Taiichi Otsuji

We report on detection of terahertz radiation by using bilayer graphene-based FET with asymmetric grating gates. The device was fabricated with a stack of h-BN/Graphene/h-BN with a back gate as well as an asymmetric dual grating top gates. It was subjected to terahertz radiation at frequencies of 150 and 300 GHz at 4K and a clear photocurrent was obtained.

Interband photoexcitation in monolayer graphene can produce a weak gain in the terahertz range by only up to 2.3%, but exciting the surface plasmon polaritons mediates the light-matter interaction, resulting in a giant terahertz gain. Nonlinear carrier relaxation/recombination dynamics and resultant stimulated terahertz (THz) photon emission with excitation of surface plasmon polaritons (SPPs) in photoexcited monolayer graphene has been experimentally studied using optical-pump/THz-probe and optical-probe measurement. We observed the spatial distribution of the THz probe pulse intensities under linear polarization of optical pump and THz probe pulses. It was clearly observed that intense THz probe pulse was detected only at the area where the incoming THz probe pulse takes a TM mode being capable of exciting the SPPs. The observed gain factor is in fair agreement with theoretical calculations.

Summary form only given. Double-graphene-layer (DGL) heterostructures have recently attracted much attention due to their potential applications in high speed modulators of terahertz (THz) and infrared (IR) radiation, transistors, and THz photomixers. In this work we report experimental observation of THz emission and detection in the DGL device structures. We demonstrate that the photon-assisted resonant radiative inter-GL transitions enable the applications of such devices for THz/IR lasers and photodetectors (PDs).The main element of both the lasers and PDs under consideration is a D-GL heterostructure with the independently contacted GLs (in red) separated by the thin transparent tunnel-barrier layer. The bias voltage V applied between the GL's contacts induces the electron and hole gases in the opposing GLs. The electron and hole densities in GLs are also controlled by the gate voltage Vg. SEM image of the fabricated devices were shown where the metal contact connected to the upper and lower graphene sheets can be seen as well as the active area of devices (700 nm × 1500 nm). The voltage-dependent band-offset energy between the Dirac points of the GLs and the depolarization shift determine the energies of the photons emitted (in the lasers) or absorbed (in the PDs) in the resonant-tunneling inter-GL transitions. The tunneling causes all excess charges in the n-type GL to recombine with the holes in the p-type GL. The inter-GL radiative C-C and V-V transitions in D-GL laser were shown.

Current-injection pumping in graphene makes carrier population inversion enabling lasing and/or amplification of terahertz (THz) radiation. We have recently demonstrated single-mode THz lasing at 100K in graphene-channel transistor laser structures. Introduction of a gated double-graphene-layered (G-DGL) van der Waals heterostructure is a promising route to further increase the operation temperature and radiation intensity via plasmon- and/or photon-assisted quantummechanical tunneling. We have proposed a cascading of the G-DGL unit element working as a new type of THz quantumcascade lasers. Numerical analyses demonstrate further increase of the quantum efficiency of THz lasing by orders of magnitude compared to a transistor or single G-DGL structure.

ABSTRACT We develop device models of a terahertz (THz) photomixer based on a high-electron mobility transistor (HEMT) structure utilizing the excitation of electron plasma oscillations in the HEMT channel by the electrons and holes photogenerated by optical signals. We use hydrodynamic equations both for electrons in the channel and for photoelectrons and photoholes in the absorption layer, or hydrodynamic equations for electrons in the channel combined with a kinetic description of the photogenerated carriers, and the Poisson equation for the self-consistent electric field. The models are used for an analytical as well as numerical (based on an ensemble Monte Carlo particle technique) analysis of the HEMT-photomixer operation in the THz frequency range.

We report on amplified spontaneous broadband terahertz emission in 1–7.6 THz range at 100 K via current injection in a distributed-feedback dual-gate graphene-channel field effect transistor (DFB-DG GFET). The device exhibited a nonlinear threshold-like behavior with respect to the current-injection level. A precise DFB cavity design is expected to transcend the observed spontaneous broadband emission to single-mode THz lasing.

