Terahertz Research Articles

Machine Learning Enabled Compact Frequency‐Tunable Triple‐Band Hexagonal‐Shaped Graphene Antenna for THz Communication

ABSTRACTIn this article, a compact triple‐band frequency‐tunable (FT) hexagonal‐shaped graphene antenna through a machine learning (ML) approach for terahertz (THz) application is presented. The proposed THz antenna is designed on a polyamide () substrate with a thickness of 10 μm, and graphene is used as an antenna radiator. The size of the substrate is 38 × 46 μm2. The FT is achieved by changing the chemical potential of graphene material. The performance of the proposed THz antenna has been investigated, and the impacts of several conducting materials like gold, aluminum, copper, and graphene and dielectric materials like Rogers RT/duroid 5880, polyamide, quartz, and SiO2 are explored. The proposed THz antenna provides three operating bands. The frequency of operation in Band‐1 is 2.51–5.05 THz, Band‐2 is 5.99–7.43 THz, and Band‐3 is 7.94–9.63 THz. The bandwidth in Band‐1, Band‐2, and Band‐3 are 2.54, 1.44, and 1.69 THz, respectively. The % of impedance bandwidth in Band‐1, Band‐2, and Band‐3 are 67.19%, 24.02%, and 21.28% respectively. The proposed antenna has a maximum peak gain of 5 dBi. The proposed antenna is optimized through various ML algorithms like random forest (RF), extreme gradient boosting (XGB), K‐nearest neighbor (KNN), decision tree (DT), and artificial neural network (ANN). The RF algorithm gives more than 99% accuracy compared to other ML algorithms and accurately predicts the S11 of the proposed antenna. The proposed THz antenna would be suitable for applications related to imaging, medical, sensing, and ultra‐speed short‐distance communication applications in the THz region.

Time-Domain-Filtered Terahertz Nanoscopy of Intrinsic Light-Matter Interactions.

Terahertz (THz) technology holds great potential across diverse applications, including biosensing and information communications, but conventional far-field techniques are limited by diffraction. Near-field optical microscopy overcomes this barrier through a sharp tip that concentrates incident THz waves into nanometric volumes, detecting scattered near-field to reveal nanoscale optical properties. However, owing to the large THz wavelengths, resonant surface waves arising on the tip and cantilever obscure the intrinsic response. Here we combine near-field microscopy with THz time-domain spectroscopy and implement time-domain filtering with an elongated cantilever to eliminate this artifact, achieving intrinsic nanospectroscopy and nanoimaging. By applying this technique, we distinguish and characterize historical pigments of an ancient sculpture, such as vermilion and red lead, on the nanoscale. We also unravel deep-subwavelength localized resonance modes in THz optical antennas, demonstrating capabilities for THz nanophotonics. Our work advances THz nanoimaging and nanospectroscopy techniques to probe intrinsic nanoscale THz light-matter interactions.

Multifunctional Reconfigurable Vanadium Dioxide Integrated Metasurface for Reflection, Asymmetric Transmission and Cross‐Polarization Conversion in Terahertz Region

AbstractIntegrating reconfigurable and diverse functionalities into a single metasurface at terahertz (THz) frequencies is an emerging research topic that faces significant challenges. Here, a reconfigurable THz metasurface is proposed, offering diversified functionalities based on the phase transition of vanadium dioxide (). The proposed metasurface can switch from wideband reflection to wideband cross‐polarization conversion (CPC) and asymmetric transmission (AT) for linearly polarized waves. When is in its fully metallic state, the metasurface efficiently reflects normally incident waves ranging from 0.2 to 2.8 THz, with total reflection exceeding 0.8. When is in its insulating state, the metasurface achieves nearly perfect wideband cross‐polarization conversion with CPC efficiency above 0.99 from 0.46 to 2.67 THz, and excellent AT effect with efficiency over 0.9 from 0.65 to 2.29 THz for normal incidence of linearly polarized waves. Moreover, the high efficiency of CPC and AT effects is maintained over a wide range of incident angles. The proposed switchable metasurface with diverse functionalities is expected to enable cutting‐edge research and innovative applications in THz communication, sensing, and imaging.

GaAs-based antenna-coupled field effect transistors as direct THz detectors across a wide frequency range from 0.2 to 29.8 THz

High-power coherent terahertz (THz) radiation from accelerator facilities such as free-electron lasers (FELs) is frequently used in pump-probe experiments where the pump or probe (or both) signals are intense THz pulses. Detectors for these applications have unique requirements that differ from those of low-power table-top systems. In this study, we demonstrate GaAs antenna-coupled field effect transistors (FETs) as a direct THz detector operating across a broad frequency spectrum ranging from 0.2 THz to 29.8 THz. At approximately 0.5 THz, the maximum current responsivity (ℜ I ) of 0.59 mA/W is observed, signifying a noise equivalent power (NEP) of 2.27 nW/√Hz. We report an empirical roll-off of f−3 for an antenna-coupled GaAs TeraFET detector. Still, NEP of 0.94 μW/√Hz and a current responsivity ℜ I = 1.7μA/W is observed at 29.8 THz, indicating that with sufficient power the FET can be used from sub-mm wave to beyond far-infrared frequency range. Current and voltage noise floor of the characterized TeraFET is 2.09 pA and 6.84 μV, respectively. This characteristic makes GaAs FETs more suitable for applications requiring higher frequencies, ultra-broadband capabilities and robustness in the THz domain, such as beam diagnostics and alignment at particle accelerators.

