Plasmonic Terahertz Research Articles

Research on the Application of Terahertz Communication Technology in 5G and Future Networks

Abstract: As the limitations of 5G in spectrum scarcity and massive data transmission capability become evident, the vision of 6G technology emerges, aiming for global coverage, abundant spectrum resources, ultra-high-speed data transmission, minimal latency, and high reliability. Terahertz (THz) communication technology, with its rich spectrum resources, is seen as a key to achieving 6G objectives. This paper explores the application and challenges of THz communication in 5G and future networks, analyzes its basic theory, key technologies, channel characteristics, and modeling, and discusses its prospects and challenges in 6G and beyond. The paper provides an overview of THz communication, including its position in the electromagnetic spectrum, bandwidth potential, and advantages of short wavelengths. It discusses THz-related technologies, particularly the roles of THz electronics, photonics, and plasmonics in device development, and their contributions to 6G advancements. Future research directions are outlined, including optimizing transmission windows, applying AI in wireless communication, and the potential of 6G as a new generation of intelligent digital infrastructure. While THz communication shows great potential in 6G networks, it faces technical and economic challenges. This paper provides a solid theoretical foundation for understanding THz technology, but future work requires deeper exploration in experimental validation, technical detail research, and cost-benefit analysis.

Control of Surface Plasmon Propagation and Terahertz Near-Field Waveforms in a Scanning Tunneling Microscope.

Coupling subcycle THz pulses to a scanning tunneling microscope (STM) enables ultrafast spectroscopy at the atomic scale. This technique critically depends on the shape of the THz near-field waveform in the tunnel junction. We characterize the THz electric field waveform in the STM junction by electro-optic sampling of tip-scattered THz light (s-EOS) and pulse correlation using the THz-induced current. Combined with full-wave simulations, we identify THz spectral distortions and reflections arising from THz surface plasmon propagation along the tip wire and cavity modes at the tip apex. By optimizing the tip shape, tip holder geometry and materials, we achieve a drastically flattened THz near-field waveform. This optimization ensures point-like coupling to the far-field and, thus, allows precise Gouy phase control at the STM tip. The improved THz waveforms facilitate atomically-resolved THz time-domain spectroscopy in the STM with high dynamic range for investigating local electron and phonon dynamics on surfaces.

Plasmonic antennas based on rectangular graphene nanoribbons with controlled polarization of terahertz and infrared radiation

Background. To develop new terahertz wireless communication systems with high throughput and transmission speeds, such as 6G and above, effective control of the polarization direction of emitted terahertz waves is necessary, but most methods are technologically complex and expensive. The implementation of terahertz antennas and devices based on 2D materials such as graphene solves the problem associated with developing effective control. Aim. Study of the possibility of controlling the polarization of terahertz and IR radiation of plasmonic antennas based on rectangular graphene nanoribbons by changing the chemical potential (application of an external electric field). Methods. This important scientific problem related to the design of terahertz antennas can largely be solved by simulation using the electrodynamic simulation program CST MWS 2023. Results. Plasmon terahertz antennas based on rectangular graphene nanoribbons were chosen as the object of analysis and the possibility of emitting waves of two orthogonal polarizations was shown. Methods have been identified for controlling the polarization of terahertz and IR radiation from such antennas, based on the selection of operating frequencies corresponding to the resonances of the modes of surface plasmon-polaritons, and the application of metallization to the dielectric substrate. Conclusion. The ability to control the polarization of terahertz and IR radiation makes it possible to create both new elements of plasmonic antenna arrays and new communication technologies, including future 6G networks.