We report on amplified spontaneous broadband terahertz emission in 1–7.6 THz range at 100 K via current injection in a distributed-feedback (DFB) dual-gate graphene-channel transistor. The device exhibited a nonlinear threshold-like behavior with respect to the current-injection level. A precise DFB cavity design is expected to transcend the observed spontaneous broadband emission to single-mode THz lasing.

We evaluate the operation of vertical hot-electron graphene-base transistors (HET-GBTs) as detectors of terahertz (THz) radiation using the developed device model. The model accounts for the carrier statistics, tunneling injection from the emitter, electron propagation across the barrier layer with partial capture into the graphene-layer (GL) base, and the self-consistent plasma oscillations of the electric potential and the hole density in the GL-base. The calculated responsivity of the HET-GBT THz detectors as a function of the signal frequency 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 HET-GBTs can compete with and even surpass other plasmonic THz detectors.

ABSTRACT We develop a numerical model for simulation of plasmons in graphene with grating-gate structures, based on the Boltzmann equation and self-consistent Poisson equation. Using the model, we study the effect of coupling between plasmons in gated and ungated regions on the frequency tunability by the gate voltage and the damping of plasmons due to carrier scattering and due to source and drain contacts.

We study instability of plasmons in a dual-grating-gate graphene field-effect transistor induced by dc current injection using self-consistent simulations with the Boltzmann equation. With ultimately high-quality graphene where the electron scattering is only limited by acoustic phonons, it is demonstrated that a total growth rate of the plasmon instability, with the terahertz/mid-infrared range of the frequency, can exceed 4 X 1012 s-1 at room temperature, which is an order of magnitude larger than in two-dimensional electron gases based on usual semiconductors. We show that the giant total growth rate originates from cooperative promotion of the so-called Dyakonov-Shur and Ryzhii-Satou-Shur instabilities.

We explore current-driven Dirac plasmon dynamics in monolayer graphene metasurfaces. DC-current-induced complete suppression of the graphene absorption is experimentally observed in a broad frequency range followed by a giant amplification (up to ∼ 9 % gain) of an incoming terahertz radiation at room temperature.

The generation and amplification of terahertz (THz) electromagnetic waves by plasmonic instabilities in conventional two-dimensional (2D) electron systems (2DESs) have been actively investigated since 1980 [1]. However, after about forty years, we are still a long way from the realization of efficient emitters and amplifiers [2]. The rise of graphene and its extremely strong light-plasmon coupling and superior carrier transport properties make this work worth to be revisited [3]. We investigate dc current driven plasmonic instabilities in high mobility graphene-channel field-effect transistors (GFETs) working for tunable THz amplifier at room temperature (RT).

Unique properties of graphene are combined to enable graphene plasmonic devices that could revolutionize the terahertz (THz) electronic technology. A high value of the carrier mobility allows us to excite resonant plasma waves. The graphene bipolar nature allows for different mechanisms of plasma wave excitation. Graphene bilayer and multilayer structures make possible improved THz device configurations. The ability of graphene to form a high quality heterostructure with h-BN, black phosphorus, and other materials systems supports advanced heterostructure devices comprised of the best properties of graphene and other emerging materials. In particular, using black phosphorus compounds for cooling electron–hole plasma in graphene could dramatically improve the conditions for THz lasing. High optical phonon energy allows for reaching higher plasma frequencies that are supported by high sheet carrier densities in graphene. Recent improvements in graphene technology combined with a bette...

We study theoretically and experimentally the plasmonic THz detection by the asymmetric dual-grating-gate HEMT at room temperature without source-to-drain bias. We derive the analytical expressions of photocurrents due to the plasmonic drag and ratchet effects, and we discuss about their frequency dependences. We also compare the theory to the experimentally obtained frequency dependence. It is demonstrated that they agree qualitatively well.

Nanometer size field effect transistors can operate as efficient detectors of terahertz radiation that means far beyond their fundamental cut-of frequency. This work is an overview of some recent results concerning the low temperatures operation, linearity, circular polarization studies and double grating gate structures of nanometer scale field effect transistors working as terahertz detectors.

ABSTRACT We report on a broadband terahertz emission from a doubly interdigitated grating gates high electron mobility transistor. The observed emission was explained as due to the excitation of multi mode of plasmons: thermally excited incoherent modes and instability-driven coherent modes. The experiment was performed using Fourier spectrometer system coupled with high sensitive 4K Silicon bolometer under the vacuum. To enhance the efficiency, the device was subjected, from the backside, to a CW 1.5 μm laser beam. Dependence of the emission on the gate bias was observed and interpreted as due to the self-oscillation of the plasma waves.

