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The investigation on the temporal dynamics of graphene-wrapped core–shell nanoparticles under the illumination of a Gaussian impulse have been carried out. By altering the graphene layers and the aspect ratio of the core–shell structure, we can adjust the resonant modes into typical cases in regime of terahertz. Accordingly, different scenarios for the temporal evolution are detected, which include two kinds of ultrafast oscillation with exponential decay tendency, pure exponential decay, and Gaussian shape, when the pulse duration of the incident pulse is much shorter than, similar to, and much longer than the localized surface plasmon lifetime. To one's interest, when the coupling between two resonant modes exists, one predicts the long-periodic oscillation, whose period is just the difference between the frequencies of the resonant modes. Hence, the intrinsic properties of the ultrafast oscillation can be hardly influenced by the input signals. Further quantitative calculation demonstrate that the periods of the ultrafast oscillations can be tuned by different physical mechanisms, which are, respectively, based on the self-interacting correction of a single resonance and the strong coupling between the resonant modes in frequency domain. Our results may be applicable in the fields of optical sensors, optical information processing, and other nanophotonic devices. [ABSTRACT FROM AUTHOR]

Non-destructive, reliable, and accurate measurements of low doping levels and carrier lifetimes in small direct-bandgap semiconductors such as indium arsenide (InAs) at room temperature are challenging due to intrinsic limitations of conventional experimental techniques. In this article, we introduce a contactless optical-terahertz (THz) pump–probe technique in a continuous-wave regime measuring low doping levels in the 10 15 − 10 16 cm − 3 range and lifetimes down to 8 ns at room temperature in InAs. We show that a single-parameter material model can be used to retrieve the free carrier density from THz transmission measurements of InAs layers. Additionally, we show that lifetime of photogenerated excess carriers can be measured by modulating this carrier density using an amplitude-modulated optical pump. [ABSTRACT FROM AUTHOR]

We propose and analyze the performance of terahertz (THz) room-temperature bolometric detectors based on the graphene channel field-effect transistors (GC-FET). These detectors comprise the gate barrier layer (BL) composed of the lateral hexagonal-boron nitride black-phosphorus/hexagonal-boron nitride (h-BN/b-P/h-BN) structure. The main part of the GC is encapsulated in h-BN, whereas a short section of the GC is sandwiched between the b-P gate BL and the h-BN bottom layer. The b-P gate BL serves as the window for the electron thermionic current from the GC. The electron mobility in the GC section encapsulated in h-BN can be fairly large. This might enable a strong resonant plasmonic response of the GC-FET detectors despite relatively lower electron mobility in the GC section covered by the b-P window BL. The narrow b-P window diminishes the Peltier cooling and enhances the detector performance. The proposed device structure and its operation principle promote elevated-temperature GC-FET THz detector responsivity values and other characteristics, especially at the plasmonic resonances. [ABSTRACT FROM AUTHOR]

This work develops a polarization-dependent analytical model using terahertz time-domain spectroscopy (THz-TDS) for computing strain in materials. The model establishes a correlation between volumetric strain and the change in time of arrival for a THz pulse by using the dielectrostrictive properties, variations in doping particle density, and changes in the thickness of the sample resulting from Poisson's effects. The analytical model is validated through strain mapping of polydimethylsiloxane (PDMS) doped with passive highly dielectrostrictive strontium titanate (STO). Two experiments, using an open-hole tensile and a circular edge-notch specimen are conducted to show the efficacy of the proposed. The stress relaxation behavior of the composite is measured and accounted for to prevent changes in strain during the measurement window. The THz strain mapping results are compared with the finite element model (FEM) and surface strain measurements using the digital image correlation (DIC) method. The experimental findings exhibit sensitivity to material features such as particle clumping and edge effects. The THz strain map shows a strong agreement with FEM and DIC results, thus demonstrating the applicability of this technique for surface and sub-surface strain mapping in polymeric composites. [ABSTRACT FROM AUTHOR]

Acoustic graphene plasmons (GPs) exhibit an exceptional density of electromagnetic states within the mid-infrared (MIR) and terahertz frequency ranges, leading to a pronounced near-field amplification and localization at the nanometer scale. This characteristic renders them highly promising for the development of ultra-sensitive plasmon-enhanced MIR sensing and spectroscopic applications. However, the tight spatial confinement inherent to acoustic GPs results in a significant momentum mismatch, which in turn leads to poor coupling efficiency with light in free space. To overcome this limitation, we leverage numerical simulations to show that GPs can act as an intermediary to facilitate efficient excitation of acoustic GPs by incident MIR radiation, achieving an extinction spectrum peak value of approximately 0.9. The proposed gas sensor based on the acoustic GP resonator is composed of pristine, large-area graphene, an array of periodic metal nanocubes, and a slender polyethylenimine (PEI) layer that adsorbs CO2, situated between the nanocubes and the graphene sheet. The sensing performance of the proposed sensor is numerically investigated. It is demonstrated that by incorporating a CO2-absorbing material into the acoustic GPs, we can perform highly sensitive assessments of the absorption bands within the PEI layer upon CO2 adsorption. The observed behavior of the acoustic GPs indicates a weakening and broadening with increasing CO2 concentrations, a phenomenon attributed to the alteration in the permittivity of the PEI in the interstitial layer due to CO2 adsorption. Numerical simulation results indicate that the sensitivity of the proposed hybrid gas sensor can reach up to 0.0183% ppm−1, which offers a remarkable 95-fold enhancement over the gas sensor based on graphene nanoribbons. Our findings underscore the potential of acoustic GP nanoresonators as a transformative platform for ultrasensitive plasmon-enhanced sensing applications, particularly when integrated with various gas adsorption layers or molecular agents. [ABSTRACT FROM AUTHOR]

This paper introduces a novel, cost-effective solution designed to achieve large absorption bandwidth within the THz spectrum employing a miniaturized, single-layer metamaterial structure. The designed structure features a single circular ring composed of an ohmic resistive sheet with notably higher sheet resistance than traditional metallic resonators. This distinctive design is implemented on a lossy dielectric polyimide substrate with a backing of metallic gold. Our developed absorbing structure demonstrates the capability to achieve a substantial absorption bandwidth ranging from 3.78 to 4.25 THz, maintaining a consistent absorption rate of over 90%. Moreover, we conducted an analysis to assess its absorption performance under various sheet resistance values within the top layer. Additionally, we characterized its angular stability and polarization insensitivity through oblique incident and polarization angle analysis. Finally, an RLC circuital and interference theory approach is adopted to justify its simulated results. The proposed absorber shows potential for a broad spectrum of applications, encompassing communication, imaging, and diverse integrated circuits operating within the THz band. [ABSTRACT FROM AUTHOR]