InAs Terahertz Metalens Emitter for Focused Terahertz Beam Generation

Metasurfaces have opened doors to combining multiple photonic functionalities in a single compact device. In particular, the ability to generate short terahertz (THz) pulses with precise wavefront engineering in a single THz metasurface redefined the role metasurfaces can play in THz systems. Here, an InAs metalens emitter which generates and focuses a THz pulse beam is demonstrated using a 130 nm thick InAs metasurface designed as a binary‐phase Fresnel zone plate. The THz beam is focused to a spot of ≈430 μm at 1 THz with a short focal length of 5 mm and large numerical aperture of 0.5. Nanoscale InAs Mie resonators comprising the metasurface enable THz generation with an amplitude as high as 20 times compared to plasmonic THz emitters and several times compared to a 1 mm thick ZnTe crystal. This InAs metasurface emitter provides a new paradigm for designing THz imaging, spectroscopy, and communication systems, where THz beam generation and shaping are performed with a single device without compromising the generation efficiency, while eliminating losses and avoiding limitations of phase matching of conventional nonlinear optics approaches.

Negative damping of terahertz plasmons in counter-streaming double-layer two-dimensional electron gases

Abstract The plasmon excitation in two-dimensional electron gases (2DEGs) is a significant way of achieving micro-nanoscale terahertz (THz) devices. Here, we establish a kinetic simulation model to study the THz plasmons amplification in a semiconductor double-quantum-well system with counter-streaming electron drift velocities. By comparing the simulation results with theoretical dispersion relations, we confirm two competing mechanisms of negative damping suitable for THz amplification: Cherenkov-type two-stream instability and a new non-Cherenkov mechanism called kinetic relaxation instability. The former is caused by the interlayer coupling of two slow plasmon modes and only exists when the drift velocities are much greater than the fermi velocities. The latter is a statistical effect caused by the momentum relaxation of electron-impurity scattering and predominates at lower drift velocities. We show that an approximate kinetic dispersion relation can accurately predict the wave growth rates of the two mechanisms. The results also indicate that the saturated plasmonic waves undergo strong nonlinearities such as wave distortion, frequency downshift, wave-packet formation, and spectrum broadening. The nonlinear evolution can be interpreted as the merging of bubble structures in the electron phase-space distribution. The present results not only reveal the potential mechanisms of the plasmonic instabilities in double-layer 2DEGs, but also provide a new guideline for the design of on-chip THz amplifiers.

A Retrospective Study: Are the Multi-Dips in the THz Spectrum during Laser Filamentation Caused by THz–Plasma Interactions?

During the process of terahertz (THz) wave generation via femtosecond laser filamentation in air, as well as through the mixing of THz waves with externally injected plasma filaments, THz waves engage in interactions with the plasma. A characteristic feature of this interaction is the modulation of the THz radiation spectrum by the plasma, which includes the generation of THz spectral dips. This information is essential for understanding the underlying mechanisms of THz–plasma interactions or for inferring plasma parameters. However, a current debate exists on the number of THz spectral dips observed after the interaction, with different opinions of single versus multiple dips, thus leaving the interaction mechanisms still ambiguous. In this work, we retrospectively analyzed the experimental appearance of multiple dips in the THz spectrum and found that the current observations of such dips are predominantly a result of the water vapor absorption with a low spectral resolution. Additionally, we observed that altering the acquisition width of the temporal THz signal also influenced the dips’ number. Hence, in future research, simultaneous attention should be paid to the following two aspects of THz–plasma interactions: (1) It is necessary to ensure a sufficiently wide time-domain window to accurately represent the spectral dip characteristics. (2) The spectral dips should be carefully distinguished from the water absorption lines before being further studied. On the other hand, for the case of a single dip in the THz spectrum, we also put forward a new viewpoint of the resonance between surface plasmon waves and THz waves, which should also be taken into consideration in future studies.

Study on mode properties of GaAs-based hybrid plasmonic terahertz waveguides

Terahertz (THz) fields are increasingly being used to address the critical challenges associated with achieving high data rates and rapid communication. In this study, a hybrid plasmonic THz waveguide is designed and analysed operating in the 2.5–3.5 THz frequency range. The waveguide is constructed using gallium arsenide as the high-refractive-index core, surrounded by aluminium arsenide and silver placed on a high-density polyethylene (HDPE) substrate. Graphene is strategically positioned between the HDPE layers to enhance light confinement. The mode properties of the proposed waveguide are simulated with Comol Multiphysics using the finite-element method and show unique characteristics. Observation of the simulated results at 2.5–3.5 THz reveals a high effective refractive index of 3.79, a maximum effective mode area of 1.88 mm2, a high birefringence of 0.2, a low dispersion of 0.10 ps THz−1 cm−1, a high mode field diameter of 15.8 mm, a high beat length of 123 mm and a low confinement loss of 1.79 × 10−9 mm−1. These features make the proposed waveguide suitable for applications in photonic integrated circuits for THz communications.