Terahertz emission by the photon-assisted resonant radiative transitions between graphene layers (GLs) in double-GL structures is theoretically and experimental demonstrated. Devices such as terahertz/infrared lasers base on this technology are very promising for terahertz optoelectronics.

We show that the ballistic electron injection from the n+ source region through the i-region into the gated n-region of the n+-i-n-n+ graphene field-effect transistor (GFET) leads to the effective drag of quasi-equilibrium electrons toward the drain. The drag results in the positive feedback between the ballistic injection and the reverse injection from the n+ drain region and can lead to the negative real part of the GFET source-drain impedance accompanied with the change of the impedance imaginary part sign. As a result, the steady-state current flow along the GFET channel can be unstable giving rise to the current driven self-excitation of the electron density high-frequency oscillations (plasma instability). The related oscillations of the current feeding an antenna can be used for the terahertz radiation emission.

This paper reviews recent advances in the terahertz (THz) graphene-based 2D-heterostructure lasers and amplifiers. The linear gapless graphene energy spectrum enables population inversion under optical and electrical pumping giving rise to the negative dynamic conductivity in a wide THz frequency range. We first theoretically discovered these phenomena and recently reported on the experimental observation of the amplified spontaneous THz emission and single-mode THz lasing at 100K in the current-injection pumped graphene-channel field-effect transistors (GFETs) with a distributedfeedback dual-gate structure. We also observed the light amplification of stimulated emission of THz radiation driven by graphene-plasmon instability in the asymmetric dual-grating gate (ADGG) GFETs by using a THz time-domain spectroscopy technique. Integrating the graphene surface plasmon polariton (SPP) oscillator into a current-injection graphene THz laser transistor is the most promising approach towards room-temperature intense THz lasing.

This paper reviews recent advances in the double-graphene-layer (DGL) active plasmonic heterostructures for the terahertz (THz) device applications. The DGL consists of a core shell in which a thin tunnel barrier layer is sandwiched by the two GLs being independently connected with the side contacts and outer gate stack layers at both sides. The DGL core shell works as a nano-capacitor, exhibiting inter-GL resonant tunneling (RT) when the band offset between the two GLs is aligned. The RT produces a strong nonlinearity with a negative differential conductance in the DGL current-voltage characteristics. The excitation of the graphene plasmons by the THz radiation resonantly modulates the tunneling currentvoltage characteristics. When the band offset is aligned to the THz photon energy, the DGL structure can mediate photonassisted RT, resulting in resonant emission or detection of the THz radiation. The cooperative double-resonant excitation with structure-sensitive graphene plasmons gives rise to various functionalities such as rectification (detection), photomixing, higher harmonic generation, and self-oscillation, in the THz device implementations.

This paper reviews recent advances in terahertz wave generation using graphene and compound semiconductor nano-heterostructures. The excitation of two-dimensional (2D) plasmons in high-electron mobility transistors (HEMTs) and related semiconductor nano-heterostructures has been used for emission of THz electromagnetic radiation. Plasmons in graphene (which is one or several monolayers of a honeycomb carbon lattice) have a higher velocity and peculiar transport properties owing to the massless and gapless energy spectrum of graphene. Optical and/or injection pumping of graphene results in a negative-dynamic conductivity in the THz spectral range, which may enable new types of THz lasers. Fundamental physics behind the device operation mechanisms and experimental results are demonstrated including coherent monochromatic THz radiation from InP-based HEMT-type emitters and stimulated emission of THz radiation with a giant gain via excitation of surface plasmon polaritons in optically pumped monolayer intrinsic graphene.

Linear and gapless energy spectrum of graphene carriers enables population inversion under optical and electrical pumping. We first theoretically discovered this phenomenon and demonstrated experimental observation of single-mode THz lasing with rather weak intensity at 100K in current-injection pumped graphene-channel field-effect transistors (GFETs). We introduce graphene surface plasmon polariton (SPP) instability to substantially boost the THz gain. We demonstrate our experimental observation of giant amplification of THz radiation at 300K stimulated by graphene plasmon instabilities in asymmetric dual-grating gate (ADGG) GFETs. Integrating the graphene SPP amplifier into a GFET laser will be a promising solution towards room-temperature intense THz lasing.

