Shallow water analogy for a ballistic field effect transistor: New mechanism of plasma wave generation by dc current

Two-dimensional topological semimetals: an emerging candidate for terahertz detectors and on-chip integration

The importance of terahertz (THz) detection lies in its ability to provide detailed information in a non-destructive manner, making it a valuable tool across various domains including spectroscopy, communication, and security. The ongoing development of THz detectors aims to enhance their sensitivity, resolution and integration into compact and portable devices such as handheld scanners or integrated communication chips. Generally, two-dimensional (2D) materials are considered potential candidates for device miniaturization but detecting THz radiation using 2D semiconductors is generally difficult due to the ultra-small photon energy. However, this challenge is being addressed by the advent of topological semimetals (TSM) with zero-bandgap characteristics. These semimetals offer low-energy excitations in proximity to the Dirac point, which is particularly important for applications requiring a broad detection range. Their distinctive band structures with linear energy-momentum dispersion near the Fermi level also lead to high electron mobility and low effective mass. The presence of topologically protected dissipationless conducting channels and self-powered response provides a basis for low-energy integration. In order to establish paradigms for semimetal-based THz detectors, this review initially offers an analytical summary of THz detection principles. Then, the review demonstrates the distinct design of devices, the excellent performance derived from the topological surface state and unique band structures in TSM. Finally, we outline the prospective avenues for on-chip integration of TSM-based THz detectors. We believe this review can promote further research on the new generation of THz detectors and facilitate advancements in THz imaging, spectroscopy, and communication systems.

Terahertz-frequency plasmonic-crystal instability in field-effect transistors with asymmetric gate arrays

We present a theory for plasmonic crystal instability in a semiconductor field-effect transistor with a dual grating gate array, designed with strong asymmetry in the elementary cell of this "crystal". We demonstrate that, under the action of a dc current bias, the Bloch plasma waves in the plasmonic crystal formed in this transistor develop the Dyakonov-Shur instability. By calculating the energy spectrum and instability increments/decrements-which govern the growth/decay of excitations within the plasmonic crystal-we analyze the dependence of the latter on the electron drift velocity and the extent of the structural asymmetry. In contrast with the corresponding problem for gate arrays with symmetric unit cells, the presence of finite plasma instability increments across the entire Brillouin zone is established. This important difference points to the possibility of exciting sustained, radiating, non-linear electron plasma oscillations in the instability endpoint of the asymmetric array. These structures should be readily implementable in common semiconductor heterostructures, using standard nanofabrication techniques, enabling operation at room temperature. Long-range coherence of the unstable plasma oscillations, generated in the elementary cells of the crystal, should dramatically increase the radiated THz electromagnetic power, making this approach a promising pathway to the generation of THz signals.

Diamond for High-Power, High-Frequency, and Terahertz Plasma Wave Electronics

High thermal conductivity and a high breakdown field make diamond a promising candidate for high-power and high-temperature semiconductor devices. Diamond also has a higher radiation hardness than silicon. Recent studies show that diamond has exceptionally large electron and hole momentum relaxation times, facilitating compact THz and sub-THz plasmonic sources and detectors working at room temperature and elevated temperatures. The plasmonic resonance quality factor in diamond TeraFETs could be larger than unity for the 240-600 GHz atmospheric window, which could make them viable for 6G communications applications. This paper reviews the potential and challenges of diamond technology, showing that diamond might augment silicon for high-power and high-frequency compact devices with special advantages for extreme environments and high-frequency applications.

Mapping the slow and fast photoresponse of field-effect transistors to terahertz and infrared radiation

Field-effect transistors are capable of detecting electromagnetic radiation from less than 100 GHz up to very high frequencies reaching well into the infrared spectral range. Here, we report on frequency coverage of up to 30THz, thus reaching the technologically important frequency regime of CO2 lasers, using GaAs/AlGaAs high-electron-mobility transistors. A detailed study of the speed and polarization dependence of the responsivity allows us to identify a cross over of the dominant detection mechanism from ultrafast non-quasistatic rectification at low Terahertz frequencies to slow rectification based on a combination of the Seebeck and bolometric effects at high frequencies, occurring at about the boundary between the Terahertz frequency range and the infrared at 10THz.

Feedback enhanced Dyakonov-Shur instability in graphene field-effect transistors

Graphene devices are known to have the potential to operate THz signals. In particular, graphene field-effect transistors (gFETs) have been proposed as devices to host plasmonic instabilities in the THz realm; for instance, Dyakonov-Shur (DS) instability which relies upon dc excitation. In this work, starting from a hydrodynamical description of the charge carriers, we extend the transmission line description of gFETs to a scheme with a positive feedback loop, also considering the effects of delay, which leads to the transcendental Laplace-transform transfer function, with complex frequencys, with terms of the forme-assechk(s)/s, for a givena∈R0+arising from the delay time and withk∈N. Applying the conditions for the excitation of DS instability, we report an enhanced voltage gain in the linear regime that is corroborated by our simulations of the nonlinear hydrodynamic model for the charge carriers. This translates to both greater saturation amplitude-often up to 50% increase-and faster growth rate of the self-oscillations. Thus, we bring forth a prospective concept for the realization of a THz oscillator suitable for future plasmonic circuitry.

On-Chip Integration of a Plasmonic FET Source and a Nano-Patch Antenna for Efficient Terahertz Wave Radiation

Graphene-based Field-Effect Transistors (FETs) integrated with microstrip patch antennas offer a promising approach for terahertz signal radiation. In this study, a dual-stage simulation methodology is employed to comprehensively investigate the device's performance. The initial stage, executed in MATLAB, delves into charge transport dynamics within a FET under asymmetric boundary conditions, employing hydrodynamic equations for electron transport in the graphene channel. Electromagnetic field interactions are modeled via Finite-Difference Time-Domain (FDTD) techniques. The second stage, conducted in COMSOL Multiphysics, focuses on the microstrip patch antenna's radiative characteristics. Notably, analysis of the S11 curve reveals minimal reflections at the FET's resonant frequency of 1.34672 THz, indicating efficient impedance matching. Examination of the radiation pattern demonstrates the antenna's favorable directional properties. This research underscores the potential of graphene-based FETs for terahertz applications, offering tunable impedance matching and high radiation efficiency for future terahertz devices.