Near-field detection of gate-tunable anisotropic plasmon polaritons in black phosphorus at terahertz frequencies

Polaritons in two-dimensional layered crystals offer an effective solution to confine, enhance and manipulate terahertz (THz) frequency electromagnetic waves at the nanoscale. Recently, strong THz field confinement has been achieved in a graphene-insulator-metal structure, exploiting THz plasmon polaritons (PPs) with strongly reduced wavelength (λp ≈ λ0/66) compared to the photon wavelength λ0. However, graphene PPs propagate isotropically, complicating the directional control of the THz field, which, on the contrary, can be achieved exploiting anisotropic layered crystals, such as orthorhombic black-phosphorus. Here, we detect PPs, at THz frequencies, in hBN-encapsulated black phosphorus field effect transistors through THz near-field photocurrent nanoscopy. The real-space mapping of the thermoelectrical near-field photocurrents reveals deeply sub-wavelength THz PPs (λp ≈ λ0/76), with dispersion tunable by electrostatic control of the carrier density. The in-plane anisotropy of the dielectric response results into anisotropic polariton propagation along the armchair and zigzag crystallographic axes of black-phosphorus. The achieved directional subwavelength light confinement makes this material system a versatile platform for sensing and quantum technology based on nonlinear optics.

Sensing and slow light applications of graphene plasmonic terahertz structure

In this work, Ansys FDTD is used to design and simulate a terahertz metamaterial structure based on periodic continuous pattern graphene monolayer, and the high-quality PIT phenomena are obtained by continuously adjusting structural parameters. To validate the designed structure, the simulated transmission curve (reflection curve) obtained is compared with the theoretical transmission curve (reflection curve) derived from coupled-mode theory. It is observed that these two results exhibit a remarkably high degree of overlap. The resonant frequency and Fermi energy reveals a perfect linear correlation between them with the resonant frequency increasing proportionally with Fermi energy increasing. Dynamic tuning of PIT can be realized by adjusting the Fermi energy of graphene. For a more in-depth study of its sensing characteristics, the structure is placed in different environments. As the refractive index of the detection medium increases, the resonant frequency gradually decreases, demonstrating a redshift phenomenon. By manipulating the resonant frequency of the PIT sensor, the selective detection of specific target can berealized. After analyzing the sensitivity and FOM values of the structure, it is found that the maximum sensitivity is 1.457 THz/RIU. At a resonant frequency of 6.8174 THz, FOM reaches 30.5652. In summary, the sensor structure designed in this work has dual frequency sensing characteristics and can be used for dual frequency detection. Moreover, compared with other sensor structures, it demonstrates superior sensing performance. Additionally, in studying the slow light effect of the structure, it is found that as the Fermi energy increases, the group index and phase shift at the transparency window continue to increase. At the Fermi energy of 1.2 eV, the group index reaches a high value of 584. This is because in the PIT phenomenon, transparent peaks are formed due to multimodal coupling. This coupling will significantly improve the dispersion characteristics near the transparent peak, resulting in a large group index near the transparent peak. Furthermore, with the increase of carrier mobility, the group index and phase shift of the structure also gradually increase. At a carrier mobility of 0.75 m²/(V·s), the group refractive index is 456, and reaches 1010 at 2.0 m²/(V·s). In this study, the slow-light performance of graphene structure can be optimized through jointly adjusting the Fermi energy and carrier mobility. This research provides theoretical support and methods for designing advanced graphene-based sensors and devices for slow-light applications.