Electrons and holes in graphene behave as relativistic charged massless Dirac fermions due to the graphene unique gapless electronic band structure with a linear dispersion law. Dirac plasmons - the quanta of the plasma oscillation of the Dirac electrons - can dramatically enhance the interaction of terahertz (THz) photons with graphene. We have proposed an original current-injection graphene THz laser transistor, demonstrated single-mode THz laser oscillation at low temperatures [1-3], and discovered and demonstrated the THz giant gain enhancement effect by the graphene Dirac plasmons [4-8]. However, further breakthroughs are needed to realize room-temperature high-intensity THz lasing and ultrafast modulation operation for the next generation wireless 6G and 7G communications. In this paper, we will present new ideas on the operating principle and device structures of the THz graphene plasmonic laser transistors with a high radiation intensity and ultrafast modulation capability operating at room temperature. To dramatically improve the quantum efficiency and gain, we utilize the Coulomb drag effect in lateral n+ - i – n - n+ graphene diode/transistor structures with the ballistic injection of the graphene Dirac fermions [9-11]. Such injection strongly modifies the current-voltage characteristics producing “plasmonic gain” in the THz frequency range applicable for THz oscillations and amplifications. This phenomenon is associated with the specifics of the ballistic electron scattering on quasi-equilibrium electrons in graphene. Depending on the device structural parameters (in particular, the gated region length and its electron Fermi energy), the graphene diode/transistor structures can exhibit either S-shaped or monotonic current-voltage characteristics [9]. In the former case, the resulting hysteresis and current filamentation effects can be used for the implementation of the voltage-switching devices. The feedback between the amplified dragged electrons current and the injected ballistic electrons current can lead to the negative THz dynamic conductivity. The self-excitation of the THz plasma oscillations in the gated region enables the realization of the graphene transistor-based sources of THz radiation [11]. To realize ultrafast modulation of lasing intensity/phase, we introduce actively controlling the parity and time-reversal (PT) symmetry [12] of the graphene Dirac plasmons (GDPs) in the dual-grating-gate graphene-channel field effect transistor (DGG-GFET) nanostructures for the ultrafast modulation of the lasing intensity and phase. [13]. The PT symmetry is expressed by a pair of complementary gain and loss elements. This gain–loss balance leads to the exceptional points at the real frequency axis in the exact PT-phase resulting in the extraordinary frequency response of “unidirectionality” [14]. The DGG-GDP metasurface, which consists of a unit cell comprising a pair of gain and loss regions and its periodical arrangement, promotes the GDP instability. The PT symmetry can be controlled (to be held or broken) by altering the gate or drain bias voltages. Our numerical simulations showed that the laser cavity Q values can be dynamically controllable in a DGG-GDP transistor structure [6] demonstrating 100-Gbit/s-class ultrafast modulation capabilities [13]. The authors thank A.A. Dubinov, D. Yadav, T. Watanabe, T. Suemitsu, W. Knap, V. Kachorovskii, and V.V. Popov for their contributions. This work was supported by JSPS-KAKENHI No. 21H04546, and No. 20K20349, Japan. V. Ryzhii, M. Ryzhii, and T. Otsuji, J. Appl. Phys. 101, 083114 (2007). T. Otsuji et al., IEEE J. Sel. Top. Quantum Electron. 19, 8400209 (2013). D. Yadav et al., Nanophoton. 7, 741-752 (2018). A.A. Duvinov el al., J. Phys.: Cond. Matters 23, 145302 (2011). T. Watanabe et al., New J. Phys. 15, 075003 (2013). Y. Koseki et al., Phys. Rev. B 93, 245408 (2016). S. Boubanga-Tombet et al., Phys. Rev. X 10, 031004 (2020). S. Boubanga-Tombet et al., Front. Phys. 9, 726806 (2021). V. Ryzhii et al., Phys. Rev. Appl. 16, 014001 (2021). V. Ryzhii et al., Appl. Phys. Lett. 119, 093501 (2021). V. Ryzhii et al., Physica Status Solidi A 218, 2100535 (2021). M.-A. Miri and A. Alu, Science 363, eaar7709 (2019). T. Otsuji et al., Nanophoton. under review. H. Ramezani and T. Kottos, Phys. Rev. A 82, 04383 (2010). Figure 1