Topological quantum devices: a review

The introduction of the concept of topology into condensed matter physics has greatly deepened our fundamental understanding of transport properties of electrons as well as all other forms of quasi particles in solid materials. It has also fostered a paradigm shift from conventional electronic/optoelectronic devices to novel quantum devices based on topology-enabled quantum device functionalities that transfer energy and information with unprecedented precision, robustness, and efficiency. In this article, the recent research progress in topological quantum devices is reviewed. We first outline the topological spintronic devices underlined by the spin-momentum locking property of topology. We then highlight the topological electronic devices based on quantized electron and dissipationless spin conductivity protected by topology. Finally, we discuss quantum optoelectronic devices with topology-redefined photoexcitation and emission. The field of topological quantum devices is only in its infancy, we envision many significant advances in the near future.

The Microscopic Mechanisms of Nonlinear Rectification on Si-MOSFETs Terahertz Detector

Studying the nonlinear photoresponse of different materials, including III-V semiconductors, two-dimensional materials and many others, is attracting burgeoning interest in the terahertz (THz) field. Especially, developing field-effect transistor (FET)-based THz detectors with preferred nonlinear plasma-wave mechanisms in terms of high sensitivity, compactness and low cost is a high priority for advancing performance imaging or communication systems in daily life. However, as THz detectors continue to shrink in size, the impact of the hot-electron effect on device performance is impossible to ignore, and the physical process of THz conversion remains elusive. To reveal the underlying microscopic mechanisms, we have implemented drift-diffusion/hydrodynamic models via a self-consistent finite-element solution to understand the dynamics of carriers at the channel and the device structure dependence. By considering the hot-electron effect and doping dependence in our model, the competitive behavior between the nonlinear rectification and hot electron-induced photothermoelectric effect is clearly presented, and it is found that the optimized source doping concentrations can be utilized to reduce the hot-electron effect on the devices. Our results not only provide guidance for further device optimization but can also be extended to other novel electronic systems for studying THz nonlinear rectification.

Two-Dimensional Plasmons in Laterally Confined 2D Electron Systems

The collective oscillations of charge density (plasmons) in conductive solids are basic excitations that determine the dynamic response of the system. In infinite two-dimensional (2D) electron systems, plasmons have gapless dispersion covering a broad spectral range from subterahertz to infrared, which is promising in light-matter applications. We discuss the state-of-the-art physics of 2D plasmons, especially in confined 2D electron systems in stripe and disk geometry, using the simplest approach for conductivity. When the metal gate is placed in the vicinity of the 2D electron system, an analytical description of the plasmon frequency and damping can be easily obtained. We also analyze gated plasmons in the disk when it was situated at various distances from the gate, and discuss in detail the nontrivial behavior of the damping. We predict that it is not a simple sum of the radiative and collisional dampings, but has a nonmonotonic dependence on the system parameters. For high-mobility 2D systems, this opens the way to achieve the maximal quality factor of plasma resonances. Lastly, we discuss the recently discovered near-gate 2D plasmons propagating along the laterally confined gate, even without applied bias voltage and having gapless dispersion when the gate has the form of a stripe, and discrete spectrum when the gate is in the form of disk. It allows for one to drive the frequency and spatial propagation of such plasmons.

Numerical Investigation of GaN HEMT Terahertz Detection Model Considering Multiple Scattering Mechanisms

GaN high-electron-mobility transistor (HEMT) terahertz (THz) detectors have been widely studied and applied in the past few decades. However, there are few reports about the influence of GaN/AlGaN heterostructure material properties on the detection model at present. In this paper, a response voltage model for a GaN HEMT THz detector that considers the carrier scattering in a GaN/AlGaN heterostructure is proposed. The phonon scattering, dislocation scattering, and interface roughness scattering mechanisms are taken into account in the classic THz response voltage model; furthermore, the influence of various material parameters on the response voltage is studied. In a low-temperature region, acoustic scattering plays an important role, and the response voltage drops with an increase in temperature. In a high temperature range, optical phonon scattering is the main scattering mechanism, and the detector operates in a non-resonant detection mode. With an increase in carrier surface density, the response voltage decreases and then increases due to piezoelectric scattering and optical phonon scattering. For dislocation and interface roughness scattering, the response voltage is inversely proportional to the dislocation density and root mean square roughness (RMS) but is positively related to lateral correlation length. Finally, a comparison between our model and the reported models shows that our proposed model is more accurate.

How Electron Hydrodynamics Can Eliminate the Landauer-Sharvin Resistance

It has long been realized that even a perfectly clean electronic system harbors a Landauer-Sharvin resistance, inversely proportional to the number of its conduction channels. This resistance is usually associated with voltage drops on the system's contacts to an external circuit. Recent theories have shown that hydrodynamic effects can reduce this resistance, raising the question of the lower bound of resistance of hydrodynamic electrons. Here, we show that by a proper choice of device geometry, it is possible to spread the Landauer-Sharvin resistance throughout the bulk of the system, allowing its complete elimination by electron hydrodynamics. We trace the effect to the dynamics of electrons flowing in channels that terminate within the sample. For ballistic systems this termination leads to back-reflection of the electrons and creates resistance. Hydrodynamically, the scattering of these electrons off other electrons allows them to transfer to transmitted channels and avoid the resistance. Counterintuitively, we find that in contrast to the ohmic regime, for hydrodynamic electrons the resistance of a device with a given width can decrease with its length, suggesting that a long enough device may have an arbitrarily small total resistance.

Nonscattering Photodetection in the Propagation of Unidirectional Surface Plasmon Polaritons Embedded with Graphene

Recently, nanoscale light manipulation using surface plasmon polaritons (SPPs) has received considerable research attention. The conventional method of detecting SPPs is through light scattering or using bulky Si or Ge photodetectors. However, these bulky systems limit the application of nanophotonic circuits. In this study, the light-matter interaction between graphene and SPP was investigated. For realizing an improved integration in nanocircuits, single-layer graphene was added to asymmetric SPP nanoantenna arrays for nonscattering detection in the near field. The developed device is capable of detecting the controlled propagation of SPPs with a photoresponsivity of 15 mA/W, which paves the way for the new-generation on-chip optical communication.