This paper introduces a novel, cost-effective solution designed to achieve large absorption bandwidth within the THz spectrum employing a miniaturized, single-layer metamaterial structure. The designed structure features a single circular ring composed of an ohmic resistive sheet with notably higher sheet resistance than traditional metallic resonators. This distinctive design is implemented on a lossy dielectric polyimide substrate with a backing of metallic gold. Our developed absorbing structure demonstrates the capability to achieve a substantial absorption bandwidth ranging from 3.78 to 4.25 THz, maintaining a consistent absorption rate of over 90%. Moreover, we conducted an analysis to assess its absorption performance under various sheet resistance values within the top layer. Additionally, we characterized its angular stability and polarization insensitivity through oblique incident and polarization angle analysis. Finally, an RLC circuital and interference theory approach is adopted to justify its simulated results. The proposed absorber shows potential for a broad spectrum of applications, encompassing communication, imaging, and diverse integrated circuits operating within the THz band. [ABSTRACT FROM AUTHOR]

Nowadays, metamaterial absorbers still suffer from limited bandwidth, poor bandwidth scalability, and insufficient modulation depth. In order to solve this series of problems, we propose a metamaterial absorber based on graphene, VO2, gallium silver sulfide, and gold-silver alloy composites with dual-control modulation of temperature and electric field. Then we further investigate the optical switching performance of this absorber in this work. Our proposed metamaterial absorber has the advantages of broad absorption bandwidth, sufficient modulation depth, and good bandwidth scalability all together. Unlike the single inspired layer of previous designs, we innovatively adopted a multi-layer excitation structure, which can realize the purpose of absorption and bandwidth width regulation by a variety of means. Combined with the finite element analysis method, our proposed metamaterial absorber has excellent bandwidth scalability, which can be tuned from 2.7 THz bandwidth to 12.1 THz bandwidth by external electrothermal excitation. Meanwhile, the metamaterial absorber can also dynamically modulate the absorption from 3.8% to 99.8% at a wide incidence angle over the entire range of polarization angles, suggesting important potential applications in the field of optical switching in the terahertz range. [ABSTRACT FROM AUTHOR]

Accurate monitoring of steel plate coating thickness is crucial in construction quality control and durability assessments. To address this challenge, this study introduces a terahertz time-domain reflection spectroscopy based on a BP neural network model to achieve a quantitative visualization characterization of coating thickness. The BP neural network eliminates the inherent dependence of terahertz reflection spectroscopy on the refractive index value in thickness calculation. This trained BP neural network model effectively establishes a functional relationship between signal feature parameters and the corresponding thickness values. Additionally, the proposed model can innovatively measure different coating materials' refractive indexes, revealing the corresponding values for the black paint, white paint, epoxy resin, and rubber as 2.212, 1.967, 1.924, and 2.185, respectively. The experimental results demonstrate the trained BP neural network model possesses remarkable accuracy in predicting coating thickness within the scanning area, achieving a precision level exceeding 96%. This method enables the visualization of coating thickness and the extraction of thickness characterization values. Furthermore, using the thickness imaging results as a reference, the method can accurately identify the thickness abnormalities across the scanning area, locating the position and size of potential defects such as internal scratches and foreign object defects. This innovative approach offers a superior means of monitoring and assessing the thickness distribution quality of the steel plate coating layer materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The optical properties of a graphene-based cylindrical photonic crystal are theoretically investigated based on the generalized cylindrical transfer matrix method to include the out-of-plane propagation of the cylindrical waves. It is found that Maxwell's equations have solutions for both E- and H-polarized waves only at the azimuthal mode number $ m = 1 $ m = 1. We study the reflectance spectrum of the structure to obtain its photonic bandgaps. We show that the reflection spectrum, in addition to the usual Bragg gaps, contains an omnidirectional bandgap at low terahertz frequencies for both E and H-polarized cylindrical waves. The robustness of this bandgap under random variations of geometrical parameters is investigated. This few-period structure is an ideal candidate for guiding waves at the low terahertz region because of its weak absorption and transmission losses. In contrast to the usual Bragg gap, the results show that in both polarizations, the width of graphene-produces bandgap reaches its maximum value when the refractive indices of the two cylindrical shells have their lowest possible values. The width of this unusual omnidirectional bandgap increases by decreasing the thicknesses of the cylindrical shells and increasing the chemical potential of the graphene monolayers. [ABSTRACT FROM AUTHOR]

In this paper, numerical investigations are carried out to explore the characteristics of hybrid plasmon modes at chiroferrite-graphene interface. Kubo formula is utilized for the modeling of monolayer graphene conductivity, and the impedance boundary condition approach is applied at the chiroferrite-graphene interface to obtain the dispersion relation. Frequency dependent normalized propagation constant under the different values of graphene's chemical potential (μ ), relaxation time (τ ), number of graphene layers (N ), temperature (T ), and constitutive parameter of chiroferrite medium such as chirality and gyrotropy are analyzed. Control on graphene and chiroferrite parameters may be useful for chiral sensing, microwave devices, antennas, and magnetic resonance imaging in the terahertz (THz) frequency band. [ABSTRACT FROM AUTHOR]

The increasing utilization of terahertz (THz) bandwidth in both industrial and private sectors highlights the significance of efficient terahertz shielding and absorption devices. These devices play a crucial role in safeguarding electronic components from disruptive effects and rendering objects less detectable by radar systems. However, the limited availability of materials and devices hinders progress in this field. In this study, a strain engineering route is presented for the active control of terahertz shielding and absorption properties in 3D graphene through the application of mechanical strain. A straintronic modulator based on 3D graphene is demonstrated, capable of modulating absorption and reflection of THz radiation in real‐time over a wide range of 0.1–3 THz. The modulator can be tuned to exhibit either shielding capability with a specific shielding effectiveness of 0.3 × 105 dB cm2 g−1 or stealth characteristics with an average reflection loss of 25 dB and 99.4% absorption. These findings open new avenues for leveraging 2D materials in their 3D porous form, where strain‐induced changes in interlayer interactions enable control over the properties of these materials. This discovery unveils vast unexplored physical phenomena with immense potential for advanced THz imaging, radar, and electromagnetic applications. [ABSTRACT FROM AUTHOR]

