sapphire laser. The THz pulse waveform generated by the ZGP antenna was measured using a time-domain spectroscopy technique. The antenna’s THz pulse energy dependence on the optical pump energy was measured, and saturation fluence and carrier mobility were estimated. The ZGP and a chemical vapor deposited ZnSe-based PCA were compared.

Hybrid Perovskite Terahertz Photoconductive Antenna.

Hybrid organic–inorganic perovskites, while well examined for photovoltaic applications, remain almost completely unexplored in the terahertz (THz) range. These low-cost hybrid materials are extremely attractive for THz applications because their optoelectronic properties can be chemically engineered with relative ease. Here, we experimentally demonstrate the first attempt to apply solution-processed polycrystalline films of hybrid perovskites for the development of photoconductive terahertz emitters. By using the widely studied methylammonium-based perovskites MAPbI3 and MAPbBr3, we fabricate and characterize large-aperture photoconductive antennas. The work presented here examines polycrystalline perovskite films excited both above and below the bandgap, as well as the scaling of THz emission with the applied bias field and the optical excitation fluence. The combination of ultrafast time-resolved spectroscopy and terahertz emission experiments allows us to determine the still-debated room temperature carrier lifetime and mobility of charge carriers in halide perovskites using an alternative noninvasive method. Our results demonstrate the applicability of hybrid perovskites for the development of scalable THz photoconductive devices, making these materials competitive with conventional semiconductors for THz emission.

Intense terahertz generation from photoconductive antennas.

In this paper, we review the past and recent works on generating intense terahertz (THz) pulses from photoconductive antennas (PCAs). We will focus on two types of large-aperture photoconductive antenna (LAPCA) that can generate high-intensity THz pulses (a) those with large-aperture dipoles and (b) those with interdigitated electrodes. We will first describe the principles of THz generation from PCAs. The critical parameters for improving the peak intensity of THz radiation from LAPCAs are summarized. We will then describe the saturation and limitation process of LAPCAs along with the advantages and disadvantages of working with wide-bandgap semiconductor substrates. Then, we will explain the evolution of LAPCA with interdigitated electrodes, which allows one to reduce the photoconductive gap size, and thus obtain higher bias fields while applying lower voltages. We will also describe recent achievements in intense THz pulses generated by interdigitated LAPCAs based on wide-bandgap semiconductors driven by amplified lasers. Finally, we will discuss the future perspectives of THz pulse generation using LAPCAs.

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.

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.

Influence of Gap Size on Intense THz Generation From ZnSe Interdigitated Large Aperture Photoconductive Antennas (October 2016)

We report on the effect of gap size on the generation of intense THz pulses emitted from ZnSe interdigitated large-aperture photoconductive antennas. The use of a relatively large gap size (460, 700, and 932 μ m) was found to slightly influence the radiated THz waveforms and the spectrum but strongly affects the scaling of the radiated THz electric field where stronger saturation appeared for smaller gap sizes, contrary to the behavior expected for operation in the THz field-screening regime. We attributed these variations to the effect of the relatively large capacitance of the antennas, which tends to limit the maximum radiated THz power, and which can be linked to space charge screening. The response of the high-voltage pulse was simulated as a function of the gap size and capacitance, revealing that these variations occur on a time scale faster than the THz pulse duration, thereby impacting the radiation emission characteristics.

Intense THz Pulses with large ponderomotive potential generated from large aperture photoconductive antennas.

We report the generation of free space terahertz (THz) pulses with energy up to 8.3 ± 0.2 µJ from an encapsulated interdigitated ZnSe Large Aperture Photo-Conductive Antenna (LAPCA). An aperture of 12.2 cm2 is illuminated using a 400 nm pump laser with multi-mJ energies at 10 Hz repetition rate. The calculated THz peak electric field is 331 ± 4 kV/cm with a spectrum characterized by a median frequency of 0.28 THz. Given its relatively low frequency, this THz field will accelerate charged particles efficiently having very large ponderomotive energy of 15 ± 1 eV for electrons in vacuum. The scaling of the emission is studied with respect to the dimensions of the antenna, and it is observed that the capacitance of the LAPCA leads to a severe decrease in and distortion of the biasing voltage pulse, fundamentally limiting the maximum applied bias field and consequently the maximum energy of the radiated THz pulses. In order to demonstrate the advantages of this source in the strong field regime, an open-aperture Z-scan experiment was performed on n-doped InGaAs, which showed significant absorption bleaching.

Intense terahertz generation at low frequencies using an interdigitated ZnSe large aperture photoconductive antenna

We demonstrate the generation of intense THz pulses at low frequencies, and THz pulse shaping, using a ZnSe interdigitated large aperture photoconductive antenna. We have experimentally measured a THz pulse energy of 3.6 ±0.8 μJ, corresponding to a calculated peak THz electric field of 143 ± 17 kV/cm. We also used a binary phase mask instead of a traditional shadow mask with our interdigitated photoconductive antenna, which allows us to generate THz field profiles that range from a symmetric single-cycle THz pulse to an asymmetric half-cycle THz pulse.