THz properties of grating-gate plasmonic crystals

This study reviews recent advances in the modern field of terahertz plasmonics concerning the control of resonant properties of grating-gate plasmonic crystal structures. Particularly, we conducted both experimental and theoretical investigations of AlGaN/GaN grating-gate structures with a focus on investigations of the resonant structure of transmission spectra associated with plasmon excitations in two-dimensional electron gas at different modulation degree of concentration profiles. Two distinct resonant phases of the plasmonic crystal structure were analyzed. The first one, the delocalized phase, is observed in the case of a small modulation degree of electron gas. In this phase, we found that plasmonic resonant absorption of incident radiation occurs across the entire grating-gate structure, with domination in the gated regions of the electron gas. In contrast, the second phase, the localized one, is realized at a strong modulation of the electron concentration profiles when the gated regions of the electron gas are completely depleted. Here, plasmon resonances are characterized by the spatial localization of absorption of incident radiation exclusively within the ungated regions of the electron gas. Moreover, in the localized phase, we observed the unexpected blue shift of plasmon resonant frequency with an increase of gate voltage. This observation was explained by the result of ‘edge gating effect’ and additional shrinking of the concentration profile of the electron gas in the ungated region. We demonstrate that the correct description of both phases requires rigorous electrodynamic simulations and cannot be achieved solely in the frameworks of simplified single-mode or single-cavity models.

Excitation of Terahertz Plasmonic Resonance on a Subwavelength Metallic Grating in an Attenuated Total Reflection Scheme Using a Monochromatic Radiation Source

Excitation of Terahertz Plasmonic Resonance on a Subwavelength Metallic Grating in an Attenuated Total Reflection Scheme Using a Monochromatic Radiation Source

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.

Graphene based waveguide fed hybrid plasmonic terahertz patch antenna

Abstract The terahertz (THz) technology has fascinated lot of attention due to its enormous potential for a wide range of applications in the public, private and enterprise domains. This research work demonstrates the design structure and analysis of graphene based waveguide fed hybrid plasmonic THz patch antenna (HPTPA) constructed at around 3 THz. Hybrid plasmonic THz waveguide (HPTW) as a feeding line for the proposed THz patch antenna can increases antenna efficiency. The graphene is sandwiched between gallium arsenide (GaAs) and silver (Ag) to confine THz waves efficiently. Using mode analysis in finite element method for the proposed HPTW the propagation length, effective refractive index has been thoroughly examined. Based on the finite element method (FEM) approach, the results of the designed graphene-based waveguide fed HPTPA shows a high effective refractive index of 2.9, large propagation length of 230 µm, gain of 2.29 dBi, bandwidth of 200 GHz and efficiency of 85 % has obtained. The proposed graphene-based waveguide fed HPTPA could be beneficial to enable several photonic integrated circuit applications in next-generation wireless communication.

A plasmonic terahertz perfect absorber based on L-shaped graphene patches and gold rods

A plasmonic terahertz perfect absorber based on L-shaped graphene patches and gold rods

Plasmons in AlGaN/GaN grating-gate structure probing with 300 K background illumination

We show that terahertz plasmons in AlGaN/GaN grating-gate structures efficiently modulate the reflection of room temperature thermal radiation, leading to spectra that are in agreement with the measurements of plasmon absorption using high-power external sources. For typical samples of a few square millimeters in size, the reflected radiation intensity is relatively weak, and measurements need the use of gate voltage plasmon modulation and lock-in detection techniques. We show that unintentional use of lock-in techniques may lead to artifacts and demonstrate what kind of special precautions need to be taken into account. We show that drain voltage modulation also leads to modulation of the reflected thermal radiation by plasmons. Our results are of key importance for the research on new resonant plasmon-based terahertz sources because of the always present superposition of electrically excited terahertz emission and background radiation reflected from the structures.

Investigation of temperature-dependent polarization properties in grating-gate AlGaN/GaN heterostructures by Drude conductivity at THz frequency

We describe the temperature-dependence polarization properties of grating-gated AlGaN/GaN heterostructures at terahertz frequencies. Using the finite-difference time-domain method, it was demonstrated that as the temperature increases, the resonant frequency of the incident light was red-shifted. Simultaneously, a shorter gate length leads to a higher resonant frequency. In addition, at a lower temperature, the coupling efficiency of terahertz radiation and plasmon is higher. In our simulation results, the maximum modulation depth is 88%; at a gate length of 800 nm and lattice temperature of 77 K. For the same gate length, with higher electron gas concentrations and filling factor, greater modulation depths were produced. Studies on these properties may help in the design and optimization of terahertz detectors and modulators.