High-performance alloy thin films and large-sized thin film wafers for infrared applications are the focus of international researchers. In this study, doped Ge1−xSnx and Ge1−yBiy semiconductor alloy films were grown on a 5-in. silicon (Si) wafer using high-quality Ge films as buffer layers. An efficient technique is presented to reduce the dark current density of near-infrared photoelectric devices. By using boron for p-type doping in Ge1−xSnx films and bismuth (Bi) for n-type doping in Ge1−yBiy films, an all-thin film planar nano-p-i-n optoelectronic device with the structure n-Ge1−yBiy/i-GeSn/p-Ge1−xSnx/Ge buffer/Si substrate has been successfully fabricated. The photoelectric performance of the device was tested, and it was found that the insertion of p-Ge1−xSnx/Ge films reduced the dark current density by 1–2 orders of magnitude. The maximum photoresponsivity reached up to 0.8 A/W, and the infrared photocurrent density ranged from 904 to 935 μA/cm2 under a +1 V bias voltage. Furthermore, the device is capable of modulating a terahertz wave using a voltage signal with a modulation bandwidth of 1.2 THz and a modulation depth of ∼83%, while the modulation rate is 0.5 MHz. This not only provides a clear demonstration of how doped alloy films and the development of nano-p-i-n heterojunctions will improve photoelectric devices' performance in the near-infrared and terahertz bands, but it also raises the possibility of optoelectronic interconnection applications being achieved through a single device. [ABSTRACT FROM AUTHOR]

Conventional chiral metasurfaces are constrained by predetermined functionalities and have limited versatility. To address these constraints, we propose a novel chirality-switchable terahertz (THz) metasurface with integrated heating control circuits tailored for spin-selective anomalous reflection, leveraging the phase-change material vanadium dioxide (VO2). The reversible and abrupt insulator-to-metal phase transition feature of VO2 is exploited to facilitate a chiral meta-atom with spin-selectivity capabilities. By employing the Pancharatnam–Berry phase principle, complete 2 π reflection phase coverage is achieved by adjusting the orientation of the chiral structure. At the resonant frequency of 0.137 THz, the designed metasurface achieves selective absorption of a circularly polarized wave corresponding to the state of the VO2 patches. Concurrently, it reflects the circularly polarized wave of the opposite chirality anomalously at an angle of 28.4° while maintaining its handedness. This chirality-switchable THz metasurface exhibits promising potential across various applications, including wireless communication data capacity enlargement, polarization modulation, and chirality detection. [ABSTRACT FROM AUTHOR]

A broadband absorber based on metamaterials of graphene and vanadium dioxide (VO2) is proposed and investigated in the terahertz (THz) regime, which can be used for switch applications with a dynamically variable bandwidth by electrically and thermally controlling the Fermi energy level of graphene and the conductivity of VO2, respectively. The proposed absorber turns 'on' from 1.5 to 5.4 THz, with the modulation depth reaching 97.1% and the absorptance exceeding 90% when the Fermi energy levels of graphene are set as 0.7 eV, and VO2 is in the metallic phase. On the contrary, the absorptance is close to zero and the absorber turns 'off' with the Fermi energy level setting at 0 eV and VO2 in the insulating phase. Furthermore, other four broadband absorption modes can be achieved utilizing the active materials graphene and VO2. The proposed terahertz absorber may benefit the areas of broadband switch, cloaking objects, THz communications and other applications. [ABSTRACT FROM AUTHOR]

Collective spin arrangements manifest diverse spin textures, encompassing ferromagnetism, antiferromagnetism, and chiral vortices. However, mapping these spin textures in ultrathin magnetic multilayers has remained elusive. A reconfigurable chiral spintronic terahertz emission method is introduced through investigations on a model system of synthetic antiferromagnet and the spin information of individual ferromagnet (FM) layers is extracted. Upon femtosecond photoexcitation of the synthetic antiferromagnet (FM1/Ru/FM2), the ferromagnets generate a pair of linearly polarized ultrafast spin currents, which, after relaxation at the FM/Ru interface, emit corresponding THz pulses. The Ruderman–Kittel–Kasuya–Yosida interactions between the two FMs give rise to magnetic‐field‐controlled spin textures causing a spin relaxation imbalance at the interfaces and induce a phase shift between the orthogonal components of the emitted terahertz fields, consequently reconfiguring the polarization of the emitted terahertz field from linear to circular. This approach offers a novel means of investigating electronic and magnetic states in ultrathin spintronic heterostructures, low‐dimensional quantum materials, topological insulators, and Weyl semimetals. Furthermore, it enables the exploration of spin textures, including skyrmions, merons, and solitons. [ABSTRACT FROM AUTHOR]

Plastic foams, near-ubiquitous in everyday life and industry, show properties that depend primarily on density. Density measurement, although straightforward in principle, is not always easy. As such, while several methods are available, plastic foam industry is not yet supported with a standard technique that effectively enables to control density maps. To overcome this issue, this paper proposes Terahertz (THz) time-of-flight imaging using normal reflection measurements as a fast, relatively cheap, contactless, non-destructive and non-dangerous way to map plastic foam density, based on the expected relationship between density and refractive index. The approach is demonstrated in the case of polypropylene foams. First, the relationship between the estimated effective refractive index and the polypropylene foam density is derived by characterizing a set of carefully crafted samples having uniform density in the range 70–900 kg/m3. The obtained calibration curve subtends a linear relationship between the density and the refractive index in the range of interest. This relationship is validated against a set of test samples, whose estimated average densities are consistent with the nominal ones, with an absolute error lower than 10 kg/m3 and a percentage error on the estimate of 5%. Exploiting the calibration curve, it is possible to build quantitative images depicting the spatial distribution of the sample density. THz images are able to reveal the non-uniform density distribution of some samples, which cannot be appreciated from visual inspection. Finally, the complex spatial density pattern of a graded foam sample is characterized and quantitatively compared with the density map obtained via X-ray microscopy. The comparison confirms that the proposed THz approach successfully determines the density pattern with an accuracy and a spatial scale variability compliant with those commonly required for plastic foam density estimate. [ABSTRACT FROM AUTHOR]

