Accurate permittivity characterization at terahertz frequencies is important for material analysis and device design, yet it remains challenging for small-volume samples and compact test structures. In this work, a terahertz permittivity sensor based on a spoof surface plasmon polariton (SSPPs) transmission line coupled to a backside split-ring resonator (SRR) is proposed and numerically studied. The SSPPs line is patterned on the top side of the substrate, while the SRR is etched on the backside, with the sample loaded into the SRR gap. The SSPPs mode penetrates through the substrate and excites the SRR, producing a pronounced transmission notch. Changes in the sample permittivity modulate the effective capacitance of the resonator, resulting in a monotonic shift in the notch center frequency. For relative permittivities from 1 to 8, the notch center frequency decreases from 152.1 GHz to 117.8 GHz, corresponding to a total shift of 34.3 GHz and an average sensitivity of about 4.90 GHz/εr. The minimum S21 remains within approximately −23.80 to −21.56 dB, while the Q-factor stays in the range of 94.33–108.23, indicating good spectral readability. Tolerance analysis further shows that the resonance frequency is sensitive to critical structural dimensions and layer alignment, and practical implementation is therefore more suitable for single-device calibrated frequency-shift sensing. These results demonstrate the feasibility of the proposed dual-layer SSPPs–SRR configuration for compact permittivity sensing in the terahertz regime.

Antiferromagnets, capable of hosting terahertz (THz)-frequency spin waves, are promising candidates for ultrafast, low-dissipation information processing. However, the systematic control of spin modes in antiferromagnetic orthoferrites remains relatively scarce. Here, we demonstrate the emission and detection of narrowband sub-THz radiation from quasi-ferromagnetic (q-FM) and quasi-antiferromagnetic (q-AFM) modes in ErxTm1−xFeO3 (x = 0, 0.5, and 1) single crystals. By varying the angle θ between the incident THz magnetic field (HTHz) and the c-axis, we can selectively excite either the q-AFM or q-FM mode and modulate their amplitude and relaxation dynamics. Furthermore, we show that the resonant THz frequencies in b-cut crystals are remarkably robust, exhibiting dependence on temperature and external magnetic fields (HDC). We calculated the magnetic anisotropy constants based on the temperature dependence of the spin waves. Finally, we generate narrowband THz emission via the inverse magneto-refraction effect by optically exciting q-AFM spin resonances.

Terahertz metasurface lenses are widely used due to their high design flexibility and ultra-thin characteristics, but they have an inherent bottleneck of a significant decrease in efficiency when turning large-angle beams. Therefore, this paper proposes a metagrating, aiming to develop terahertz devices with wideband response, large-angle control, and efficient beam deflection. The results demonstrate that when the deflection angle is 71.3°, the efficiency exceeds 70%. In addition, within a relative bandwidth of 15%, the metagrating maintains a diffraction efficiency exceeding 50%, and the beam deflection range is 56.79° to 84.88°. This research has confirmed the outstanding potential of metagrating to achieve high efficiency, broadband, and large-angle beam deflection.

The design of terahertz (THz) metamaterial absorbers has evolved from simple structures to complex composites to meet the demands for multi-frequency absorption, broadband absorption, and polarization independence. However, the nonlinear interactions and expanded design spaces of composite structures pose significant challenges, making traditional design methods time-consuming and labor-intensive. To address these issues, this study proposes a data-driven framework that integrates a hybrid variational autoencoder-transformer-long short-term memory (VTL) model. The architecture is specifically designed to capture structural-spectral relationships, where the transformer models global dependencies among structural parameters, the long short-term memory (LSTM) network enhances the modeling of sequential spectral features in the terahertz frequency range, and the variational autoencoder (VAE) improves feature representation by learning implicit latent distributions. This integrated design enables effective characterization of the complex electromagnetic responses of composite metamaterials. The proposed approach achieves high prediction accuracy, with a mean squared error (MSE) of 0.0009, a coefficient of determination (R2) of 0.9725, and a mean absolute error (MAE) of 0.0161. It predicts a perfect absorption rate of 99.9% and optimally adjusts structural parameters to achieve targeted frequency responses. By addressing the limitations of traditional methods, this framework not only shortens the design cycle and reduces experimental costs but also offers a robust solution for the efficient design of high-performance THz metamaterial absorbers.