Plasmon absorption reduction in multiple quantum well structures

The damping of two-dimensional plasmons in structures with several quantum wells due to absorption by free carriers is studied theoretically. Both gate structures and structures without a gate are considered. It is shown by the example of structures with GaAs quantum wells that an increase in the number of quantum wells while maintaining the electron concentration in each of them leads to a decrease in the damping coefficient of two-dimensional plasmons. The physical reasons for the decrease in the absorption of plasmons are discussed. It is shown that an increase in the number of quantum wells should lead to a decrease in the decay of plasmons in systems with a finite gate width as well.

Dissipation-enabled hydrodynamic conductivity in a tunable bandgap semiconductor

Electronic transport in the regime where carrier-carrier collisions are the dominant scattering mechanism has taken on new relevance with the advent of ultraclean two-dimensional materials. Here, we present a combined theoretical and experimental study of ambipolar hydrodynamic transport in bilayer graphene demonstrating that the conductivity is given by the sum of two Drude-like terms that describe relative motion between electrons and holes, and the collective motion of the electron-hole plasma. As predicted, the measured conductivity of gapless, charge-neutral bilayer graphene is sample- and temperature-independent over a wide range. Away from neutrality, the electron-hole conductivity collapses to a single curve, and a set of just four fitting parameters provides quantitative agreement between theory and experiment at all densities, temperatures, and gaps measured. This work validates recent theories for dissipation-enabled hydrodynamic conductivity and creates a link between semiconductor physics and the emerging field of viscous electronics.

Asymmetrically Engineered Nanoscale Transistors for On-Demand Sourcing of Terahertz Plasmons

Terahertz (THz) plasma oscillations represent a potential path to implement ultrafast electronic devices and circuits. Here, we present an approach to generate on-chip THz signals that relies on plasma-wave stabilization in nanoscale transistors with specific structural asymmetry. A hydrodynamic treatment shows how the transistor asymmetry supports plasma-wave amplification, giving rise to pronounced negative differential conductance (NDC). A demonstration of these behaviors is provided in InGaAs high-mobility transistors, which exhibit NDC in accordance with their designed asymmetry. The NDC onsets once the drift velocity in the channel reaches a threshold value, triggering the initial plasma instability. We also show how this feature can be made to persist beyond room temperature (to at least 75 °C), when the gating is configured to facilitate a transition between the hydrodynamic and ballistic regimes (of electron-electron transport). Our findings represent a significant step forward for efforts to develop active components for THz electronics.

Giant inverse Faraday effect in a plasmonic crystal ring

Circularly polarized electromagnetic wave impinging on a conducting ring with a two-dimensional electron channel generates a circulating DC plasmonic current resulting in an inverse Faraday effect in nanorings. We show that a large ring with periodically modulated width on a nanoscale, smaller or comparable with the plasmonic mean free path, supports plasmon energy bands. When circularly polarized radiation impinges on such a plasmonic ring, it produces resonant DC plasmonic current on a macro scale resulting in a giant inverse Faraday effect. The systems comprised of the concentric variable-width rings ("plasmonic disks") and stacked plasmonic disks ("plasmonic solenoids") amplify the generated constant magnetic field by orders of magnitude.

Quantum emitters and detectors based on 2D van der Waals materials

Light plays an essential role in our world, with several technologies relying on it. Photons will also play an important role in the emerging quantum technologies, which are primed to have a transformative effect on our society. The development of single-photon sources and ultra-sensitive photon detectors is crucial. Solid-state emitters are being heavily pursued for developing truly single-photon sources for scalable technology. On the detectors' side, the main challenge lies in inventing sensitive detectors operating at sub-optical frequencies. This review highlights the promising research being conducted for the development of quantum emitters and detectors based on two-dimensional van der Waals (2D-vdW) materials. Several 2D-vdW materials, from canonical graphene to transition metal dichalcogenides and their heterostructures, have generated a lot of excitement due to their tunable emission and detection properties. The recent developments in the creation, fabrication and control of quantum emitters hosted by 2D-vdW materials and their potential applications in integrated photonic devices are discussed. Furthermore, the progress in enhancing the photon-counting potential of 2D material-based detectors, viz. 2D photodetectors, bolometers and superconducting single-photon detectors functioning at various wavelengths is also reported.

Diffusion of Photoexcited Holes in a Viscous Electron Fluid

The diffusion of photogenerated holes is studied in a high-mobility mesoscopic GaAs channel where electrons exhibit hydrodynamic properties. It is shown that the injection of holes into such an electron system leads to the formation of a hydrodynamic three-component mixture consisting of electrons and photogenerated heavy and light holes. The obtained results are analyzed within the framework of ambipolar diffusion, which reveals characteristics of a viscous flow. Both hole types exhibit similar hydrodynamic characteristics. In such a way the diffusion lengths, ambipolar diffusion coefficient, and the effective viscosity of the electron-hole system are determined.

High Performance of Room-Temperature NbSe2 Terahertz Photoelectric Detector

Photoelectric detection is developing rapidly from ultraviolet to infrared band. However, terahertz (THz) photodetection approaches is constrained by the bandgap, dark current, and absorption ability. In this work, room-temperature photoelectric detection is extended to the THz range implemented in a planar metal-NbSe₂-metal structure based on an electromagnetic induced well (EIW) theory, exhibiting an excellent broadband responsivity of 5.2 × 10⁷ V W^-1 at 0.027 THz, 7.8 × 10⁶ V W^-1 at 0.173 THz, and 9.6 × 10⁵ V W^-1 at 0.259 THz. Simultaneously, the NbSe₂ photoelectric detector (PD) with ultrafast response speed (∼610 ns) and ultralow equivalent noise power (4.6 × 10^-14 W Hz^-1/2) in the THz region is realized, enabling high-resolution imaging. The figure of merit (FOM) characterizing the detection performance of the device is 2 orders of magnitude superior to that of the reported THz PDs based 2D materials. Furthermore, the THz response speed is 2 orders of magnitude faster than that of the visible due to the different response mechanisms of the device. Our results exhibit promising potential to achieve highly sensitive and ultrafast photoelectric detection.