In this communication, the design and fabrication of optical active metamaterials were developed by the incorporation of graphene and joining it to different substrates with variable spectroscopical properties. It focuses on how graphene and its derivatives could generate varied optical setups and materials considering modified and enhanced optics within substrates and surfaces. In this manner, it is discussed how light could be tuned and modified along its path from confined nano-patterned surfaces or through a modified micro-lens. In addition to these optical properties generated from the physical interaction of light, it should be added that the non-classical light pathways and quantum phenomena could participate. In this way, graphene and related carbon-based materials with particular properties, such as highly condensed electronics, pseudo-electromagnetic properties, and quantum and luminescent properties, could be incorporated. Therefore, the modified substrates could be switched by photo-stimulation with variable responses depending on the nature of the material constitution. Therefore, the optical properties of graphene and its derivatives are discussed in these types of metasurfaces with targeted optical active properties, such as within the UV, IR, and terahertz wavelength intervals, along with their further properties and respective potential applications. [ABSTRACT FROM AUTHOR]

Water and graphene have been used separately to effectively broaden absorption bandwidth in the design of metamaterial absorbers (MAs). In this paper, a broad terahertz MA is presented, in which water and graphene are used together for improving absorption performance. The absorptivity of the presented MA exceeds 90% over the frequency range of 2.81–6.74 THz. It is found that the proposed MA has superior absorption performance to those using water or graphene separately. The impedance matching theory as well as the distribution of the field and power loss are used to provide a deep insight into the absorption mechanism. Furthermore, the proposed MA exhibits excellent thermal stability as well as good insensitivity to polarization and incident angle. This work provides a reference for designing broadband MA in terahertz frequency band. [ABSTRACT FROM AUTHOR]

The diversity and practicability of terahertz metamaterials have experienced rapid development in the past decade due to the increasing demand for various devices. This topic has attracted significant interest from researchers. Among the key functional devices in terahertz metamaterial systems, the active control ability of terahertz metamaterials is highly valuable and captivating. This implies that the electromagnetic properties of metamaterials can be modulated over a wide dynamic range by external stimuli. This review categorizes the different types of tunable terahertz metamaterials based on the external stimuli to which they respond, namely, mechanical modulation, electrical modulation, magnetic modulation, and optical modulation. Mechanically modulated devices offer simple yet efficient modulation, while electrical and magnetic modulation provide effective active modulation through electrical mechanisms. Optical modulation, in contrast, focuses on incorporating various materials to achieve active modulation. [ABSTRACT FROM AUTHOR]

In this study, we explored the manipulation of optical properties in the terahertz (THz) frequency band of radio-frequency (RF) sputtered indium tin oxide (ITO) thin films on highly resistive silicon substrate by rapid thermal annealing (RTA). The optical constants of as-deposited and RTA-processed ITO films annealed at 400 °C, 600 °C and 800 °C are determined in the frequency range of 0.2 to 1.0 THz. The transmittance can be changed from ~27% for as-deposited to ~10% and ~39% for ITO films heat-treated at different annealing temperatures (Ta's). Such variations of optical properties in the far infrared for the samples under study are correlated with their mobility and carrier concentration, which are extracted from Drude–Smith modeling of THz conductivity with plasma frequency, scattering time and the c-parameters as fitting parameters. Resistivities of the films are in the range of 10−3 to 10−4 Ω-cm, confirming that annealed ITO films can potentially be used as transparent conducting electrodes for photonic devices operating at THz frequencies. The highest mobility, μ = 47 cm2/V∙s, with carrier concentration, Nc = 1.31 × 1021 cm−3, was observed for ITO films annealed at Ta = 600 °C. The scattering times of the samples were in the range of 8–21 fs, with c-values of −0.63 to −0.87, indicating strong backscattering of the carriers, mainly by grain boundaries in the polycrystalline film. To better understand the nature of these films, we have also characterized the surface morphology, microscopic structural properties and chemical composition of as-deposited and RTA-processed ITO thin films. For comparison, we have summarized the optical properties of ITO films sputtered onto fused silica substrates, as-deposited and RTA-annealed, in the visible transparency window of 400–800 nm. The optical bandgaps of the ITO thin films were evaluated with a Tauc plot from the absorption spectra. [ABSTRACT FROM AUTHOR]

Water is vital for life, and without it, biomolecules and cells cannot maintain their structures and functions. The remarkable properties of water originate from its ability to form hydrogen-bonding networks and dynamics, which the connectivity constantly alters because of the orientation rotation of individual water molecules. Experimental investigation of the dynamics of water, however, has proven challenging due to the strong absorption of water at terahertz frequencies. In response, by employing a high-precision terahertz spectrometer, we have measured and characterized the terahertz dielectric response of water from supercooled liquid to near the boiling point to explore the motions. The response reveals dynamic relaxation processes corresponding to the collective orientation, single-molecule rotation, and structural rearrangements resulting from breaking and reforming hydrogen bonds in water. We have observed the direct relationship between the macroscopic and microscopic relaxation dynamics of water, and the results have provided evidence of two liquid forms in water with different transition temperatures and thermal activation energies. The results reported here thus provide an unprecedented opportunity to directly test microscopic computational models of water dynamics. [ABSTRACT FROM AUTHOR]

We evaluate the terahertz (THz) detectors based on field-effect transistor (FET) with the graphene channel (GC) and a floating metal gate (MG) separated from the GC by a black-phosphorus (b-P) or black-arsenic (b-As) barrier layer. The operation of these GC-FETs is associated with the heating of the two-dimensional electron gas in the GC by impinging THz radiation leading to thermionic emission of the hot electrons from the GC to the MG. This results in the variation of the floating gate potential, which affects the source–drain current. At the THz radiation frequencies close to the plasmonic resonance frequencies in the gated GC, the variation of the source–drain current and, hence, the detector responsivity can be resonantly large. [ABSTRACT FROM AUTHOR]

In this study we investigate a relatively new material for terahertz (THz) photoconductive antennas (PCAs): GaAs epitaxial films grown on (111)A-oriented semi-insulated GaAs substrates and doped with Si. GaAs:Si (111)A films with the required high resistance without postgrowth annealing have been grown by molecular-beam epitaxy. The Si doping of (111)A-oriented GaAs was expected to lead to the formation of acceptors that facilitate the activation of As Ga + traps. The relaxation times of nonequilibrium charge carriers in the films were measured in pump-probe experiment, and, finally, the electron mobility was estimated from the carrier lifetime and current–voltage characteristics measured under pulsed pumping by Ti:sapphire femtosecond laser. Dipole and bow-tie PCAs were fabricated, and the THz emission spectra and the total THz emission power were measured with varying applied voltage and optical excitation power. The properties of GaAs:Si (111)A films and the PCAs based on them are compared with LTG-GaAs (100) and (111)A-oriented films. [ABSTRACT FROM AUTHOR]