Abstract This study presents a dual-band metamaterial perfect absorber (MPA) operating in the terahertz frequency range. The proposed MPA features a periodic structure with a top octagonal-shaped resonator with side arms. The absorber achieves two resonant modes at 6.735 THz and 7.312 THz, with peak absorption values close to 100%. The electric and magnetic field analysis shows that at the resonant frequencies the octagonal structure facilitates strong electromagnetic coupling, with distinct patterns for each mode. Impedance matching analysis was conducted to demonstrate the consistency and validate the simulation methods. The sensitivity and Q-factors are optimized for different cancer cell detection by tuning the structural parameters, such as arm length, dielectric thickness, and incident angle. The final optimized design shows sensitivity of 657 GHz/RIU and a Q-factor of 112, demonstrating its capability for precise sensing. The absorber also demonstrates polarization insensitivity and stable performance, making it a strong candidate for cancer detection, biosensing, and environmental monitoring.

ABSTRACT Graphene plasmons confine incident terahertz fields far below the diffraction limit and, when hosted by a gate‐defined Fabry–Perot cavity, they enable electrically tunable, frequency‐selective photodetectors. In a magnetic field, these plasmons hybridize with the cyclotron motion to form magnetoplasmons, offering a platform for fundamental studies and for nonreciprocal, spectrally selective, and ultrasensitive terahertz photonics. However, implementing magnetoplasmon‐assisted resonant transistors at terahertz frequencies has remained challenging so far. Here, we extend the resonant Dyakonov–Shur graphene TeraFET framework into the magnetoplasmonic regime and use gate‐dependent, on‐chip terahertz photocurrent spectroscopy combined with a perpendicular magnetic field to resolve and probe the evolution of resonant magnetoplasmons in antenna‐coupled monolayer and bilayer graphene TeraFETs. In monolayer graphene, the dispersion reflects the Dirac nature of the carriers, exhibiting a nonmonotonic density dependence due to the interplay of plasma resonance and cyclotron motion, with an inflection point at maximal plasmon–cyclotron coupling. In contrast, in bilayer graphene, we recover and map a magnetoplasmon dispersion consistent with the conventional Schrödinger‐type picture. These results establish graphene TeraFET devices as a robust on‐chip platform for resonant magnetoplasmonics at terahertz frequencies, opening avenues toward magnetically programmable, frequency‐selective terahertz photodetectors.

In this paper, a broadband graphene-metal hybrid frequency-selective surface (FSS) is designed and numerically investigated for tunable band-pass-filtering applications in the terahertz range. The unit cell of the proposed FSS comprises a gold nanostructure and a distinctive graphene pattern. These layers are separated by a thin silicon dioxide (SiO2) layer and are uniformly applied on both sides of a thick foam block. The results demonstrate that the proposed FSS exhibits broadband transmission in the range of 7.71-9.89THz. This corresponds to a fractional bandwidth of 24.77%. The originality of the proposed work, to our knowledge, lies in the incorporation of the dual-patterned graphene layers in the design, enabling the proposed FSS to provide an extended tunable filtering response. The design facilitates the preservation of continuity in the graphene pattern, promoting an efficient way of electrical biasing of the device. Simulations reveal that it can achieve a tunable transmission band from 6.67 to 10.20THz (∼41.85%), with an adequate out-of-band attenuation when the graphene's chemical potential is adjusted between 0 and 1.5eV. The design is analyzed in a detailed manner with the aid of various simulated results, which have been subsequently validated by an in-house equivalent circuit model approach. The proposed FSS is polarization independent and exhibits angular stability under oblique incidence up to 40° for both transverse electric and transverse magnetic wave polarizations. Owing to these unique features, it has huge potential to be employed in EM shielding, 6G communication systems, and cognitive radio-based futuristic devices.

Introduction: Quantum light–matter interaction provides an excellent application of Rydberg levels. Rydberg states are special in the sense that they have a long lifetime and large dipole moments. The large number of these levels and the spacing among them enable the detection of the incoming signals in the broad region of the electromagnetic spectrum, encompassing microwave (MW), terahertz (THz) and radio frequency (RF) ranges. Electromagnetically Induced Absorption (EIA) and Transparency (EIT) can be achieved at these levels. MW and THz signals can quantum-mechanically split the relevant spectral lines in which the splitting is proportional to the strength of an incoming em field.

Conventional metasurfaces are inherently frequency-selective, limiting wavefront control to a single operating frequency. Achieving independent manipulation at multiple frequencies remains a significant challenge for expanding the capabilities of integrated optics. Here, we propose and demonstrate an all-silicon metasurface platform that enables arbitrary independent wavefront shaping at two distinct terahertz frequencies. Our design leverages meta-molecules that synergize geometric and propagation phases to decouple the phase profiles for each frequency under a single polarization. This work provides a versatile platform for spatial-domain multiplexing, paving the way for high-capacity communication and multifunctional terahertz photonic devices.

This article presents the design of a graphene-based microstrip patch antenna, operating frequency range: (1-5) THz for terahertz (THz) applications. This paper presents simulations and a machine learning (ML) approach to characterize the performance characteristics, such as S11, Voltage Standing Wave Ratio (VSWR), gain, radiation, and total efficiencies, as well as the radiation pattern in horizontal (H, XZ) and vertical (V, XY) planes. The Computer Simulation Tool (CST) full microwave studio is used to model the antenna with dimensions of Length (L) and × Width (W): 93 μm × 113 μm, a return loss around - 40 dB, with 11 multi-band frequencies, achieving a maximum gain of 7.5 dBi at 3.2 THz. To analyze the effect of geometric parameters like length ([Formula: see text]) and width ([Formula: see text]) of the patch and graphene properties such as chemical potential ([Formula: see text]) and relaxation time (τ) on the performance characteristics of the patch antenna, three ML models are developed, such as Artificial Neural Networks (ANN), Random Forest ([Formula: see text]), and Support Vector Machine (SVM). The training data is collected for 784 simulations. The ANN architecture is built with four features in the input layer and one output layer to predict performance characteristics. The performance of the developed models is evaluated using metrics such as Mean Squared Error (MSE) and R-Squared (R2). Out of three developed models, ANN predicts the performance within 0.7 milliseconds (ms) with high accuracy, achieving an R2 of 0.99 for all performance characteristics. The predicted results discuss that the regression-based predictive models can capture the nonlinear relationship between antenna geometry and electromagnetic (EM) responses. These advantages, such as faster predictions and higher prediction accuracy, make these models, especially the ANN model, a replacement for traditional EM simulations by reducing computation time. Such qualities made the proposed ML models a powerful alternative to traditional simulation tools, making these antennas useful for next-generation wireless communication systems in the THz frequency range and beyond 6G.

Abstract Terahertz (THz) absorbers have been the subject of extensive research because of their promising applications in advanced technologies. However, their widespread use is limited by an inherently narrow operating bandwidth. We report an innovative absorber that employs metamaterial concepts to achieve high efficiency across a broad terahertz (THz) frequency range, effectively addressing this challenge. The absorber features a three-layer design, consisting of a top vanadium dioxide (VO 2 ) film, a SiO 2 dielectric middle layer, and an Au ground layer. The upper VO 2 layer is precisely patterned into a configuration that includes crosscircular and split square rings, thus improving impedance matching and broadening absorption performance. The simulation results reveal that the proposed structure reaches an absorption rate over 90% under normal incidence in a broad frequency range of 2.82 to 6.79 THz, with a central resonance peak at 4.8 THz. Furthermore, this corresponds to an impressive relative bandwidth of approximately 83%. Moreover, by modulating the conductivity of the VO 2 , the absorption can be dynamically adjusted, achieving peak values between 0.83% and 99.85%. The structure is independent of the polarization angle and shows angular stability for both (TE) and (TM) modes, while maintaining perfect absorption efficiency across various incident angles.The perfect absorption mechanism is comprehensively explained through three key approaches, relative impedance matching with free space, interference theory and validation using the equivalent circuit model(ECM). Owing to its structurally simple design, ease of integration, and superior electromagnetic performance, the proposed absorber holds significant potential for applications in terahertz filtering, electromagnetic cloaking, sensing, communication systems, and optoelectronic switching.