Multi-Segment TFT Compact Model for THz Applications

We present an update of the Rensselaer Polytechnic Institute (RPI) thin-film transistor (TFT) compact model. The updated model implemented in Simulation Program with Integrated Circuit Emphasis (SPICE) accounts for the gate voltage-dependent channel layer thickness, enables the accurate description of the direct current (DC) characteristics, and uses channel segmentation to allow for terahertz (THz) frequency simulations. The model introduces two subthreshold ideality factors to describe the control of the gate voltage on the channel layer and its effect on the drain-to-source current and the channel capacitance. The calculated field distribution in the channel is used to evaluate the channel segment parameters including the segment impedance, kinetic inductance, and gate-to-segment capacitances. Our approach reproduces the conventional RPI TFT model at low frequencies, fits the measured current–voltage characteristics with sufficient accuracy, and extends the RPI TFT model applications into the THz frequency range. Our calculations show that a single TFT or complementary TFTs could efficiently detect the sub-terahertz and terahertz radiation.

Concurrent-Mode CMOS Detector IC for Sub-Terahertz Imaging System

A CMOS detector with a concurrent mode for high-quality images in the sub-terahertz region has been proposed. The detector improves output-signal coupling characteristics at the output node. A cross-coupling capacitor is added to isolate the DC bias between the drain and gate. The detector is designed to combine a 180° phase shift based on common source operation and an in-phase output signal based on the drain input. The circuit layout and phase shift occurring in the cross-coupled capacitor during phase coupling are verified using an EM simulation. The detector is fabricated using the TSMC 0.25-μm mixed-signal 1-poly 5-metal layer CMOS process, where the size, including the pad, is 1.13 mm × 0.74 mm. The detector IC comprises a folded dipole antenna, the proposed detector, a preamplifier, and a voltage buffer. Measurement results using a 200-GHz gyrotron source demonstrate that the proposed detector voltage responsivity is 14.13 MV/W with a noise-equivalent power of 34.42 pW/√Hz. The high detection performance helps resolve the 2-mm line width. The proposed detector exhibits a signal-to-noise ratio of 49 dB with regard to the THz imaging performance, which is 9 dB higher than that of the previous CMOS detector core circuits with gate-drain capacitors.

Terahertz Reconfigurable Intelligent Surfaces (RISs) for 6G Communication Links

The forthcoming sixth generation (6G) communication network is envisioned to provide ultra-fast data transmission and ubiquitous wireless connectivity. The terahertz (THz) spectrum, with higher frequency and wider bandwidth, offers great potential for 6G wireless technologies. However, the THz links suffers from high loss and line-of-sight connectivity. To overcome these challenges, a cost-effective method to dynamically optimize the transmission path using reconfigurable intelligent surfaces (RISs) is widely proposed. RIS is constructed by embedding active elements into passive metasurfaces, which is an artificially designed periodic structure. However, the active elements (e.g., PIN diodes) used for 5G RIS are impractical for 6G RIS due to the cutoff frequency limitation and higher loss at THz frequencies. As such, various tuning elements have been explored to fill this THz gap between radio waves and infrared light. The focus of this review is on THz RISs with the potential to assist 6G communication functionalities including pixel-level amplitude modulation and dynamic beam manipulation. By reviewing a wide range of tuning mechanisms, including electronic approaches (complementary metal-oxide-semiconductor (CMOS) transistors, Schottky diodes, high electron mobility transistors (HEMTs), and graphene), optical approaches (photoactive semiconductor materials), phase-change materials (vanadium dioxide, chalcogenides, and liquid crystals), as well as microelectromechanical systems (MEMS), this review summarizes recent developments in THz RISs in support of 6G communication links and discusses future research directions in this field.

Electrostatic tapering for efficient generation of radiation

We demonstrate the feasibility of electrostatic (dc) tapering such that the net average force of the electromagnetic (ac) field is compensated by a dc field, which at resonance may be interpreted as "direct" energy transfer from the dc to the ac field. This combination persists in all three components of the setup-e-gun, resonant zone, and collector, in each one playing a different role. In equilibrium, the two field components field-emit at the cathode a density modulated cylindrical beam which is accelerated along the e-gun by the dc field; the latter also focuses the e-beam. Radiation confinement perpendicular to the e-beam is ensured by an array of dielectric Bragg-mirrors and an array of metallic hollow electrodes impose synchronous bunches and wave. This double periodic structure ensures the coexistence of dc and ac fields in the same volume. Numerical simulations demonstrate the feasibility of generation of order of 1[W] power at 1 THz from a volume on a scale of few mm^{3} with efficiency of the order of 25%.

Plasmonic Field-Effect Transistors (TeraFETs) for 6G Communications

Ever increasing demands of data traffic makes the transition to 6G communications in the 300 GHz band inevitable. Short-channel field-effect transistors (FETs) have demonstrated excellent potential for detection and generation of terahertz (THz) and sub-THz radiation. Such transistors (often referred to as TeraFETs) include short-channel silicon complementary metal oxide (CMOS). The ballistic and quasi-ballistic electron transport in the TeraFET channels determine the TeraFET response at the sub-THz and THz frequencies. TeraFET arrays could form plasmonic crystals with nanoscale unit cells smaller or comparable to the electron mean free path but with the overall dimensions comparable with the radiation wavelength. Such plasmonic crystals have a potential of supporting the transition to 6G communications. The oscillations of the electron density (plasma waves) in the FET channels determine the phase relations between the unit cells of a FET plasmonic crystal. Excited by the impinging radiation and rectified by the device nonlinearities, the plasma waves could detect both the radiation intensity and the phase enabling the line-of-sight terahertz (THz) detection, spectrometry, amplification, and generation for 6G communication.