We demonstrate a metamaterial (MM) design capable of showing linear broadband polarization conversion over the terahertz (THz) frequency range. The building block of the proposed MM structure is composed of a strip and four split ring resonators (SRRs), which are coupled through their near fields. To examine co- and cross-polarization transmission amplitudes, we gradually increase the distance between the strip and SRRs. When the SRRs are near (S = 2 μ m) the strip, maximum cross-polarization conversion is attained with a resonance mode hybridization effect in the co-polarization transmission due to strong near-field coupling between the strip and SRRs. When the SRRs moved away from the strip (S = 22 μ m), minimum cross-polarization conversion is attained due to weak coupling between the strip and SRRs. This MM system exhibits a transition from a strongly coupled state to a weakly coupled state with the rise in displacement between the strip and SRRs. The ability to tune the linear polarization conversion can be useful in the improvement of efficient THz polarization rotation devices. The proposed MM structure can be used in other frequency domains, like the microwave and visible range, by scaling up/down the structure. [ABSTRACT FROM AUTHOR]

Optical elements based on nanomaterials are becoming major avenues to satisfy the technological requirements of compact, lightweight, and tunable elements of the emerging terahertz (THz) field. A new generation of diffractive components integrating specific geometry with additional features (flexibility, stretchability, rotation, and other approaches for tuning properties) extends the functionality of wavefront control. Here, an innovative approach is demonstrated to control the THz wavefront via a layered composition of spiral zone plates (SZPs) with tunable mutual orientation and scaling. As a proof of concept, the SZP is designed using Laguerre‐Gauss mode analysis with further fabrication and experimental characterization of the resultant vortex beams. For each single SZP, a flexible element is proposed based on a thin film of single‐walled carbon nanotubes deposited on a stretchable substrate. Thus, this diffraction element can be tuned not only by rotation (along the azimuthal direction), but also by its stretching (in the radial direction). The spatial tuning of the developed SZPs (spiral zone plates) opens up an efficient, convenient, and highly customizable approach for the manipulation of vortex beams. [ABSTRACT FROM AUTHOR]

Graphene can support surface plasmon polaritons (SPPs) in the terahertz band, and graphene SPP sensors are widely used in the field of terahertz micro- and nano-optical devices. In this paper, we propose an H-shaped graphene metasurface and investigate the plasmon-induced transparency (PIT) phenomenon in the proposed structure using the finite-difference time-domain (FDTD) method. Our results show that the Fermi energy levels, as well as certain shape parameters, can effectively modulate the PIT phenomenon in the proposed structure. Interestingly, changing some of these shape parameters can excite two dips into three. In terms of sensing performance, the maximum values of sensitivity and figure of merit (FOM) are 1.4028 THz/RIU and 17.97, respectively. These results offer valuable guidance for the use of terahertz optical graphene SPP sensors. [ABSTRACT FROM AUTHOR]

Metasurfaces represent a new frontier in materials science paving for unprecedented methods of controlling electromagnetic waves, with a range of applications spanning from sensing to imaging and communications. For pulsed terahertz (THz) generation, metasurfaces offer a gateway to tuneable thin emitters that can be utilized for large‐area imaging, microscopy, and spectroscopy. In literature, THz‐emitting metasurfaces generally exhibit high absorption, being based either on metals or on semiconductors excited in highly resonant regimes. Here, the use of a fully dielectric semiconductor exploiting morphology‐mediated resonances and inherent quadratic nonlinear response is proposed. This system exhibits a remarkable 40‐fold efficiency enhancement compared to the unpatterned at the peak of the optimized wavelength range, demonstrating its potential as a scalable emitter design. [ABSTRACT FROM AUTHOR]

Wireless communication networks could become quicker and more dependable as sixth generation (6G) antennas develop. One difficult development in the field of wearable technology is wearable textile antennas. Wearable textile antennas require flexible building materials, primarily textiles with planar structures. This study will concentrate on the design and specification of microstrip rectangular patch antennas that use a variety of fabrics as the substrate, such as lycra, polyester, and washed cotton. Using two-dimensional (2D) materials in the terahertz (THz) range, the study presented here will help in the construction of appropriate wearable antennas. This work may significantly improve materials science and engineering by investigating and using 2D materials, such as tungsten disulfide, in antenna design. The suggested antennas’ resonance frequencies are 1.1254 THz for polyester substrates, 4.4019 THz for washed cotton, and 2.9861 THz for lycra substrates. For substrate materials such as lycra, polyester, and washed cotton, the measured return loss was -44.92 dB, -38.17 dB, and -20.75 dB. This study could lead to the creation of new technologies and materials, such tungsten disulfide, which would have far-reaching uses outside of wearable electronics and provide significant advantages for society. [ABSTRACT FROM AUTHOR]

Metasurfaces suitable for the terahertz gap are alternatives to naturally occurring materials and could accelerate the development of terahertz flat optics integrated with terahertz continuous-wave sources. However, metasurfaces have yet to be commonly adopted in terahertz devices that require a number of specific material properties because of the many choices in meta-atom design. In this paper, we demonstrate that simple dimension control of a single kind of meta-atom enables the design of a wide range of refractive indices from large positive to negative values in the 0.3-THz band. Measurements by terahertz time-domain spectroscopy verify three kinds of metasurfaces with (1) an extremely high refractive index of 12.3 + j0.88 and reflectance of 5.1% at 0.31 THz, (2) a zero refractive index of − 0.44 + j0.12 and reflectance of 2.6% at 0.34 THz, and (3) a negative refractive index of − 5.4 + j0.32 and reflectance of 22.7% at 0.31 THz. The 0.3-THz band is a frequency band for candidates to 6G wireless communications. Our results offer an accessible material platform for terahertz flat optics such as metalenses, antennas, and phase plates. Terahertz flat optics based on our presented metasurfaces would be a welcome contribution to the development of terahertz industries, including 6G wireless communications. [ABSTRACT FROM AUTHOR]