Unidirectional electromagnetic modes can be immune to backscattering, as no backward-propagating mode exists in the system. For broadband unidirectional modes (typically supported by uniform guiding structures), an additional striking property may emerge: exotic dispersion, in which a monotonically varying dispersion curve crosses the entire air light cone. However, in the terahertz regime, uniform unidirectional waveguides proposed previously do not exhibit such behavior. In this work, we show that a semiconductor-silicon-opaque medium structure can support a terahertz unidirectional surface magnetoplasmon (USMP) mode with exotic dispersion under certain conditions. We also reveal why previous USMP waveguides do not exhibit exotic dispersion. USMPs with exotic dispersion enable a class of metasurfaces whose extracted phases and amplitudes can be set arbitrarily and continuously. As examples, we numerically demonstrate terahertz wave focusing and Airy-beam radiation.

This Letter describes a method for synthesizing arbitrary biaxial metamaterials based on an interleaved metallic patch-rod structure. By tailoring the patch lengths and rod spacing, the effective permittivity tensor (εx, εy, εz) can be independently engineered while maintaining μ≈1. The suggested structure exhibits nearly dispersionless, low-loss electromagnetic behavior by operating in the deeply subwavelength regime. The homogenization model is validated through full-wave simulations that show excellent agreement between the synthesized and equivalent bulk media. Although our approach has been demonstrated in the microwave region, it is scalable to terahertz and optical frequencies and hence represents a general framework for designing anisotropic media with a customizable biaxial response.

Transverse magnetic (TM) and transverse electric (TE) surface electromagnetic waves supported by a graphene–hypercrystal interface are studied in the presence of an external static magnetic field. The system consists of a graphene monolayer placed at the interface between vacuum and a magnetoactive ferrite–semiconductor layered metamaterial described within the effective medium approximation. The magnetic field is applied parallel to the graphene plane (Voigt configuration), so that the Hall conductivity in graphene is absent and the graphene response is governed by a scalar surface conductivity. The optical conductivity of graphene is described using the intraband (Drude) limit of the Kubo formula, which is valid in the considered frequency range. Based on Maxwell’s equations, analytical dispersion relations for TM- and TE-polarized surface waves are derived. The obtained expressions explicitly demonstrate the distinct roles of graphene in the two polarizations, leading to different conductivity-dependent contributions to the dispersion laws. It is shown that the combined action of graphene and the anisotropic magnetoactive hypercrystal enables flexible control over the existence domains, dispersion characteristics, and field localization of surface waves. These results highlight the potential of graphene–hypercrystal interfaces as tunable platforms for controlling surface electromagnetic modes in the terahertz and mid-infrared frequency ranges.

Abstract We systematically investigated terahertz photoconductive antennas on GaAs incorporating coplanar striplines of varying widths and two types of contact metallizations (AuGe/Ni/Au and standalone Ti). Measurements of the emitted terahertz power show that AuGe contacts yield stronger emission under low-bias conditions, whereas Ti contacts - initially constrained by Schottky barriers - exhibit superior performance at high bias due to barrier lowering. Terahertz time-domain spectroscopy further confirms the expected classical antenna behavior: decreasing the stripline width shifts the resonance frequency to higher values. At the shortest dipole lengths, however, the emitted spectra of the two metallizations diverge, with Ti-metalized antennas exhibiting higher resonance frequencies. These findings demonstrate that the performance of terahertz photoconductive antennas is constrained by the impedance of metallic contacts, providing essential design considerations for next-generation devices intended to operate at higher terahertz frequencies.

Low-Field Non-Reciprocal Terahertz Devices Indium antimonide (InSb) rod arrays enable robust unidirectional propagation at terahertz frequencies under low magnetic fields (0.1 T). By exploiting the Voigt effect and dispersive scaling, this work realizes magnetic-field-controlled non-reciprocal devices that exhibit topological immunity to defects and sharp bends. More information can be found in the Research Article by Sai Chen and co-workers (10.1002/adpr.202500253).

Innovative MIMO antenna for terahertz band 6G communications: Performance evaluation and machine learning integration

This paper puts forward a novel proportional structure for a tunable broadband absorber in the terahertz (THz) frequency band, with graphene as its core material. In the pursuit of an optimal design, we maintain a proportional distance between the top graphene structure and the periodic edges. This design enables excellent absorptance, surpassing 90% in the 2.89–6.43 (3.54) THz frequency range. Notably, at f = 4.25 THz, perfect absorption is achieved. At f = 5.8 THz, the absorptance nears 95%, and at f = 5.47 THz, it is around 91%. The average absorptance within the 2.89–6.43 THz range reaches 94.5%. By calculating the impedance of the absorber, the favorable absorption frequency band is demonstrated. Moreover, analyzing the internal electric field of the absorber reveals a strong electric field coupling effect between different regions. This effect leads to the overlap of absorption peaks, thus forming a broadband response. This characteristic allows for the selection and combination of regions according to application requirements. By adjusting the absorber's parameters, it shows coordinated performance and manufacturing tolerance. Additionally, the absorber exhibits a certain tolerance to the incident angle of electromagnetic waves. These findings highlight the potential of this absorber in applications such as optoelectronic devices, THz detection, and stealth technology.

The exponential growth in satellite data traffic demands communication systems exceeding current microwave capacity limitations, while the terahertz (THz) frequency band (0.1-10THz) offers unprecedented bandwidth potential with superior weather resilience compared to optical systems, particularly when combined with ultra-low Earth orbit (ULEO) satellite deployments below 300km altitude. This paper presents a comprehensive performance evaluation for ULEO-THz satellite-to-ground communications, analyzing three distinct transmission architectures: direct satellite-to-ground (S2G), satellite-relay-ground (SRG) forwarding, and satellite-to-high-altitude base station (S2H) with fiber backhaul. Our analysis leverages altitude-resolved atmospheric propagation models validated using yearlong meteorological data from four high-altitude stations in Tibet and Qinghai, China. It incorporates frequency-dependent atmospheric absorption using ITU-R standards, free-space path loss with curved atmospheric modeling, and regional atmospheric variations to derive total channel path loss, available bandwidth capacity, and bit error rate (BER) performance under both AWGN and Weibull fading conditions across multiple THz frequencies. Results demonstrate that direct S2G transmission at lower THz frequencies achieves optimal practical performance with maximum available bandwidth under QPSK modulation, while SRG suffers prohibitive cumulative losses from multiple hops, and S2H is rendered impractical for long-haul links by substantial electro-optical conversion and fiber transmission losses.

Abstract A 1-bit coding metamaterial work focused on the terahertz frequency region, resulting in a compact, novel design. Two unit cells, element 0 and element 1, having phase responses of 0° and 180°, were designed in the frequency range of 1 to 7 THz. The reflection coefficient, transmission coefficient, and phase response characteristics of element 1 were all carefully examined. At 2.65 THz and 6 THz, the necessary phase response was achieved. The validation was implemented by using CST Microwave Studio 2021. The proposed unit cell offers reduced electrical size with strong electromagnetic control. Seven coding sequences were adopted by exploiting phase response properties and used for the smaller lattices (4, 6, and 8) and the larger lattices (20, 24, and 28) in the parametric studies. Each coding sequence (CS) was analyzed for its monostatic and bistatic curves. The coding sequence 4 offers excellent Radar cross section(RCS) reduction of −76.12 dBm2 for the smaller lattice. The geometry of the designed unit cell ensures strong excitation of the current at the corners, which leads to wide-band RCS reduction.