Carbon nanotubes for polarization sensitive terahertz plasmonic interferometry

We report on helicity sensitive photovoltaic terahertz radiation response of a carbon nanotube made in a configuration of a field-effect transistor. We find that the magnitude of the rectified voltage is different for clockwise and anticlockwise circularly polarized radiation. We demonstrate that this effect is a fingerprint of the plasma waves interference in the transistor channel. We also find that the presence of the helicity- and phase-sensitive interference part of the photovoltaic response is a universal phenomenon which is obtained in the systems of different dimensionality with different single-particle spectrum. Its magnitude is a characteristic of the plasma wave decay length. This opens up a wide avenue for applications in the area of plasmonic interferometry.

Entropy Wave Instability in Dirac and Weyl Semimetals

Hydrodynamic instabilities driven by a direct current are analyzed in 2D and 3D relativisticlike systems with the Dyakonov-Shur boundary conditions supplemented by a boundary condition for temperature. Besides the conventional Dyakonov-Shur instability for plasmons, we find an entropy wave instability in both 2D and 3D systems. The entropy wave instability is a manifestation of the relativisticlike nature of electron quasiparticles and a nontrivial role of the energy current in such systems. These two instabilities occur for the opposite directions of fluid flow. While the Dyakonov-Shur instability is characterized by the plasma frequency in 3D and the system size in 2D, the frequency of the entropy wave instability is tunable by the system size and the flow velocity.

A Terahertz Detector Based on Double-Channel GaN/AlGaN High Electronic Mobility Transistor

A double-channel (DC) GaN/AlGaN high-electron-mobility transistor (HEMT) as a terahertz (THz) detector at 315 GHz frequency is proposed and fabricated in this paper. The structure of the epitaxial layer material in the detector is optimized, and the performance of the GaN HEMT THz detector is improved. The maximum responsivity of 10 kV/W and minimum noise equivalent power (NEP) of 15.5 pW/Hz^0.5 are obtained at the radiation frequency of 315 GHz. The results are comparable to and even more promising than the reported single-channel (SC) GaN HEMT detectors. The enhancement of THz response and the reduction of NEP of the DC GaN HEMT detector mainly results from the interaction of 2DEG in the upper and lower channels, which improves the self-mixing effect of the detector. The promising experimental results mean that the proposed DC GaN/AlGaN HEMT THz detector is capable of the practical applications of THz detection.

Recent Progress in the Development of Graphene Detector for Terahertz Detection

Terahertz waves are expected to be used in next-generation communications, detection, and other fields due to their unique characteristics. As a basic part of the terahertz application system, the terahertz detector plays a key role in terahertz technology. Due to the two-dimensional structure, graphene has unique characteristics features, such as exceptionally high electron mobility, zero band-gap, and frequency-independent spectral absorption, particularly in the terahertz region, making it a suitable material for terahertz detectors. In this review, the recent progress of graphene terahertz detectors related to photovoltaic effect (PV), photothermoelectric effect (PTE), bolometric effect, and plasma wave resonance are introduced and discussed.

Optimized response of the AlGaN/GaN heterostructure with asymmetric gratings at oblique incidence in the terahertz regime

The performance of terahertz (THz) detectors based on two-dimensional electron gas (2DEG) with the aid of a subwavelength gratings coupler depends critically on the amplitude difference of an electric field with positive and negative scattering orders. An efficient method was proposed to enhance the current response of the AlGaN/GaN material in the frequency range from 0 to 5 THz under oblique incidence of THz radiation with asymmetric gratings, where the optimal incidence angle exists. The case of symmetric gratings was also studied for comparison. The results will be useful for the theoretical and experimental optimization of grating-assisted THz detectors without electrical bias.

Fizeau drag in graphene plasmonics

Dragging of light by moving media was predicted by Fresnel¹ and verified by Fizeau's celebrated experiments² with flowing water. This momentous discovery is among the experimental cornerstones of Einstein's special relativity theory and is well understood^3,4 in the context of relativistic kinematics. By contrast, experiments on dragging photons by an electron flow in solids are riddled with inconsistencies and have so far eluded agreement with the theory^5-7. Here we report on the electron flow dragging surface plasmon polaritons^8,9 (SPPs): hybrid quasiparticles of infrared photons and electrons in graphene. The drag is visualized directly through infrared nano-imaging of propagating plasmonic waves in the presence of a high-density current. The polaritons in graphene shorten their wavelength when propagating against the drifting carriers. Unlike the Fizeau effect for light, the SPP drag by electrical currents defies explanation by simple kinematics and is linked to the nonlinear electrodynamics of Dirac electrons in graphene. The observed plasmonic Fizeau drag enables breaking of time-reversal symmetry and reciprocity¹⁰ at infrared frequencies without resorting to magnetic fields^11,12 or chiral optical pumping^13,14. The Fizeau drag also provides a tool with which to study interactions and nonequilibrium effects in electron liquids.

Rapid prototyping of a bispectral terahertz-to-infrared converter

Conversion of terahertz radiation into thermal radiation is a promising approach for the development of low cost terahertz instruments. Here, we experimentally demonstrate bispectral terahertz-to-infrared conversion using metamaterials fabricated using a rapid prototyping technique. The converter unit cell is composed of two metal-insulator-metal (MIM) antennas absorbing independently the terahertz radiation at 96 and 130 GHz and a thin carbon nanotubes (CNT) layer used as a thermal emitter. The converter unit cell has a typical λ/100 thickness and sub-wavelength lateral dimensions. The terahertz absorption of the converter was observed by monitoring its thermal emission using an infrared camera. Within the first hundred milliseconds of the terahertz pulse, thermal radiation from the CNTs only increases at the location of the MIM antennas, thus allowing to record the terahertz response of each MIM antenna independently. Beyond 100 ms, thermal diffusion causes significant cross-talk between the pixels, so the spectral information is more difficult to extract. In a steady state regime, the minimum terahertz power that can be detected is 5.8 µW at 130 GHz. We conclude that the converter provides a suitable low-cost solution for fast multi-spectral terahertz imaging with resolution near the diffraction limit, using an infrared camera in combination with a tunable source.

Novel Broadband Slot-Spiral Antenna for Terahertz Applications

We report a novel broadband slot-spiral antenna that can be integrated with high-electron-mobility transistor (HEMT) terahertz (THz) detectors. The effect of various antenna parameters on the transmission efficiency of the slot-spiral structure at 150–450 GHz is investigated systematically. The performances of the slot-spiral antenna and the spiral antenna both integrated with HEMTs are compared. The results show that the slot-spiral structure has a better transmission and miniaturization capability than the spiral structure. A formula for the responsivity is derived based on the transmission line principle and antenna theory, and results show that the detector responsivity is correlated with the antenna absorptivity. Additionally, guidelines for HEMT THz detector design are proposed. The results of this study indicate the excellent application prospects of the slot-spiral antenna in THz detection and imaging.

Photodetectors of 2D Materials from Ultraviolet to Terahertz Waves

2D materials are considered to be the most promising materials for photodetectors due to their unique optical and electrical properties. Since the discovery of graphene, many photodetectors based on 2D materials have been reported. However, the low quantum efficiency, large noise, and slow response caused by the thinness of 2D materials limit their application in photodetectors. Here, recent progress on 2D material photodetectors is reviewed, covering the spectrum from ultraviolet to terahertz waves. First the interaction of 2D materials with light is analyzed in terms of optical physics. Then the present methods to improve the performance of 2D material photodetectors are summarized, such as defect engineering, p-n junctions and hybrid detectors, and the issue of serious overestimation of the performance in reported photodetectors based on 2D materials is discussed. Next, a comparison of 2D material photodetectors with traditional commercially available detectors shows that it is difficult to balance the current 2D material photodetectors with regard to having simultaneously both high sensitivity and fast response. Finally, a possible novel EIW mechanism is suggested to advance the performance of 2D material photodetectors in the future.

Antenna-coupled field-effect transistors as detectors for terahertz near-field microscopy

We report the successful implementation of antenna-coupled terahertz field-effect transistors (TeraFETs) as homodyne detectors in a scattering-type scanning near-field optical microscope (s-SNOM) operating with radiation at 246.5 GHz. The devices were fabricated in Si CMOS foundry technology with two different technologies, a 90 nm process, which provides a better device performance, and a less expensive 180 nm one. The high sensitivity enables s-SNOM demodulation at up to the 10th harmonic of the cantilever's oscillation frequency. While we demonstrate application of TeraFETs at a fixed radiation frequency, this type of detector device is able to cover the entire THz frequency range.

Plasmonic semiconductor nanogroove array enhanced broad spectral band millimetre and terahertz wave detection

High-performance uncooled millimetre and terahertz wave detectors are required as a building block for a wide range of applications. The state-of-the-art technologies, however, are plagued by low sensitivity, narrow spectral bandwidth, and complicated architecture. Here, we report semiconductor surface plasmon enhanced high-performance broadband millimetre and terahertz wave detectors which are based on nanogroove InSb array epitaxially grown on GaAs substrate for room temperature operation. By making a nanogroove array in the grown InSb layer, strong millimetre and terahertz wave surface plasmon polaritons can be generated at the InSb-air interfaces, which results in significant improvement in detecting performance. A noise equivalent power (NEP) of 2.2 × 10^-14 W Hz^-1/2 or a detectivity (D^*) of 2.7 × 10¹² cm Hz^1/2 W^-1 at 1.75 mm (0.171 THz) is achieved at room temperature. By lowering the temperature to the thermoelectric cooling available 200 K, the corresponding NEP and D^* of the nanogroove device can be improved to 3.8 × 10^-15 W Hz^-1/2 and 1.6 × 10¹³ cm Hz^1/2 W^-1, respectively. In addition, such a single device can perform broad spectral band detection from 0.9 mm (0.330 THz) to 9.4 mm (0.032 THz). Fast responses of 3.5 µs and 780 ns are achieved at room temperature and 200 K, respectively. Such high-performance millimetre and terahertz wave photodetectors are useful for wide applications such as high capacity communications, walk-through security, biological diagnosis, spectroscopy, and remote sensing. In addition, the integration of plasmonic semiconductor nanostructures paves a way for realizing high performance and multifunctional long-wavelength optoelectrical devices.

Tuning electrical and thermal conductivities of the two-dimensional electron gas in AlN/GaN heterostructures by piezoelectricity

We investigate the electrical and thermal conductivities of the two-dimensional electron gas (2DEG) confined in the quantum well formed at the heterojunction between a thin GaN layer and an AlN layer strained by an Al _x Ga_1-x N capping layer in the temperature range from 10 to 360 K. The experimental protocol developed to deduce from calorimetric and Hall-effect measurements at a variable temperature the critical characteristics and transport properties of the confined 2DEG is presented. It is found that, in the measured temperature range (10-360 K), the electrical conductivity of the 2DEG is temperature-independent, due to the predominance of scattering processes by interface defects. However, the thermal conductivity shows a linear temperature dependence, mirroring the specific heat of free electrons. The temperature-independent relaxation time associated with the overall electron scattering means that the values obtained for electrical and thermal conductivities are in excellent agreement with those stipulated by the Weidemann-Franz law. It is also found that for weak strain fields in the AlN layer, both the electrical and thermal conductivities of the two-dimensional interfacial electrons increase exponentially with strain. The importance of 2DEG in AlN/GaN quantum wells lies in the fact that the strong piezoelectricity of AlN allows the transport properties of the 2DEG to be tuned or modulated by a weak electric field even with the high density of lattice mismatch induced defects at the AlN-GaN interface .

Modified rigorous coupled-wave analysis for grating-based plasmonic structures with a delta-thin conductive channel: far- and near-field study

The modified rigorous coupled-wave analysis technique is developed to describe the optical characteristics of the plasmonic structures with the grating-gated delta-thin conductive channel in the far- and near-field zones of electromagnetic waves. The technique was applied for analysis of the resonant properties of AlGaN/GaN heterostructures combined with a deeply subwavelength metallic grating, which facilitates the excitation of the two-dimensional plasmons in the terahertz (THz) frequency range. The convergence of the calculations at the frequencies near the plasmon resonances is discussed. The impact of the grating's parameters, including filling factor and thickness of the grating, on resonant absorption of the structure was investigated in detail. The spatial distributions of the electromagnetic field in a near-field zone were used for the evaluation of total absorption of the plasmonic structures separating contributions of the grating-gated two-dimensional electron gas and the grating coupler.

Numerical Study of the Coupling of Sub-Terahertz Radiation to n-Channel Strained-Silicon MODFETs

This paper reports on a study of the response of a T-gate strained-Si MODFETs (modulation-doped field-effect transistor) under continuous-wave sub-THz excitation. The sub-THz response was measured using a two-tones solid-state source at 0.15 and 0.30 THz. The device response in the photovoltaic mode was non-resonant, in agreement with the Dyakonov and Shur model for plasma waves detectors. The maximum of the photoresponse was clearly higher under THz illumination at 0.15 THz than at 0.3 THz. A numerical study was conducted using three-dimensional (3D) electromagnetic simulations to delve into the coupling of THz radiation to the channel of the transistor. 3D simulations solving the Maxwell equations using a time-domain solver were performed. Simulations considering the full transistor structure, but without taking into account the bonding wires used to contact the transistor pads in experiments, showed an irrelevant role of the gate length in the coupling of the radiation to the device channel. Simulations, in contradiction with measurements, pointed to a better response at 0.3 THz than under 0.15 THz excitation in terms of the normalized electric field inside the channel. When including four 0.25 mm long bonding wires connected to the contact pads on the transistor, the normalized internal electric field induced along the transistor channel by the 0.15 THz beam was increased in 25 dB, revealing, therefore, the important role played by the bonding wires at this frequency. As a result, the more intense response of the transistor at 0.15 THz than at 0.3 THz experimentally found, must be attributed to the bonding wires.

A modulation-doped heterostructure-based terahertz photoconductive antenna emitter with recessed metal contacts

We present the implementation of an efficient terahertz (THz) photoconductive antenna (PCA) emitter design that utilizes high mobility carriers in the two-dimensional electron gas (2DEG) of a modulation-doped heterostructure (MDH). The PCA design is fabricated with recessed metal electrodes in direct contact with the 2DEG region of the MDH. We compare the performance of the MDH PCA having recessed contacts with a PCA fabricated on bulk semi-insulating GaAs, on low temperature-grown GaAs, and a MDH PCA with the contacts fabricated on the surface. By recessing the contacts, the applied bias can effectively accelerate the high-mobility carriers within the 2DEG, which increases the THz power emission by at least an order of magnitude compared to those with conventional structures. The dynamic range (62 dB) and bandwidth characteristics (3.2 THz) in the power spectrum are shown to be comparable with the reference samples. Drude-Lorentz simulations corroborate the results that the higher-mobility carriers in the MDH, increase the THz emission. The saturation characteristics were also measured via optical fluence dependence, revealing a lower saturation value compared to the reference samples. The high THz conversion efficiency of the MDH-PCA with recessed contacts at low optical power makes it an attractive candidate for THz-time domain spectroscopy systems powered by low power fiber lasers.

Helicity-Sensitive Plasmonic Terahertz Interferometer

Plasmonic interferometry is a rapidly growing area of research with a huge potential for applications in the terahertz frequency range. In this Letter, we explore a plasmonic interferometer based on graphene field effect transistor connected to specially designed antennas. As a key result, we observe helicity- and phase-sensitive conversion of circularly polarized radiation into dc photovoltage caused by the plasmon-interference mechanism: two plasma waves, excited at the source and drain part of the transistor, interfere inside the channel. The helicity-sensitive phase shift between these waves is achieved by using an asymmetric antenna configuration. The dc signal changes sign with inversion of the helicity. A suggested plasmonic interferometer is capable of measuring the phase difference between two arbitrary phase-shifted optical signals. The observed effect opens a wide avenue for phase-sensitive probing of plasma wave excitations in two-dimensional materials.

Simulation of THz Oscillations in Semiconductor Devices Based on Balance Equations

Instabilities of electron plasma waves in high-mobility semiconductor devices have recently attracted a lot of attention as a possible candidate for closing the THz gap. Conventional moments-based transport models usually neglect time derivatives in the constitutive equations for vectorial quantities, resulting in parabolic systems of partial differential equations (PDE). To describe plasma waves however, such time derivatives need to be included, resulting in hyperbolic rather than parabolic systems of PDEs; thus the fundamental nature of these equation systems is changed completely. Additional nonlinear terms render the existing numerical stabilization methods for semiconductor simulation practically useless. On the other hand there are plenty of numerical methods for hyperbolic systems of PDEs in the form of conservation laws. Standard numerical schemes for conservation laws, however, are often either incapable of correctly handling the large source terms present in semiconductor devices due to built-in electric fields, or rely heavily on variable transformations which are specific to the equation system at hand (e.g. the shallow water equations), and can not be generalized easily to different equations. In this paper we develop a novel well-balanced numerical scheme for hyperbolic systems of PDEs with source terms and apply it to a simple yet non-linear electron transport model.

Plasmonic Nonreciprocity Driven by Band Hybridization in Moiré Materials

We propose a new current-driven mechanism for achieving significant plasmon dispersion nonreciprocity in systems with narrow, strongly hybridized electron bands. The magnitude of the effect is controlled by the strength of electron-electron interactions α, which leads to its particular prominence in moiré materials, characterized by α≫1. Moreover, this phenomenon is most evident in the regime where Landau damping is quenched and plasmon lifetime is increased. The synergy of these two effects holds great promise for novel optoelectronic applications of moiré materials.

Carbon Nanotube Detectors and Spectrometers for the Terahertz Range

We present the compact unified charge control model (UCCM) for carbon nanotube field-effect transistors (CNTFETs) to enable the accurate simulation of the DC characteristics and plasmonic terahertz (THz) response in the CNTFETs. Accounting for the ambipolar nature of the carrier transport (n-type and p-type conductivity at positive and negative gate biases, respectively), we use n-type and p-type CNTFET non-linear equivalent circuits connected in parallel, representing the ambipolar conduction in the CNTFETs. This allows us to present a realistic non-linear model that is valid across the entire voltage range and is therefore suitable for the CNTFET design. The important feature of the model is that explicit equations for gate bias, current, mobility, and capacitance with smoothing parameters accurately describe the device operation near the transition from above- to below-threshold regimes, with scalability in device geometry. The DC performance in the proposed compact CNTFET model is validated by the comparison between the SPICE simulation and the experimental DC characteristics. The simulated THz response resulted from the validated CNTFET model is found to be in good agreement with the analytically calculated response and also reveals the bias and power dependent sub-THz response and relatively wide dynamic range for detection that could be suitable for THz detectors. The operation of CNTFET spectrometers in the THz frequency range is further demonstrated using the present model. The simulation exhibits that the CNT-based spectrometers can cover a broad THz frequency band from 0.1 to 3.08 THz. The model that has been incorporated into the circuit simulators enables the accurate assessment of DC performance and THz operation. Therefore, it can be used for the design and performance estimation of the CNTFETs and their integrated circuits operating in the THz regime.

Graphene Hybrid Structures for Integrated and Flexible Optoelectronics

Graphene (Gr) has many unique properties including gapless band structure, ultrafast carrier dynamics, high carrier mobility, and flexibility, making it appealing for ultrafast, broadband, and flexible optoelectronics. To overcome its intrinsic limit of low absorption, hybrid structures are exploited to improve the device performance. Particularly, van der Waals heterostructures with different photosensitive materials and photonic structures are very effective for improving photodetection and modulation efficiency. With such hybrid structures, Gr hybrid photodetectors can operate from ultraviolet to terahertz, with significantly improved R (up to 10⁹ A W^-1 ) and bandwidth (up to 128 GHz). Furthermore, integration of Gr with silicon (Si) complementary metal-oxide-semiconductor (CMOS) circuits, the human body, and soft tissues is successfully demonstrated, opening promising opportunities for wearable sensors and biomedical electronics. Here, the recent progress in using Gr hybrid structures toward high-performance photodetectors and integrated optoelectronic applications is reviewed.

Direct nanoscopic observation of plasma waves in the channel of a graphene field-effect transistor

Plasma waves play an important role in many solid-state phenomena and devices. They also become significant in electronic device structures as the operation frequencies of these devices increase. A prominent example is field-effect transistors (FETs), that witness increased attention for application as rectifying detectors and mixers of electromagnetic waves at gigahertz and terahertz frequencies, where they exhibit very good sensitivity even high above the cut-off frequency defined by the carrier transit time. Transport theory predicts that the coupling of radiation at THz frequencies into the channel of an antenna-coupled FET leads to the development of a gated plasma wave, collectively involving the charge carriers of both the two-dimensional electron gas and the gate electrode. In this paper, we present the first direct visualization of these waves. Employing graphene FETs containing a buried gate electrode, we utilize near-field THz nanoscopy at room temperature to directly probe the envelope function of the electric field amplitude on the exposed graphene sheet and the neighboring antenna regions. Mapping of the field distribution documents that wave injection is unidirectional from the source side since the oscillating electrical potentials on the gate and drain are equalized by capacitive shunting. The plasma waves, excited at 2 THz, are overdamped, and their decay time lies in the range of 25-70 fs. Despite this short decay time, the decay length is rather long, i.e., 0.3-0.5 μm, because of the rather large propagation speed of the plasma waves, which is found to lie in the range of 3.5-7 × 106 m/s, in good agreement with theory. The propagation speed depends only weakly on the gate voltage swing and is consistent with the theoretically predicted1 4 power law.

Stokes flow around an obstacle in viscous two-dimensional electron liquid

The electronic analog of the Poiseuille flow is the transport in a narrow channel with disordered edges that scatter electrons in a diffuse way. In the hydrodynamic regime, the resistivity decreases with temperature, referred to as the Gurzhi effect, distinct from conventional Ohmic behaviour. We studied experimentally an electronic analog of the Stokes flow around a disc immersed in a two-dimensional viscous liquid. The circle obstacle results in an additive contribution to resistivity. If specular boundary conditions apply, it is no longer possible to detect Poiseuille type flow and the Gurzhi effect. However, in flow through a channel with a circular obstacle, the resistivity decreases with temperature. By tuning the temperature, we observed the transport signatures of the ballistic and hydrodynamic regimes on the length scale of disc size. Our experimental results confirm theoretical predictions.

Grating Metamaterials Based on CdTe/CdMgTe Quantum Wells as Terahertz Detectors for High Magnetic Field Applications

The cyclotron and magnetoplasmon resonances were studied at 2 K in grating metamaterials fabricated on wafers with one or two modulation doped CdTe/CdMgTe quantum wells. The gratings (with the period varied between 2 μ m and 8 μ m) were prepared with an electron beam lithography either by etching or by evaporation of Au. The gratings were studied with an atomic force microscope which revealed a correlation between the depth and width of etched grooves at a constant time of etching. The sharpest resonances observed are due to excitation of magnetoplasmon in the case of Au gratings on a wafer with one quantum well. Etched samples with two quantum wells showed the strongest tuneability of magnetoplasmon resonances with the period of the grating and illumination with white light. We showed that the samples studied can be used as resonant or quasi-resonant terahertz detectors tuneable with magnetic field and white light.

Terahertz Frequency Comb in Graphene Field-Effect Transistors

Graphene Field-effect transistors (GFETs) are excellent candidates for all-electric, low-power radiation sources and detectors based on integrated circuit technology. In this work, we show that a hydrodynamic instability can be ex¬plored (the Dyakonov–Shur instability) to excite the graphene plasmons. The instability can be sustained with the help of a source-to-drain current and con¬trolled with the gate voltage. It is shown that the plasmons radiate a frequency comb in the Terahertz (THz) range. It is argued how this can pave the stage for a new generation of low power THz sources in integrated-circuit technology.

Large Responsivity of Graphene Radiation Detectors With Thermoelectric Readout: Results of Simulations

Simple estimations show that the thermoelectric readout in graphene radiation detectors can be extremely effective even for graphene with modest charge-carrier mobility ∼1000 cm2/(Vs). The detector responsivity depends mostly on the residual charge-carrier density and split-gate spacing and can reach competitive values of ∼103–104 V/W at room temperature. The optimum characteristics depend on a trade-off between the responsivity and the total device resistance. Finding out the key parameters and their roles allows for simple detectors and their arrays, with high responsivity and sufficiently low resistance matching that of the radiation-receiving antenna structures.