Terahertz nanospectroscopy of plasmon polaritons for the evaluation of doping in quantum devices.

Terahertz (THz) waves are a highly sensitive probe of free carrier concentrations in semiconducting materials. However, most experiments operate in the far-field, which precludes the observation of nanoscale features that affect the material response. Here, we demonstrate the use of nanoscale THz plasmon polaritons as an indicator of surface quality in prototypical quantum devices properties. Using THz near-field hyperspectral measurements, we observe polaritonic features in doped silicon near a metal-semiconductor interface. The presence of the THz surface plasmon polariton indicates the existence of a thin film doped layer on the device. Using a multilayer extraction procedure utilising vector calibration, we quantitatively probe the doped surface layer and determine its thickness and complex permittivity. The recovered multilayer characteristics match the dielectric conditions necessary to support the THz surface plasmon polariton. Applying these findings to superconducting resonators, we show that etching of this doped layer leads to an increase of the quality factor as determined by cryogenic measurements. This study demonstrates that THz scattering-type scanning near-field optical microscopy (s-SNOM) is a promising diagnostic tool for characterization of surface dielectric properties of quantum devices.

Optimization of strip-based hybrid plasmonic terahertz waveguide with distributed Bragg reflector layers

In view of recent developments in the utilization of terahertz (THz) communications technology for the increasing demands of high speed and data rates for various wireless applications, this research work targets the design and analysis of distributed Bragg reflector (DBR)-based hybrid plasmonic THz waveguide. This THz waveguide operates at a frequency range from 2.5 to 3.5 THz. The proposed DBR-based hybrid plasmonic THz waveguide consists of DBR layers of gallium arsenide (GaAs) and aluminum arsenide, high-density polyethylene (HDPE) as substrate, a GaAs stripe for wave-guidance, and few layers of graphene layers in HDPE for better confinement. To increase the light confinement between DBR and graphene, a GaAs strip is placed into HDPE. By altering the width and height of the GaAs strip, the parameters birefringence, mode field diameter (MFD), confinement loss, effective mode area, beat length, and dispersion have been fully examined. The obtained simulation results shows high birefringence of 0.6276, maximum MFD of 45 mm, low confinement loss of 7.5 × 10 − 9 mm − 1, high effective mode area of 16.5 mm2, and low anomalous dispersion of 0.0023 (ps/THz/cm) in the range of 2.5 to 3.5 THz. This optimized THz waveguide may helps to enable in guided THz applications for photonic integrated circuits.

Multifunctional Terahertz Spoof Plasmonic Devices

AbstractThe next‐generation wireless communications and intrachip and interchip communications require high‐speed terahertz (THz) interconnects. The interconnects based on spoof surface plasmon polaritons (SSPPs) provide a possible route to achieve this goal. Recently, some THz plasmonic devices are explored, but they suffer from bulk sizes and/or single function. Herein, planar multifunctional THz plasmonic devices based on the interaction between SSPPs and spoof localized surface plasmon (SLSPs) including the functions of pass‐through transmission, multifrequency resonances, and phase shift are proposed and experimentally demonstrated. The pass‐through transmission can be realized on the SSPP waveguide by increasing the loss of the coupled SLSP resonator; the multifrequency resonances are obtained by placing the SU‐8 films with different thicknesses on the SLSP resonator due to its high sensitivity; and the phase shift can be achieved by integrating the SU‐8 film on the SSPP waveguide due to the flexible dispersion behaviors of SSPPs. The proposed multifunctional THz plasmonic devices hold great promise for the THz on‐chip integrations, wireless communications, and sensing systems.

Terahertz Plasmonic Sensor based on bulk Dirac Semimetals

Terahertz Plasmonic Sensor based on bulk Dirac Semimetals