Terahertz (THz) backward wave oscillators (BWOs) hold immense potential for a broad range of industrial and military applications. This study presents a comparative analysis of 0.34 THz sheet beam (SB) and circular beam (CB) folded waveguide (FWG) BWOs. We examined the design, simulation (CST MWS and PS, HFSS), and performance, revealing that sheet beam BWO outperforms circular beam BWO in terms of interaction impedance, power, efficiency, and bandwidth. Under 20 kV beam voltage and 10 mA beam current conditions, sheet beam BWO achieves 0.47 Ω (0.34 THz) interaction impedance, 0.65 W output power, and a bandwidth of approximately 12 GHz, surpassing the 0.18 Ω , 0.23 W, and 8.5 GHz values of circular beam BWO, respectively. Furthermore, this study encompasses the fabrication and thorough characterization of the sheet beam BWO's slow-wave structure. Experimental validation confirms its effectiveness, with measured S 11 demonstrating reflection below - 10 dB and S 21 exhibiting transmission above - 2 dB. [ABSTRACT FROM AUTHOR]

Herein transparent iron nanosheets deposited by the ionic coating technique onto glass and WO3 dielectric lenses are studied and characterized. The thickness of Fe nanosheets is varied in the range of 70–350 nm. It is observed that the transmittance and reflectance of the Fe nanosheets are highly affected by the layer roughness. Coating of iron nanosheets onto WO3 dielectric lenses increases the light absorption of WO3 by more than 240 times and red‐shifts the energy bandgap. Remarkable enhancements in the dielectric constant and in the optical conductivity are achieved via Fe coatings. In addition, iron coated dielectric lenses show higher terahertz cutoff limits varying in the range of 1.0–30 THz. Iron nanosheets remarkably increase the free charge carrier density and plasmon frequency in the infrared range of light. Moreover, the temperature dependent electrical conductivity shows high temperature stability and an increased electrical conductivity by more than 7 orders of magnitude by coating WO3 with 70 nm thick Fe nanosheets. The stability of the electrical conductivity at low temperatures and the wide range of terahertz cutoff limits in addition to the well‐enhanced light absorbability makes the iron coated tungsten oxide dielectric lenses promising for multifunction optoelectronic applications. [ABSTRACT FROM AUTHOR]

In this paper, we propose a tunable metal-graphene hybrid metamaterial modulator based on the electromagnetically induced transparency (EIT) effect, which can be applied in the terahertz modulation field. The device consists of a U-shaped sliver strip and a rectangular sliver ring. The proposed structure is coupled by bright-bright EIT mode to generate a narrow-band electromagnetically induced transparent window. In the modulation process, the transparent window only changes in amplitude, keeping the resonant frequency constant and improving the device's resistance to interference. Active tunability is achieved by adjusting the bias voltage to alter the Fermi level of the graphene film layer, thereby facilitating modulation performance adjustment. The transparent window has a narrow bandwidth of 0.053 THz, with the transparent peak located at 0.773 THz. The maximum modulation depth of the proposed device is 74.1% and the maximum positive group delay in the transparent window can reach 0.01 ns. The designed structure has the advantages of terahertz modulation and slow light field due to its narrow-band tunability. This work provides a potential solution for electromagnetic wave control for 6G wireless communication. [ABSTRACT FROM AUTHOR]

In this paper, by finite-difference-time-domain (FDTD) simulation, we numerically demonstrate a plasmon-induced transparency (PIT) effect based on a carbon nanotubes (CNT) metamaterial. The proposed CNT metamaterials are regular arrays of unit cells composed of a cut wire and two rectangular split ring resonators (RSRRs). Under the excitation of a TM-polarization terahertz wave, a single PIT transparency window is achieved for the symmetry CNT metamaterial. By breaking the symmetry of the proposed structure with different sizes of RSRRs, a double PIT window is realized. Results show that the PIT effect is originated from the destructive interference of the three bright modes. A coupled harmonic oscillator model is used to describe the destructive interference between the three bright modes, and the results agree well with the FDTD simulation. The effect of geometrical sizes, like coupling distance, splitting size, and opening direction between two RSRRS on the PIT window is analyzed in detail. Further, the slow light performance of the proposed asymmetric CNT metamaterial is investigated, and a maximum time delay of 1.71 ps is obtained. Finally, an extensive sensitivity of 1.12 THz/RIU is obtained. Therefore, the proposed CNT-based device exhibits numerous potential applications in THz slow light, sensors, and modulators. [ABSTRACT FROM AUTHOR]

This research article presents, a design of a graphene-based, octagonal shaped three element series feed terahertz (THz) linear array antenna is designed for very short-distance wireless communication applications. The antenna utilizes a four-layer structure comprising a gold ground plate and patch, with quartz (SiO4) serving as the substrate with a thickness of 140 µm deposited on the gold ground plate. On this substrate, an octagonal three-element linear array patch made of gold is applied, and finally, a fourth layer of elliptical-shaped graphene, with a thickness of 10 nm, is deposited onto the gold patch plate. The THz linear array antenna demonstrates good performance, with a reflection coefficient value of less than − 10 dB in the 0.285–0.535 THz range, resulting in a substantial 250 GHz bandwidth and the peak realized gain value is 5.25 dBi. Further, the designed antenna employs material switching via change in the chemical potential of graphene to attain pattern reconfigurability and facilitates beam steering capabilities for wireless applications. [ABSTRACT FROM AUTHOR]

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A novel, highly efficient terahertz fully polarized transmission line is designed by two-dimensional tellurium photonic crystals consisting of square lattice rod arrays with a complete photonic bandgap. The TE and TM photonic bandgaps of the tellurium photonic crystals, which are computed by plane wave expansion, happen to coincide, and the complete photonic bandgap covers from 2.894 to 3.025 THz. The function of the designed waveguide is simulated by the finite element method, and the transmission characteristics are optimized by accurately adjusting its structural parameters. The transmission efficiency of the waveguide for TE mode achieves a peak value of −0.34 dB at a central frequency of 2.950 THz and keeps above −3 dB from 2.82 THz to 3.02 THz, obtaining a broad relative bandwidth of about 6.84 percent. The operating bandwidth of the tellurium photonic crystals' waveguide for TM mode is narrower than that of TE mode, whose relative bandwidth is about 4.39 percent or around 2.936 THz above −5 dB. The designed terahertz photonic crystals' waveguide can transmit both TE and TM waves, and not only can it be used as a high-efficiency transmission line, but it also provides a promising approach for implementing fully polarized THz devices for future 6G communication systems. [ABSTRACT FROM AUTHOR]

