Plasmonic Terahertz Detectors Research Articles

Gate-readout and a 3D rectification effect for giant responsivity enhancement of asymmetric dual-grating-gate plasmonic terahertz detectors.

We experimentally investigated the asymmetric dual-grating-gate plasmonic terahertz (THz) detector based on an InGaAs-channel high-electron-mobility transistor (HEMT) in the gate-readout configuration. Throughout the THz pulse detection measurement on the fabricated device, we discovered a new detection mechanism called the "3D rectification effect" at the positive gate bias application, which is a cooperative effect of the plasmonic nonlinearities in the channel with the diode nonlinearity in the heterobarrier between the InGaAs channel layer and the InAlAs spacer/carrier-supply/barrier layers, resulting in a giant enhancement of the detector responsivity. We also found that an undesired long-tail waveform observed on the temporal pulse photoresponse of the device is due to trapping of carriers to the donor levels in the silicon δ-doped carrier-supply layer when they tunnel through the barrier to the gate and can be eliminated completely by introducing the so-called inverted-HEMT structure. The internal current responsivity and noise-equivalent power are estimated to be 0.49 A/W (with the equivalent voltage responsivity of 4.9 kV/W with a high output impedance of 10 kΩ) and 196 pW/√Hz at 0.8 THz. These results pave the way towards the application of the plasmonic THz detectors to beyond-5G THz wireless communication systems.

Enhanced terahertz detection of multigate graphene nanostructures.

Terahertz (THz) waves have revealed a great potential for use in various fields and for a wide range of challenging applications. High-performance detectors are, however, vital for exploitation of THz technology. Graphene plasmonic THz detectors have proven to be promising optoelectronic devices, but improving their performance is still necessary. In this work, an asymmetric-dual-grating-gate graphene-terahertz-field-effect-transistor with a graphite back-gate was fabricated and characterized under illumination of 0.3THz radiation in the temperature range from 4.5K up to the room temperature. The device was fabricated as a sub-THz detector using a heterostructure of h-BN/Graphene/h-BN/Graphite to make a transistor with a double asymmetric-grating-top-gate and a continuous graphite back-gate. By biasing the metallic top-gates and the graphite back-gate, abrupt n+n (or p+p) or np (or pn) junctions with different potential barriers are formed along the graphene layer leading to enhancement of the THz rectified signal by about an order of magnitude. The plasmonic rectification for graphene containing np junctions is interpreted as due to the plasmonic electron-hole ratchet mechanism, whereas, for graphene with n+n junctions, rectification is attributed to the differential plasmonic drag effect. This work shows a new way of responsivity enhancement and paves the way towards new record performances of graphene THz nano-photodetectors.

Terahertz Plasmonic Technology

The terahertz (THz) technology has found applications ranging from astronomical science and earth observation to compact radars, non-destructive testing, chemical analysis, explosive detection, moisture content determination, coating thickness control, film uniformity determination, structural integrity testing wireless covert communications, medical applications (including skin cancer detection), imaging, and concealed weapons detection. Beyond 5G Wi-Fi and Internet of Things (IoT) are the expected killer applications of the THz technology. Plasmonic TeraFETs such as Si CMOS with feature sizes down to 3 nm could enable a dramatic expansion of all these applications. At the FET channel sizes below 100 nm, the physics of the electron transport changes from the collision dominated to the ballistic or quasi-ballistic transport. In the ballistic regime, the electron inertia and the waves of the electron density (plasma waves) determine the high frequency response that extends into the THz range of frequencies. The rectification and instabilities of the plasma waves support a new generation of THz and sub-THz plasmonic devices. The plasmonic electronics technology has a potential become a dominant THz electronics sensing technology when the plasmonic THz sources join the compact, efficient, and fast plasmonic TeraFET THz detectors already demonstrated and being commercialized.

Low-temperature GaAs-based plasmonic photoconductive terahertz detector with Au nano-islands

We have fabricated low-temperature grown GaAs (LT-GaAs)-based plasmonic photoconductive antennas by RF sputtering of Au nanoparticles and have evaluated their terahertz detection properties. Localized surface plasmon resonance enhances the electric fields near the surface and increases the optical absorption of nanoparticles. The resonance frequency depends on the density of electrons, the effective electron mass, and the size and shape of the nanoparticles. Therefore, we tried to develop a high-sensitivity LT-GaAs photoconductive detector (PCD), which is effective over a wide range of wavelengths, by RF sputtering of Au nano-islands with a variety of aspect ratios from 1.2 to 5.1 on the dipole gap region of the PCD. As a result, we succeeded in increasing the sensitivity by 29% and 40% in the amplitude of observed terahertz pulse for 800 nm and 1560 nm femtosecond laser excitations, respectively.

Optimization of the frequency response of a novel GaAs plasmonic terahertz detector

Previously there was reported a new type of high-speed plasmonic THz detector that can operate at room temperature. As an extension of that work, the sensitivity of the detector was investigated over a wide range of sub-THz frequencies. The measured frequency response is not purely monotonic but exhibits oscillatory behaviour with a number of maxima and minima. Our study reveals that such frequency dependence is caused by the interference of electromagnetic waves inside the detector substrate, as the frequencies of these extrema are found to be governed by the substrate thickness. We demonstrate that sensitivity of this type of detector can be optimized for the desired operating frequency within 0.06–0.7 THz spectrum by adjusting the substrate thickness. We also show that a monotonic frequency response with eliminated minima can be achieved by mounting the detector on a specially designed silicon lens.

Graphene plasmonic terahertz detector with high responsivity

Rectification of terahertz radiation in a spatially periodic graphene structure with dual grating gate is theoretically studied. The calculated current responsivity is an order of magnitude greater than the responsivity of conventional plasmonic detector based on the field-effect-transistor array.

Terahertz plasmonic rectification in a spatially periodic graphene

The plasmonic terahertz detection in a dual grating-gate graphene structure with an asymmetric unit cell is studied theoretically using a hydrodynamic approach. Two plasmonic effects of rectification of terahertz radiation—the differential plasmonic drag of charge carriers and the plasmonic electron-hole ratchet—are introduced and investigated. The plasmonic electron-hole ratchet effect appears only if the carriers of opposite signs are there in graphene under different subgratings of the dual grating gate. We demonstrate that the electron-hole ratchet effect dominates in the terahertz frequency range.

Resonant plasmonic terahertz detection in graphene split-gate field-effect transistors with lateral p–n junctions

We evaluate the proposed resonant terahertz (THz) detectors on the basis of field-effect transistors (FETs) with split gates, electrically induced lateral p–n junctions, uniform graphene layer (GL) or perforated (in the p–n junction depletion region) graphene layer (PGL) channel. The perforated depletion region forms an array of the nanoconstions or nanoribbons creating the barriers for the holes and electrons. The operation of the GL-FET- and PGL-FET-detectors is associated with the rectification of the ac current across the lateral p–n junction enhanced by the excitation of bound plasmonic oscillations in the p- and n-sections of the channel. Using the developed device model, we find the GL-FET- and PGL-FET-detector characteristics. These detectors can exhibit very high voltage responsivity at the THz radiation frequencies close to the frequencies of the plasmonic resonances. These frequencies can be effectively voltage tuned. We show that in PL-FET-detectors the dominant mechanism of the current rectification is due to the tunneling nonlinearity, whereas in the PGL-FET-detector the current rectification is primarily associated with the thermionic processes. Due to much lower p–n junction conductance in the PGL-FET-detectors, their resonant response can be substantially more pronounced than in the GL-FET-detectors corresponding to fairly high detector responsivity.

On the response time of plasmonic terahertz detectors

The response time of a plasmonic terahertz detector is investigated using two independent experimental techniques relying on time-resolved and heterodyne measurements, respectively. Both methods demonstrate that the detector response time does not exceed 110 ps. The results obtained suggest that the designed terahertz detector can be used as a component of high-speed telecommunication systems of a new generation.

Accurate Analysis and Characterization of Silicon Field Effect Transistor-Based Terahertz Wave Detector with Quasi-Plasma Two-Dimensional Electron Gas.

We report the nonresonant plasmonic terahertz (THz) wave detector based on the silicon (Si) field effect transistor (FET) with a technology computer-aided design (TCAD) platform. The plasma wave behavior has been modeled by a quasi-plasma electron box as a two-dimensional electron gas (2DEG) in the channel of the FET. The incoming alternating current (AC) signal as the THz wave radiation can induce the direct-current (DC) voltage difference between the source and drain, which is called the photoresponse. For accurate analysis of the modulation and propagation of the channel electron density as the plasma wave, we have characterized the quasi-plasma 2DEG model with two key parameters, such as quasi-plasma 2DEG length (I(QP)) and density (N(QP)). By using our normalization method, I(QP) and N(QP) is defined exactly as extracting the average point of the electron density. We also investigate the performance enhancement of the plasmonic terahertz wave detector based on Si FET by scaling down the gate oxide thickness (t(ox)), which is a significant parameter of FET-based plasmonic terahertz detector for the channel electron density modulation. According to scaling down t(ox), the responsivity (R(v)) and noise equivalent power (NEP), which are the important performance metrics of the THz wave detector, have been enhanced. The proposed methodologies will provide the advanced physical analysis and structural design platform for developing the plasmonic terahertz detectors operating in nonresonant regime.

High-Performance Plasmonic THz Detector Based on Asymmetric FET With Vertically Integrated Antenna in CMOS Technology

We report a high-performance plasmonic terahertz (THz) detector based on an antenna-coupled asymmetric FET by using the 65-nm CMOS technology. By designing an asymmetric FET on a self-aligned poly-Si gate structure, more enhanced channel charge asymmetry between the source and the drain has been obtained in comparison with the nonself-aligned metal gate structure of our previous paper. In addition, using a vertically integrated patch antenna, which is designed for a 0.2-THz resonance frequency, we demonstrated the highly enhanced detection performance with a responsivity of 1.5 kV/W and a noise-equivalent power of 15 pW/Hz0.5 at 0.2 THz.

Resonant plasmonic terahertz detection in vertical graphene-base hot-electron transistors

We analyze dynamic properties of vertical graphene-base hot-electron transistors (GB-HETs) and consider their operation as detectors of terahertz (THz) radiation using the developed device model. The GB-HET model accounts for the tunneling electron injection from the emitter, electron propagation across the barrier layers with the partial capture into the GB, and the self-consistent oscillations of the electric potential and the hole density in the GB (plasma oscillations), as well as the quantum capacitance and the electron transit-time effects. Using the proposed device model, we calculate the responsivity of GB-HETs operating as THz detectors as a function of the signal frequency, applied bias voltages, and the structural parameters. The inclusion of the plasmonic effect leads to the possibility of the GB-HET operation at the frequencies significantly exceeding those limited by the characteristic RC-time. It is found that the responsivity of GB-HETs with a sufficiently perfect GB exhibits sharp resonant maxima in the THz range of frequencies associated with the excitation of plasma oscillations. The positions of these maxima are controlled by the applied bias voltages. The GB-HETs can compete with and even surpass other plasmonic THz detectors.

Response of plasmonic terahertz detectors to amplitude modulated signals

We present theoretical study of the response of two-dimensional gated electron gas to an amplitude modulated signals with carrier frequency in the terahertz range. The model is based on complete hydrodynamic equations, and includes effects of viscosity, pressure gradients and thermal transport in the conduction channel of a high electron mobility semiconductor transistor (HEMT). The modulation response was evaluated as a function of modulation frequency fM for a wide range of mobility values. Maximum modulation frequency fMAX was evaluated as a function of channel mobility, with typical values of fMAX in the subterahertz range of frequencies. Our analysis shows that short channel field effect transistors operating in the plasmonic regime can meet all the requirements for applications as terahertz detectors and modulators in ultra high-speed wireless communication circuits.

(Invited) Plasmonic Terahertz Detectors

We review the achieved and potential performance of plasmonic terahertz detectors implemented in III-N, III-V, Si, and graphene materials systems and operating in the overdamped and resonant regimes. Such detectors have been using single transistors and transistor arrays. They operated in a wide temperature range – from cryogenic to elevated temperatures – and in the frequency range from 0.2 THz to 5 THz with Noise Equivalent Powers as low as 1 pW/Hz1/2. The modulation frequency over 10 GHz has been achieved. The predicted modulation frequencies reach into the THz range. Each materials system (III-N, III-V, and Si) has its optimum frequency range of operation. High mobility InGaAs plasmonic HEMT detectors are expected to operate efficiently up to 10 THz and higher. For the III-N plasmonic transistors, the operation range could reach or exceed 1 or 2 THz. Si CMOS could work as efficient plasmonic THz detectors up to 1 THz or higher. The optimum channel length for each frequency range depends on the materials system. The device responsivity peaks near or slightly below the threshold voltage and is greatly enhanced by driving drain or gate currents due to the increased non-linearity (in the case of the drain current) and due to the induced phase shift (in the case of the gate current). The detection of single THz pules with plasmonic detectors has also been achieved. These THz detectors are expected to find numerous applications in sensing, imaging, wireless communications, and security systems.

(Invited) Plasmonic Terahertz Detectors

We review the achieved and potential performance of plasmonic terahertz detectors implemented in III-N, III-V, and Si materials systems and operating in the overdamped and resonant regimes. Such detectors have been using single transistors and transistor arrays. They operated in a wide temperature range – from cryogenic to elevated temperatures – and in the frequency range from 0.2 THz to 5 THz with Noise Equivalent Powers as low as 1 pW/Hz1/2. The predicted modulation frequencies reach into the THz range. Each materials system (III-N, III-V, and Si) has its optimum frequency range of operation. High mobility InGaAs plasmonic HEMT detectors are expected to operate efficiently up to 10 THz and higher. For the III-N plasmonic transistors, the operation range could reach or exceed 1 or 2 THz. Si CMOS could work as efficient plasmonic THz detectors up to 1 THz or higher. The optimum channel length for each frequency range depends on the materials system. The detection of single THz pules with plasmonic detectors has also been achieved. These THz detectors are expected to find numerous applications in sensing, imaging, wireless communications, and security systems.

Performance Enhancement of Plasmonic Sub-Terahertz Detector Based on Antenna Integrated Low-Impedance Silicon MOSFET

We demonstrate the performance enhancement of field-effect transistor (FET)-based plasmonic terahertz (THz) detector with monolithic integrated antenna in low-impedance regime and report the experimental results of Si MOSFET impedance in THz regime using 0.2-THz measurement system. By designing FET with low-impedance ranges (<;1 kΩ) and integrating antennas with impedances of 50 and 100 Ω, we found that our low-impedance MOSFETs have the input impedance criterion of 50 Ω at 0.2 THz and the MOSFETs with thinner gate oxide show the highly enhanced plasmonic photoresponses at 50-Ω antenna by 325 times from the result of the detector without antenna.

Plasmonic terahertz detectors based on a high-electron mobility GaAs/AlGaAs heterostructure

In order to characterize magnetic field (B) tunable THz plasmonic detectors, spectroscopy experiments were carried out at liquid helium temperatures and high magnetic fields on devices fabricated on a high electron mobility GaAs/AlGaAs heterostructure. The samples were either gated (the gate of a meander shape) or ungated. Spectra of a photovoltage generated by THz radiation were obtained as a function of B at a fixed THz excitation from a THz laser or as a function of THz photon frequency at a fixed B with a Fourier spectrometer. In the first type of measurements, the wave vector of magnetoplasmons excited was defined by geometrical features of samples. It was also found that the magnetoplasmon spectrum depended on the gate geometry which gives an additional parameter to control plasma excitations in THz detectors. Fourier spectra showed a strong dependence of the magnetoplasmon resonance amplitude on the conduction-band electron filling factor which was explained within a model of the electron gas heating with THz radiation. The study allows to define both the advantages and limitations of plasmonic devices based on high-mobility GaAs/AlGaAs heterostructures for THz detection at low temperatures and high magnetic fields.

Theory of plasmonic terahertz detection by a dual-grating-gate field-effect transistor

Plasmonic terahertz detection in a dual-grating-gate field-effect transistor structure with an asymmetric unit cell is studied theoretically and modeled numerically. It is shown that DC bias current flowing in the structure channel strongly affects the detector responsivity.

Theory and measurement of plasmonic terahertz detector response to large signals

Electron gas in the conduction channel of a Field Effect Transistor (FET) can support collective plasma oscillations tunable by the gate voltage. In the Dyakonov-Shur terahertz (THz) detector, nonlinearities in the plasma wave propagation in the gated channel of a FET lead to a constant source-to-drain voltage providing the detector output. We present the detector theory in the frame of the hydrodynamic model using the electron plasma Navier-Stokes and thermal transport equations, thus fully accounting for the hydrodynamic non-linearity, the viscosity, and pressure gradients in the detector response. Both resonant and broadband operations of the high electron mobility transistor (HEMT) based plasmonic detectors are described by this model. The relation between the electron channel density and gate voltage was modeled by the unified charge control model applicable both above and below the threshold voltage. The theoretical results are compared with the response measured in the short channel InGaAs HEMT and the analytical approximation. The THz source was operating at 1.63 THz, and the response was measured at varying signal intensities. The response of the detector operated in the open drain mode was measured above and below the threshold, and the theoretical and experimental results are shown to be in good agreement.

Enhanced Photoresponse of Plasmonic Terahertz Wave Detector Based on Silicon Field Effect Transistors with Asymmetric Source and Drain Structures

We investigate the enhanced effects of asymmetry ratio variations of the source and drain area in silicon (Si) field-effect transistor (FET). Photoresponse according to the variation of asymmetry difference between the width of source and drain are obtained by using the plasmonic terahertz (THz) wave detector simulation based on technology computer-aided design (TCAD) with the quasi-plasma 2DEG model. The simulation results demonstrate the potential of Si FETs with asymmetric source and drain structures as the promising plasmonic THz detectors.