Terahertz-radiation-enhanced broadband terahertz generation from large aperture photoconductive antenna

A technique of enhancing and broadening terahertz (THz) wave radiation from large aperture photoconductive (PC) antenna is presented in this paper. In this technique, the PC antenna is excited by both the optical and previously generated THz pulses by a laser-induced air plasma created in front of PC antenna and an enhanced and broadened THz wave signal is obtained. The theoretical and experimental investigation shows that the superposition is the main mechanism for this enhancement. The technique shown in this paper can be very useful for THz imaging and spectroscopy.

Reshaped Terahertz waveforms from a large-aperture photoconductive antenna with millimeter scale metal hole and grid combination

Time-domain THz transmission of a metal wire grid is investigated experimentally. The transmission depends on the relative angle (θ) between the polarization of the THz wave and the direction of the wires. When the polarization is parallel to the wires (θ = 0°), the grid works as a high-pass filter with a cutoff frequency at 0.3 THz. Meanwhile, when the polarization is perpendicular to the wires (θ = 90°), the waveform of THz pulses is only slightly altered after passing through the grid. These results are reproduced accurately in computational simulations of the electromagnetic field. Simulation results indicate that strongly excited surface waves between two adjacent metal rods are responsible for the transmission peak at 0.81 THz, while surface waves slightly excited on the front surface of the grid are responsible for the low transmission at 0.3 THz. Transmission spectra of θ = 0° are interpreted qualitatively by applying the concept of effective surface plasmons.

Terahertz enhancement from terahertz-radiation-assisted large aperture photoconductive antenna

The observation of enhanced terahertz (THz) wave generation from the large aperture photoconductive (PC) antenna excited by both a femtosecond pump beam and a collinearly propagating ZnTe-pregenerated THz wave is reported within this paper. An analysis based on both the calculated and experimental results demonstrated that the superposition acts as the main physical mechanism of this THz enhancement effect due to the dominant contribution from the rapid change in photoexcited carrier density. A prerequisite for the THz enhancement requires that the polarization of the applied bias and the ZnTe-pregenerated THz should be identical in order to have a constructive superposition. Therefore, this observation introduces the possibility of recycling the unused portion of the pump beam to further improve the THz radiation. The enhancement effect could be optimized by changing the thickness of ZnTe, which could affect the photoexcited-free-carrier absorption of THz in the PC antenna and the bandwidth of final enhanced THz radiation.

Study on characteristics of terahertz radiation in the surface and far field generated by large-aperture photoconductive antennas

The time-domain characteristics of terahertz radiations emitted from a biased large-aperture photoconductive antenna trigged by an ultra-short optical pulse are studied. Succinct explicit expressions in the surface field and in the far field for the emitted THz radiation fields are deduced based on the well-known current-surge model. The expressions indicate that the THz radiations in the surface and far fields are mainly affected by the incident light intensity, the incident light wavelength, the carrier relaxation time, the reflectivity illuminated at the photoconductor, the carrier lifetime, and the incident light pulse width. The characteristics of THz radiation in the surface and far fields are discussed, and the phenomena of changes in the radiation peak values, the radiation pulse width, and the rising edges are explained in details. Especially the changes in surface fields are explained with the increasing of the ratio of luminous flux. The numerical results indicate that the emitted THz radiation intensity can be increased by enhancing the ratio of luminous flux, prolonging the carrier lifetime, or reducing the incident light pulse width. To avoid the shielding effect of the radiation field, an effectively method is presented that the intense of the incident laser pulse could be divided into multiple beams with low energy to trigger the photoconductive antenna, thus the THz radiation power can be further increased significantly.

Generation of Terahertz Radiation by Large-Aperture Photoconductive Antennas in Condition of a High Level Excitation

Theoretical and experimental results about generation coherent terahertz (THz) radiation in photoconductive medium in condition of a high level excitation are presented. Employing self-consistent analytical approach to the set of non-equilibrium Boltzmann equation for charge carriers and Maxwell equations for electromagnetic radiation we have studied the radiation phenomena in non-equilibrium e–h plasma excited in the photoconductive gap of terahertz radiating large-aperture photoconductive antenna by an ultrashort laser pulse. Equally with the effect of the radiation screening, the effect of the non-linear absorption of an optical pump pulse (bleaching of photoconductive medium) was taken into account. It was shown that the effect of nonlinear absorption reconstructs the dynamics of the generation of terahertz radiation and must take into account the effects associated with the spread of the generated wave and the wave of excitation.