Innovative active terahertz modulator based on dynamic graphene-silicon arrays

This paper presents an innovative active terahertz (THz) modulator that utilizes dynamic graphene-silicon arrays (DGSAs) to overcome the challenges of dynamically manipulating THz waves. The DGSA combines the exceptional electrical properties of graphene with the enhanced optical absorption capabilities of silicon arrays, enabling dual-mode active control through optical pumping and electrical biasing. An 808 nm laser is used to photoexcite carriers in the silicon arrays for optical pumping while a bias voltage modulates the Fermi level of graphene, subsequently varying Si conductivity and influencing the transmission properties of THz waves. Simulations have demonstrated modulation depths of up to 94% at 2.5 THz in the transmission mode and 89% at 1 THz in the reflection mode. Furthermore, this study delves into a theoretical analysis and numerical simulations, exploring the modulation mechanisms, including the role of graphene’s Drude model and PN junction effects. These insights provide a robust theoretical framework for understanding the DGSA’s operation and lay a solid foundation for the design and optimization of high-performance THz modulation devices.

Photon-Induced Ultrafast Multitemporal Programming of Terahertz Metadevices.

Dynamic terahertz (THz) metasurface can feature modulated and multiplexed electromagnetic functionalities, important for wave-based computation, six-generation communications, and other applications. The versatile dynamic switching typically relies on a series of complex or incompatible multifield activations, with excessive system complexity, additional loss, slow modulation speed, and inertial time-varying properties, limiting more widespread applications. Here, a photon-induced ultrafast programmable THz metadevice is reoprted in time-frequency dimensions with polarization-decoupled temporal responses. By the pixelated design with multi-materials and triggering switches, the multimodal modulation transcends the constraints inherent in materials, enabling the ultrafast programmable temporal evolution. All the resonances can be independently programmed at the working band from 0.6 to 2 THz. The tri-temporal (with switching time of 1.25, 1, and 4.75ps) and bi-temporal (with switching time of 2.25 and 4.75ps) dynamic manipulations are performed by all-optical driven molecularization process of hybrid metasurfaces loaded with silicon (Si)and germanium (Ge) under different polarizations. Combining these features, the temporally programmed THz logic gates are last experimentally demonstrated, possessing basic operation of XNOR, NOR, and OR,as a proof-of-concept application. This reported light-driven programmable THz flat-optics allows ultrafast hybrid molecularization processes and new possibilities for miniaturized, flexible, multifunctional, and temporally programmable integrated devices.

Terahertz Resonant Emission by Optically Excited Infrared-Active Shear Phonons in KY(MoO4)2.

The generation of monochromatic electromagnetic radiation in the terahertz (THz) frequency range has remained a challenging task for many decades. Here, the emission of monochromatic sub-THz radiation by optical phonons in the dielectric material KY(MoO4)2 is demonstrated. The layered crystal structure of KY(MoO4)2 causes infrared-active shear lattice vibrations to have energies below 3.7meV, corresponding to frequencies lower than 900GHz where solid-state-based monochromatic radiation sources are rare. Directly excited by a 5ps long broadband THz pulse, infrared-active optical vibrations in KY(MoO4)2 re-emit narrowband sub-THz radiation as a time-varying dipole for tens of picoseconds, which is exceptionally long for oscillators with frequencies below 1THz. Such a long coherent emission allows for the detection of more than 50 periods of radiation with frequencies of 568 and 860GHz. The remarkably long decay time together with the chemical stability of the employed material suggests a variety of possible applications in THztechnology.

Implementing federated learning over VPN-based wireless backhaul networks for healthcare systems

Federated learning (FL) is a popular method where edge devices work together to train machine learning models. This study introduces an efficient network for analyzing healthcare records. It uses VPN technology and applies a federated learning approach over a wireless backhaul network. The study compares different wireless backhaul channels, including terahertz (THz), E/V band (mmWave), and microwave, for their effectiveness. We looked closely at a suggested FL network that uses VPN technology over awireless backhaul network. We compared it with the standard method and found that using the FedAvg algorithm with Terahertz (THz) for communication gave the best accuracy. The time it took to reach a conclusion improved a lot, going from 55 seconds to an impressive 38 seconds. This emphasizes how having a faster communication link makes FL networks work much better. Furthermore, a three-step plan was executed to boost security, adopting a multi-layered method to safeguard the FL network and its confidential data. The first step involves integrating a private network into the current telecom infrastructure, establishing an initial layer of security. To enhance security further, licensed frequency channels are introduced, providing an extra layer of protection. The highest level of security is achieved by combining a private network with licensed frequency channels, complemented by an additional layer of security through VPN-based measures. This comprehensive strategy ensures the application of strong security protocols.