High-entropy alloys (HEAs) provide new research avenues for alloy combinations in the periodic table, opening numerous possibilities in novel-alloy applications. However, their electrical characteristics have been relatively underexplored. The challenge in establishing an HEA electrical conductivity model lies in the changes in electronic characteristics caused by lattice distortion and complexity of nanostructures. Here we show a low-frequency electrical conductivity model for the Nb-Mo-Ta-W HEA system. The cocktail effect is found to explain trends in electrical-conductivity changes in HEAs, while the magnitude of the reduction is understood by the calculated plasma frequency, free electron density, and measured relaxation time by terahertz spectroscopy. As a result, the refractory HEA Nb15Mo35Ta15W35 thin film exhibits both high hardness and excellent conductivity. This combination of Nb15Mo35Ta15W35 makes it suitable for applications in atomic force microscopy probe coating, significantly improving their wear resistance and atomic-scale image resolution. The transport behavior of high-entropy alloys (HEAs) remains unclear. Here, the authors explore the fundamentals of low-wear and high-conductivity refractory HEAs, examining the cocktail effect in conductivity, and highlight its potential applications in enhancing atomic-scale image resolution. [ABSTRACT FROM AUTHOR]

2D materials, with outstanding optical, thermal, and electric properties, are emerging as promising candidates for fabricating high‐performance photodetectors. Recently, impressive progresses have been made in this area and some challenges are remaining to improve the properties of photodetectors. As one important part in the mainstream photodetection mechanisms, photothermoelectric (PTE) effect is showing unique priorities in fabricating advanced photodetectors, especially broadband detection operating in the mid‐infrared and terahertz spectral regime. Here, recent progress on PTE photodetectors based on layered 2D materials is reviewed. The physical mechanism of PTE effect is first discussed and then the optical and thermoelectric properties of various 2D materials are analyzed. Furthermore, strategies to improve the photodetection performance of PTE detectors are summarized in two major categories including enhanced photothermal conversion and thermoelectric conversion processes. Finally, the challenges and prospects for future research in 2D thermoelectric materials and PTE detectors are also provided. [ABSTRACT FROM AUTHOR]

Here, a novel design of a microstructured quad‐band terahertz metamaterial absorber is presented consisting of a copper‐based merged circular and square split ring resonator‐integrated over a lead glass substrate intended to detect changes in the surrounding medium's refractive index for gas sensing applications through THz spectroscopy. Outstanding performance is shown by the suggested absorber, which achieves a maximum absorption rate of 99.4%, 98.7%, 99.2%, and 99.4%, at resonance frequencies of 8.055 (f1$f_1$), 8.423 (f2$f_2$), 9.257 (f3$f_3$), and 9.507 (f4$f_4$) THz, respectively. The quality factors (Q‐factors) attained are 201, 129, 298, and 271 for these four absorption bands. The obtained sensitivity values at these four distinct bands are 3898, 2927, 4719, and 4102 GHz per refractive index unit with high figure of merits (FoM) of 97, 95, 152, and 117, respectively, throughout a range of refractive indices from n = 1.00 to n = 1.05 (with a step size of 0.005). The study's novelty lies in its novel design and remarkable ability to excel in gas sensing with high sensitivity and high FoM across not just one but four distinct THz bands. Furthermore, each band demonstrates significantly superior performance in sensitivity, Q‐factor, and FoM, among other parameters compared to prior research in both gas detection and RI monitoring scenarios, hence exhibiting its potential across a wide range of applications. [ABSTRACT FROM AUTHOR]

For decades, it was considered all but impossible to perform Stark spectroscopy on molecules in a liquid solution, because their concomitant orientation to the applied electric field results in overwhelming background signals. A way out was to immobilize the solute molecules by freezing the solvent. While mitigating solute orientation, freezing removes the possibility to study molecules in liquid environments at ambient conditions. Here we demonstrate time-resolved THz Stark spectroscopy, utilizing intense single-cycle terahertz pulses as electric field source. At THz frequencies, solute molecules have no time to orient their dipole moments. Hence, dynamic Stark spectroscopy on the time scales of molecular vibrations or rotations in both non-polar and polar solvents at arbitrary temperatures is now possible. We verify THz Stark spectroscopy for two judiciously selected molecular systems and compare the results to conventional Stark spectroscopy and first principle calculations. Stark spectroscopy of molecules in liquid solutions was once challenging due to orientation effects, solved by freezing but limiting ambient studies. Now, THz Stark spectroscopy with intense terahertz pulses enables dynamic analysis of molecules in both non-polar and polar solvents at any temperature, advancing conventional methods. [ABSTRACT FROM AUTHOR]

ZnNb 2- x V x /2 O 6-1.25 x (x = 0.1, 0.2, 0.4, and 0.6) ceramics were synthesized using a conventional solid phase reaction technique. The terahertz dielectric performances of ZnNb 2- x V x /2 O 6-1.25 x (x = 0.1, 0.2, 0.4, and 0.6) ceramics have been investigated using Rietveld refinement, scanning electron microscopy, first principles, and terahertz time-domain spectroscopy. XRD and mapping data indicate that the replacement of Nb5+ ions with V5+ ions induces variations in inter-ion interactions. Combined analysis with scanning electron microscopy indicated that the authigenic low melting point phase (ZnV 2 O 6) had a positive effect on the densification of the specimen (ZnNb 1.8 V 0.1 O 5.75) at low temperatures. However, more low melting point phases (ZnV 2 O 6 and Zn 2 V 2 O 7) adversely affect the densification and terahertz dielectric performances of the specimens as the V5+ content increases. In addition, the dielectric constant of the specimen is mainly affected by the ionicity of the Nb–O bond, while the dielectric loss is jointly affected by the covalency of the Nb–O bond and the grain size. The developed materials are expected to be used in LTCC terahertz application due to the low sintering temperature (950 °C) and excellent dielectric performance (dielectric constant of ∼20 and dielectric loss of 0.02–0.04). [ABSTRACT FROM AUTHOR]