We evaluate the optical pumping efficiency of the graphene-layer (GL) heterostructures intended for the terahertz (THz) lasing using the interband transitions in the GL. The pumping of such by near- or mid-infrared (NIR or MIR) radiation leads to the creation of a substantially hot two-dimensional electron-hole plasma (2D-EHP) in the GL. This hampers the interband population inversion in the 2D-EHP and can suppress the THz lasing. To prevent the 2D-EHP overheating, we propose to use the NIR/MIR radiation pumping of the GL through a sufficiently thick layer absorbing this radiation. This layer with sufficiently small energy gap enables an increase in the quantum efficiency of the pumping accompanied by strong cooling of the electron-hole pairs injected into the GL. As shown, the absorbing-cooling layers made of black-arsenic-phosphorus can be fairly efficient if their energy gap is smaller than the optical phonon energy in the GL.

We study the optical near- and mid-infrared pumping of the heterostructure based on graphene with a black-As layer. This layer serves for the optical generation and cooling of the electron-hole pairs to be injected into the graphene layer. Due to the cooling of the electron-hole pairs, their energy in the case of the absorbing-cooling layer with the optimized thickness can be close to the energy gap of the black-As layer. Owing to a relatively narrow energy gap of the black-As layer Δ G, the energy of the injected electron-hole pairs can be smaller than the energy of optical phonons in in graphene (ℏ ω 0≃0.2 eV. This can provide the formation of the cold electron-hole plasma in the graphene-layer that is beneficial for achieving of the interband population inversion and the interband terahertz lasing. The obtained results can be used for the optimization of the terahertz lasers with the optical pumping.

Abstract. We analyze the pumping of the graphene-based laser heterostructures by infrared radiation using the numerical model. To enable the injection of sufficiently cooled carriers into the graphene layer (GL) leading to the interband population inversion, we propose to use the graded-gap black-PxAs1  −  x absorption-cooling layers. Our calculations are based on the thermodiffusion-drift carrier transport model. We demonstrate that the proposed optical pumping method can provide an efficient injection of the cool electron–hole plasma into the GL and the interband population inversion in the GL. Since the energy gap in b-As layer can be smaller than the energy of optical phonons in the GL, the injected electron–hole plasma can be additionally cooled down to the temperatures lower than the lattice temperature. This promotes a stronger population inversion that is beneficial for realization of the GL-based optically pumped terahertz and far-infrared laser, plasmon emitters, and the superluminescent downconverters. We also compare the efficiency of optical pumping through the graded-gap and uniform absorbing-cooling layers.

ABSTRACT Among different carbon materials (diamond, graphite, fullerene, carbon nanotubes), graphene and more complex graphene-based structures attracted a considerable attention. The gapless energy spectrum of graphene implies that graphene can absorb and emit photons with rather low energies corresponding to terahertz (THz) and infrared (IR) ranges of the electromagnetic spectrum. In this presentation, the discussion is focused on the double-graphene-layer (double-GL) structures. In these structures, GLs are separated by a barrier layer (Boron Nitride, Silicon Carbide, and so on). Applying voltage between GLs, one can realize the situation when one GL is filled with electrons while the other is filled with holes. The variation of the applied voltage leads to the variations of the Fermi energies and, hence, to the change of the interband and intraband absorption of electromagnetic radiation and to the variation of the tunneling current. The plasma oscillations in double-GL structures exhibit interesting features. This is mainly because each GL serves as the gate for the other GL. The spectrum of the plasma oscillations in the double-GL structures falls into the terahertz range (THz) of frequencies and can be effectively controlled by the bias voltage. In this paper, we discuss the effects of the excitation of the plasma oscillations by incoming THz radiation and by optical radiation of two lasers with close frequencies as well as negative differential conductivity of the N-type and Z-type. These effects can be used in resonant THz detectors and THz photomixers. The models of devices based on double-GL structures as well as their characteristics are discussed.