Terahertz time-domain investigation of atacamite: Spectral analysis and theoretical insights for cultural heritage applications

Spectroscopic methods based on terahertz (THz) radiation can represent an innovative approach in the Cultural Heritage field. Specifically, contemporary and historical pigments can exhibit unique fingerprints. However, despite the high selectiveness of terahertz technique, the origin of spectral characteristics is not often known. In this work, the characteristic terahertz-time-domain spectroscopy spectrum of atacamite is reported in the spectral range 0.1–3.7 THz along with ab initio calculation of lattice dynamics to assess the nature of experimental features. The obtained results enable the acquisition of reliable new data in Heritage sciences, which is essential for developing accurate reference pigment databases.

Comparison of electronic and optoelectronic signal generation for (sub-)THz communications

Abstract In recent years, the significance of terahertz (THz) and (sub-)THz communications has grown substantially due to its promising trade-off between higher capacity compared to microwave-based communication and better resilience against weather dependent influences (e.g., fog and rain). While electronic and optoelectronic techniques have been extensively explored, each offering distinct advantages and limitations, they have predominantly been demonstrated and discussed as individual experiments, making performance comparison challenging. This paper addresses this gap by systematically benchmarking electronic and optoelectronic signal generation approaches under comparable conditions. Our experiments incorporate various receiver types, revealing that best performance is achieved by combining optoelectronic signal generation techniques at the transmitter in combination with an all-electric intradyne receiver. This results in a remarkable line rate of 200 Gbit/s over a distance of 52 m. To our knowledge, this represents the highest line rate achieved for technically relevant transmission distances for indoor access or outdoor small cell networks.

Hybrid α -Ta/ β -Ta lumped element kinetic inductance detectors with photon noise limited sensitivity and stability

The terahertz (THz) band is of immense interest in astronomy as it encompasses significant energy generated following the Big Bang, offering critical insight into processes invisible in other bands, such as the earliest stages of planet, star, and galaxy formation. Kinetic inductance detectors (KIDs) have emerged as a formidable contender in the field of THz astronomy, attributed to their exceptional sensitivity and scalability. In this study, we introduce a kind of KIDs incorporating a lumped element (LE) resonator design, with inductors fabricated on β-Ta film and capacitors on α-Ta film. We characterize the noise of the hybrid α-Ta/β-Ta LEKIDs, achieving an optical noise equivalent power of 8.3 ± 5.7 × 10−19 W/Hz1/2, demonstrating high sensitivity. Additionally, the LEKIDs exhibited stability across multiple thermal cycles. The combination of high sensitivity and stability makes the hybrid LEKIDs promising for the stringent demands of THz astronomy.

The Transcriptomic Response of Cells of the Thermophilic Bacterium Geobacillus icigianus to Terahertz Irradiation

As areas of application of terahertz (THz) radiation expand in science and practice, evidence is accumulating that this type of radiation can affect not only biological molecules directly, but also cellular processes as a whole. In this study, the transcriptome in cells of the thermophilic bacterium Geobacillus icigianus was analyzed immediately after THz irradiation (0.23 W/cm2, 130 μm, 15 min) and at 10 min after its completion. THz irradiation does not affect the activity of heat shock protein genes and diminishes the activity of genes whose products are involved in peptidoglycan recycling, participate in redox reactions, and protect DNA and proteins from damage, including genes of chaperone protein ClpB and of DNA repair protein RadA, as well as genes of catalase and kinase McsB. Gene systems responsible for the homeostasis of transition metals (copper, iron, and zinc) proved to be the most sensitive to THz irradiation; downregulation of these systems increased significantly 10 min after the end of the irradiation. It was also hypothesized that some negative effects of THz radiation on metabolism in G. icigianus cells are related to disturbances in activities of gene systems controlled by metal-sensitive transcription factors.

A high-performance ultra-compact plasmonic metamaterial structure for optical THz absorption

This study presents the design and analysis of a high-performance metamaterial absorber for the optical terahertz (THz) regime. The proposed absorber utilizes a unique nested flower-shaped structure composed of nickel (Ni) and silicon dioxide (SiO2), achieving an average absorption exceeding 97.91% with a broad bandwidth of 1320 THz (180 THz − 1500 THz). A unit cell size of 66 nm × 66 nm × 24 nm makes this design highly attractive for miniaturized devices. Strong absorption originates from localized surface plasmon resonance (LSPR), where light interacts with the electrons on the Ni surface. Notably, the design maintains excellent absorption performance even at oblique incident angles up to 60° with polarization insensitivity. These results highlight the Ultra-Compact Plasmonic Metamaterial (UCPM) absorber’s potential for diverse applications due to its broad spectral response, high absorption efficiency, and minimal footprint, making it valuable for energy harvesting, infrared imaging, and electromagnetic stealth technologies.

Room temperature single-photon terahertz detection with thermal Rydberg atoms

Single-photon terahertz (THz) detection is one of the most demanding technologies for a variety of fields and could lead to many breakthroughs. