Photoconductive Terahertz Emitters Research Articles

Microstructured large-area photoconductive terahertz emitters driven at high average power.

Emitters based on photoconductive materials excited by ultrafast lasers are well-established and popular devices for THz generation. However, so far, these emitters - both photoconductive antennas and large area emitters - were mostly explored using driving lasers with moderate average powers (either fiber lasers with up to hundreds of milliwatts or Ti:Sapphire systems up to few watts). In this paper, we explore the use of high-power, MHz repetition rate Ytterbium (Yb) based oscillator for THz emission using a microstructured large-area photoconductive emitter, consist of semi-insulating GaAs with a 10 × 10 mm2 active area. As a driving source, we use a frequency-doubled home-built high average power ultrafast Yb-oscillator, delivering 22 W of average power, 115 fs pulses with 91 MHz repetition rate at a central wavelength of 516 nm. When applying 9 W of average power (after an optical chopper with a duty cycle of 50%) on the structure without optimized heatsinking, we obtain 65 µW THz average power, 4 THz bandwidth; furthermore, we safely apply up to 18 W of power on the structure without observing damage. We investigate the impact of excitation power, bias voltage, optical fluence, and their interplay on the emitter performance and explore in detail the sources of thermal load originating from electrical and optical power. Optical power is found to have a more critical impact on large area photoconductive emitter saturation than electrical power, thus optimized heatsinking will allow us to improve the conversion efficiency in the near future towards much higher emitter power. This work paves the way towards achieving hundreds of MHz or even GHz repetition rates, high-power THz sources based on photoconductive emitters, that are of great interest for example for future THz imaging applications.

Improving the efficiency of an optical-to-terahertz converter using sapphire fibers

Objectives. The study aims to improve the efficiency of a large-area photoconductive terahertz (THz) emitter based on an optical-to-terahertz converter (OTC) having a radiating area of 0.3 × 0.3 mm2 for generating high-power THz radiation by using an array of close-packed profiled sapphire fibers having a diameter in the range of 100–300 μm as focusing optics.Methods. As a photoconductive substrate, we used a semi-infinite LT-GaAs layer (low-temperature grown GaAs; GaAs layer grown by molecular beam epitaxy at a low growth temperature). Additional Si3N4 and Al2O3 layers are intended for reducing leakage currents in the OTC and reducing the reflection of the laser pump pulse from the air/semiconductor interface (Fresnel losses), respectively, at a gap width of 10 μm. For forming the antenna electrodes and feed strips, the Ti/Au metal system was used. The simulation was carried out by the finite element method in the COMSOL Multiphysics environment.Results. The use of a profiled sapphire fiber whose diameter has been optimized with respect to the gap parameters to significantly increase the concentration of charge carriers in the immediate vicinity of the electrodes of an OTC is demonstrated. The integrated efficiency of a large-area photoconductive THz emitter was determined taking into account the microstrip topology of the array with a characteristic size of feed strips proportional to the gap width in the OTC and with the upper (masking) metal layer. The maximum localization of the electromagnetic field in close proximity to the edges of electrodes at the “fiber–semiconductor” interface is achieved with a profiled sapphire fiber diameter of 220 μm.Conclusions. By optimizing the diameter of the sapphire fiber, the possibility of improving the localization of incident electromagnetic waves in close proximity to the edges of the OTC electrodes by ~40 times compared to the case without fiber, as well as increasing the overall efficiency of a large-area emitter by up to ~7–10 times, was demonstrated.

Bias-free terahertz generation from a silicon-compatible photoconductive emitter operating at telecommunication wavelengths

We present a telecommunication-compatible bias-free photoconductive terahertz emitter composed of a bilayer InAs structure directly grown on a high-resistivity silicon substrate. The bilayer InAs structure includes p+-doped and undoped InAs layers, inducing a strong built-in electric field that enables terahertz generation without requiring any external bias voltage. A large-area plasmonic nanoantenna array is used to enhance and confine optical generation inside the photoconductive region with the highest built-in electric field, leading to the generation of a strong ultrafast photocurrent and broadband terahertz radiation. Thanks to a higher terahertz transmission through the silicon substrate and a shorter carrier lifetime in the InAs layers grown on silicon, higher signal-to-noise ratios are achieved at high terahertz frequencies compared with previously demonstrated bias-free terahertz emitters realized on GaAs. In addition to compatibility with silicon integrated optoelectronic platforms, the presented bias-free photoconductive emitter provides more than a 6 THz radiation bandwidth with more than 100 dB dynamic range when used in a terahertz time-domain spectroscopy system.

High\u2013Bias\u2013Field Operation of GaAs Photoconductive Terahertz Emitters

We demonstrate experimentally the increase of optical-to-terahertz conversion efficiency for GaAs-based photoconductive terahertz emitters. This increase is achieved by preventing device breakdown through series resistors, which act as a current limiter. Pulsed photoexcitation and potential current fluctuations result in heat dissipation leading to local heating, which further increases the current and may lead to device breakdown. We manage to increase the maximum bias field before device breakdown by a factor of 3 under illuminated conditions. For a laser system with 250-kHz repetition rate, the terahertz emission amplitude increases linearly with applied bias field up to 120 kV/cm bias field, which results in 3 times higher signal as compared to the standard device. Furthermore, we have also achieved this expanded breakdown prevention at 78-MHz repetition rate, where an integrated on-chip resistance leads to an enhancement of the terahertz field amplitude by 70%. This simple technique can increase the performance of almost all photoconductive terahertz emitters by using appropriate resistances according to the emitter capacitance and laser repetition rate.

Characteristics of Bow-Tie Antenna Structures for Semi-Insulating GaAs and InP Photoconductive Terahertz Emitters.

This work presents a study of photoconductive (PC) terahertz (THz) emitters based upon varied bow-tie (BT) antenna structures on the semi-insulating (SI) forms of GaAs and InP. The BT antennas have electrodes in the form of a Sharp BT, a Broad BT, an Asymmetric BT, a Blunted BT, and a Doubled BT. The study explores the main features of PC THz emitters for spectroscopic studies and sensors application in terms of THz field amplitude and spectral bandwidth. The emitters’ performance levels are found to depend strongly upon the PC material and antenna structure. The SI-InP emitters display lower THz field amplitude and narrower bandwidth compared to the SI-GaAs emitters with the same structure (and dimensions). The characterized Doubled BT structure yields a higher THz field amplitude, while the characterized Asymmetric BT structure with flat edges yields a higher bandwidth in comparison to the sharp-edged structures. This knowledge on the PC THz emitter characteristics, in terms of material and structure, can play a key role in future implementations and applications of THz sensor technology.

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.

Photoconductive terahertz generation in semi-insulating GaAs and InP under the extremes of bias field and pump fluence.

This Letter analyzes photoconductive (PC) terahertz (THz) emitters based on the semi-insulating (SI) forms of GaAs and InP. The dependencies of the emitters are studied under the extremes of the bias field and pump fluence to reveal the underlying physics of charge carrier photoexcitation, transport, and emission. The bias field dependence shows that SI-GaAs PC THz emitters are preferentially subject to space-charge-limited current, under the influence of trap states, while SI-InP PC THz emitters are preferentially subject to sustained current, due to a prolonged charge carrier lifetime and the ensuing joule heating. The pump fluence dependence shows space-charge and near-field screening for all emitters, with SI-GaAs predisposed to near-field screening (under the influence of transient mobility) and SI-InP predisposed to space-charge screening. Such findings can support a deeper understanding of the underlying physics and optimal performance of SI-GaAs and SI-InP PC THz emitters.

Non-plasmonic improvement in photoconductive THz emitters using nano- and micro-structured electrodes.

We investigate here terahertz enhancement effects arising from micrometer and nanometer structured electrode features of photoconductive terahertz emitters. Nanostructured electrode based emitters utilizing the palsmonic effect are currently one of the hottest topics in the research field. We demonstrate here that even in the absence of any plasmonic resonance with the pump pulse, such structures can improve the antenna effect by enhancing the local d.c. electric field near the structure edges. Utilizing this effect in Hilbert-fractal and grating-like designs, enhancement of the THz field of up to a factor of ∼ 2 is observed. We conclude that the cause of this THz emission enhancement in our emitters is different from the earlier reported plasmonic-electrode effect in a similar grating-like structure. In our structure, the proximity of photoexcited carriers to the electrodes and local bias field enhancement close to the metallization cause the enhanced efficiency. Due to the nature of this effect, the THz emission efficiency is almost independent of the pump laser polarization. Compared to the plasmonic effect, these effects work under relaxed device fabrication and operating conditions.

THz power optimization and analysis of plasmonic unbiased photoconductive THz emitters coupled to a spiral-like dipole antenna

We propose a new spiral-like dipole antenna-coupled and unbiased terahertz (THz) photomixer with plasmonic unsymmetrical interdigitated electrodes (UIEs). Each electrode pair is made of subwavelength metal–semiconductor–metal (MSM) structures constituting heterogeneous Schottky contacts. From the optical view and analysis of the electrical transport in the semiconductor, by applying a systematic procedure to achieve optimum design, it is possible to benefit from advantages of having high THz photocurrent derived from the plasmonic enhanced optical absorption and, hence, photogeneration as well as severe built-in Schottky field in the photoconductor. Furthermore, from the electromagnetic view, by introducing an output resonant THz antenna having great impedance matching with the photomixer device, the efficiency of free-space coupling has significantly been enhanced. By exploiting all aspects of design, it has been provided to achieve the THz power of 409 µW from the spiral-like dipole antenna coupled to the proposed plasmonic UIEs with 10 × 10 µ m 2 active area and pitch of Λ = 395 n m . This is more than 1 order of magnitude larger THz power than the highest THz power radiated from the same area of a conventional dipole antenna-coupled unbiased array of the near-field emitters. Also, this improvement is more than 3 orders of magnitude higher THz power as compared with the power radiated from a non-plasmonic far-field operating antenna-less THz chip of the same size.

637 μ W emitted terahertz power from photoconductive antennas based on rhodium doped InGaAs

We investigate photoconductive terahertz (THz) emitters compatible with 1550 nm excitation for THz time-domain spectroscopy (TDS). The emitters are based on rhodium (Rh) doped InGaAs grown by molecular beam epitaxy. InGaAs:Rh exhibits a unique combination of ultrashort trapping time, high electron mobility, and high resistivity. THz emitters made of InGaAs:Rh feature an emitted THz power of 637 μW at 28 mW optical power and 60 kV/cm electrical bias field. In particular for a fiber coupled photoconductive emitter, this is an outstanding result. When these emitters are combined with InGaAs:Rh based receivers in a THz TDS system, 6.5 THz bandwidth and a record peak dynamic range of 111 dB can be achieved for a measurement time of 120 s.

Improved electrode design for interdigitated large-area photoconductive terahertz emitters.

We study here the effect of the electrode parameters on the terahertz emission efficiency of large-area emitters based on interdigitated electrodes. Electrode parameters are optimized to get maximum terahertz emission by optimizing the balance condition among the emission efficiency of individual electrode pairs, number of emitters per unit area, and fraction of semiconductor exposed for optical pumping. A maximum enhancement by about 50% in the peak to peak electric field is observed as compared to the previous state of the art design.

Design and Simulation of a Piezotronic GaN-Based Pulsed THz Emitter

A piezotronic pulsed GaN-based photoconductive TeraHertz (THz) emitter is proposed and simulated for the first time. In this paper, we benefit from high break down-voltage, thermal conductivity, and saturation velocity of GaN, to design a pulsed antenna-less THz emitter. The proposed emitter consists of asymmetric metal-semiconductor-metal structure. Moreover, strong coupling of semiconducting/piezoelectric properties in GaN, allows modulation of the dissimilar Schottky barriers, leading to tunable THz output power and bandwidth. Our simulation results demonstrate that applying an external strain of about 4% results in 26% enhancement in the radiated THz power, while output bandwidth is improved about 4.2%. The proposed structure is promising for emerging reconfigurable THz emitters, capable of being tunable by external strain as a controlling gate.

Gapless Broadband Terahertz Emission from a Germanium Photoconductive Emitter

Photoconductive terahertz (THz) emitters have been fulfilling many demands required for table-top THz time-domain spectroscopy up to 3–4 THz. In contrast to the widely used photoconductive materials such as GaAs and InGaAs, Ge is a nonpolar semiconductor characterized by a gapless transmission in the THz region due to absence of one-phonon absorption. We present here the realization of a Ge-based photoconductive THz emitter with a smooth broadband spectrum extending up to 13 THz and compare its performance with a GaAs-based analogue. We show that the spectral bandwidth of the Ge emitter is limited mainly by the laser pulse width (65 fs) and, thus, can be potentially extended to even much higher THz frequencies.

A polarization-insensitive plasmonic photoconductive terahertz emitter.

We present a polarization-insensitive plasmonic photoconductive terahertz emitter that uses a two-dimensional array of nanoscale cross-shaped apertures as the plasmonic contact electrodes. The geometry of the cross-shaped apertures is set to maximize optical pump absorption in close proximity to the contact electrodes. The two-dimensional symmetry of the cross-shaped apertures offers a polarization-insensitive interaction between the plasmonic contact electrodes and optical pump beam. We experimentally demonstrate a polarization-insensitive terahertz radiation from the presented emitter in response to a femtosecond optical pump beam and similar terahertz radiation powers compared to previously demonstrated polarization-sensitive photoconductive emitters with plasmonic contact electrode gratings at the optimum optical pump polarization.