The Space High-cadence Observing Telescope (SHOT) is proposed to conduct a wide-field survey in one of the frequency bands between 1 and 3 THz with an aperture of 0.8 m. It will provide complete terahertz and far-infrared photometry with low-resolution spectroscopy throughout the sky, promoting the study of fast radio bursts, the formation and evolution of stars, and other scientific goals. The telescope will adopt a superconducting kinetic inductance detector to achieve a wide survey of terahertz imaging. This work designs the optical system of SHOT to obtain a large FOV of up to 1.5 deg and match the detector with the size of 60 mm × 60 mm and pixel pitch of 0.8 mm. The primary optical system adopts a Ritchey–Chrétien design. By adding a corrector lens to the optical system and optimization, the aberrations mainly caused by the optical elements in the detector system are reduced. The influence of each component assigned to different terahertz materials is analyzed. The results show that TPX is more suitable than the other two materials. The performance of the optimized optical system is close to its diffraction limit. It will have the characteristics of high image quality and compact layout. [ABSTRACT FROM AUTHOR]

A mid-infrared to terahertz ultra-broadband all-dielectric absorber is proposed and numerically verified, which consists of a single medium of tantalum pentoxide (Ta2O5) with pyramid arrays on a film substrate. Absorption is higher than 0.9 between 0.72 and 100 THz with a bandwidth of 99.28 THz and a relative bandwidth of 197%. It is higher than 0.9999 between 2 and 100 THz. The ultra-broadband absorption in 0.72~100 THz results from the particular permittivity of Ta2O5 and the optimized structure. The inner mechanism is studied by the spectra and impedance match of different parts of the structure and the distributions of electromagnetic fields and loss at different frequencies. Furthermore, the ultra-broadband perfect absorber is insensitive to polarization angle, incident angle, structure parameters as well as structure shape. These characteristics make the proposed absorber more practical in mid-infrared to terahertz ultra-broadband stealth, imaging and thermal emission. [ABSTRACT FROM AUTHOR]

Herein, it is demonstrated that ultrashort spatially structured beams can sculpt a sample of gas‐phase molecules like a 4D material to produce a spatial pattern of aligned molecules whose shape and temporal evolution allow to restore the spatial light information on a time‐delayed reading pulse. To do so, the spatial phase and amplitude information of ultrashort light beams are encoded into rotational coherences of molecules by exploiting the interplay between spin angular momentum and orbital angular momentum. The field‐free molecular alignment resulting from the interaction leads to an inhomogeneous spatial structuring of the sample allowing to transfer the encoded information into a time‐delayed probe beam. The demonstration is conducted in CO2$\left(\text{CO}\right)_{2}$ molecules. Besides applications in terms of THz bandwidth buffer memory, the strategy features interesting prospects for establishing versatile optical processing of orbital angular momentum (OAM) fields, for studying various molecular processes, or for designing new photonic devices enabling to impart superpositions of OAM modes to light beams. [ABSTRACT FROM AUTHOR]

In this paper, an absorber with multi-band, tunable, high Q, and high sensitivity, based on terahertz periodic two-dimensional patterned graphene surface plasmon resonance (SPR), is proposed. The absorber consists of a bottom metal film separated by a periodically patterned graphene metamaterial structure and a SiO2 dielectric layer, where the patterned graphene layer is etched by "+" and "L" shapes and circles. It has simple structural features that can greatly simplify the fabrication process. We have analyzed the optical properties of a graphene surface plasmon perfect metamaterial absorber based on graphene in the terahertz region using the finite-difference method in time domain (FDTD). The results show that the absorber device exhibits three perfect absorption peaks in the terahertz bands of f1 = 1.55 THz, f2 = 4.19 THz, and f3 = 6.92 THz, with absorption rates as high as 98.70%, 99.63%, and 99.42%, respectively. By discussing the effects of parameters such as the geometrical dimensions of patterned graphene metamaterial structure "+" width W1, "L" width W2, circular width R, and the thickness of the dielectric layer on the absorption performance of absorber, as well as investigating the chemical potential and relaxation time of patterned-layer graphene material, it was found that the amplitude of the absorption peaks and the frequency of resonance of absorber devices can be dynamically adjusted. Finally, we simulated the spectra as the surrounding refractive index n varied to better evaluate the sensing performance of the structure, yielding structural sensitivities up to 382 GHz/RIU. Based on this study, we find that the results of our research will open new doors for the use of multi-band, tunable, polarization-independent metamaterial absorbers that are insensitive to large-angle oblique incidence. [ABSTRACT FROM AUTHOR]

A multifunctional switchable metamaterial device based on graphene, a gold layer, polyimide, vanadiµm dioxide (VO2), and the sapphire substrate is designed in this paper. The top layer consists of a gold wire, graphene, and two split-ring resonators with the same parameters. By adjusting the Fermi level of graphene, the regulation of BIC and quasi-BIC is realized, and the conversion between BIC and absorber is realized by adjusting the conductivity of VO2. When the device is converted into a wave-absorbing device with single-band absorption characteristics, the Fermi level of graphene at this time is 0.001 eV, the absorption peak at 0.820 THz is higher than 99.5%, and when the Fermi level of regulated graphene is 1 eV, the absorption peak at 0.667 THz is also higher than 99.5%. The peak frequency of the device is 0.640 THz when it converts to quasi-BIC. To the best of our knowledge, this is the first time that the conversion and regulation of BIC and absorber have been achieved using these two phase change materials. Moreover, by adjusting the parameters of the metamaterial structure, the working efficiency and frequency of BIC and absorber can be dynamically adjusted. The electric field distribution and surface current of metamaterials are further studied, and the physical mechanism of effective absorption and BIC is discussed. These results show that the metamaterials proposed in this paper have many advantages, such as terahertz absorption, BIC, and active device control, and are of great significance for developing terahertz multifunctional devices. [ABSTRACT FROM AUTHOR]

Terahertz (THz) waves have gained considerable attention in the rising 6G communication due to their large bandwidth. However, the cost and power consumption become the major constraints for the commercialization of 6G THz systems as the frequency increases. Reconfigurable intelligent surface (RIS) comprising active metasurfaces and digital controllers has been proposed for beamforming in the 6G multiple-input-multiple-output systems, showing good potential to suppress the system size, weight, and power consumption (SWaP). Currently, their controlling diodes can hardly work up to THz frequencies. Therefore, several active stimuli have been investigated as alternatives. Among them, chalcogenide phase-change material Ge2Sb2Te5 (GST) addresses large modulation depth, picosecond switching speed, and non-volatile properties. Notably, the non-volatile GST may enable RIS systems with memory and low control power. This work briefly reviews the advances of GST-tuned THz metamaterials (MTMs), discusses the current obstacles to overcome, and gives a perspective of GST applications in the rising 6G communications. [ABSTRACT FROM AUTHOR]