Spectral Response Tuning of Photoconductive Terahertz Emitters With Binary Phase Masks

In this paper, binary phase masks are designed and implemented for use with a photoconductive terahertz (THz) emitter in order to shape the frequency response of the emitted THz waveforms. Theoretical and experimental results are applied to study the system. The theoretical results are presented in the form of both numerical simulations (to characterise and uniquely identify the underlying time-delayed photogeneration and resulting charge-carrier dynamics) and a simplified model (to conceptualise the system). The theoretical and experimental results are found to agree. It is ultimately shown that the phase delay through the binary phase mask can be used to preferentially allocate power into bands within the THz spectrum. Such results can lay the groundwork for future studies needing precise control of power within the THz spectrum.

High accuracy terahertz time‐domain system for reliable characterization of photoconducting antennas

ABSTRACTWe report on a terahertz (THz) time‐domain system to characterize photoconductive THz emitters and detectors, that is designed for highest reproducibility. This system is excellently suited for studying the performance of THz sources and detectors in a systematic manner, either by varying the substrate materials or the geometrical parameters of metallic antenna contacts building a photoconductive switch. After confirming the reproducibility and stability of the system with errors of only 1.9% (over 3 h) and 2.6% (over 9 days), we use the system to compare the performance of five low temperature grown (LT) GaAs wafers with growth temperatures between 200°C and 300°C. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:468–472, 2017

Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter.

We report on large-area photoconductive terahertz (THz) emitters with a low-temperature-grown GaAs (LT-GaAs) active layer fabricated on quartz substrates using a lift-off transfer process. These devices are compared to the same LT-GaAs emitters when fabricated on the growth substrate. We find that the transferred devices show higher optical-to-THz conversion efficiencies and significantly larger breakdown fields, which we attribute to reduced parasitic current in the substrate. Through these improvements, we demonstrate a factor of ~8 increase in emitted THz field strength at the maximum operating voltage. In addition we find improved performance when these devices are used for photoconductive detection, which we explain through a combination of reduced parasitic substrate currents and reduced space-charge build-up in the device.

High power telecommunication-compatible photoconductive terahertz emitters based on plasmonic nano-antenna arrays.

We present a high-power and broadband photoconductive terahertz emitter operating at telecommunication optical wavelengths, at which compact and high-performance fiber lasers are commercially available. The presented terahertz emitter utilizes an ErAs:InGaAs substrate to achieve high resistivity and short carrier lifetime characteristics required for robust operation at telecommunication optical wavelengths. It also uses a two-dimensional array of plasmonic nano-antennas to offer significantly higher optical-to-terahertz conversion efficiencies compared to the conventional photoconductive emitters, while maintaining broad operation bandwidths. We experimentally demonstrate pulsed terahertz radiation over 0.1-5 THz frequency range with the power levels as high as 300 μW. This is the highest-reported terahertz radiation power from a photoconductive emitter operating at telecommunication optical wavelengths.

Plasmonic efficiency enhancement at the anode of strip line photoconductive terahertz emitters.

We investigate strip line photoconductive terahertz (THz) emitters in a regime where both the direct emission of accelerated carriers in the semiconductor and the antenna-mediated emission from the strip line play a significant role. In particular, asymmetric strip line structures are studied. The widths of the two electrodes have been varied from 2 µm to 50 µm. The THz emission efficiency is observed to increase linearly with the width of the anode, which acts here as a plasmonic antenna giving rise to enhanced THz emission. In contrast, the cathode width does not play any significant role on THz emission efficiency.

Photoconductive terahertz generation from textured semiconductor materials.

Photoconductive (PC) terahertz (THz) emitters are often limited by ohmic loss and Joule heating—as these effects can lead to thermal runaway and premature device breakdown. To address this, the proposed work introduces PC THz emitters based on textured InP materials. The enhanced surface recombination and decreased charge-carrier lifetimes of the textured InP materials reduce residual photocurrents, following the picosecond THz waveform generation, and this diminishes Joule heating in the emitters. A non-textured InP material is used as a baseline for studies of fine- and coarse-textured InP materials. Ultrafast pump-probe and THz setups are used to measure the charge-carrier lifetimes and THz response/photocurrent consumption of the respective materials and emitters. It is found that similar temporal and spectral characteristics can be achieved with the THz emitters, but the level of photocurrent consumption (yielding Joule heating) is greatly reduced in the textured materials.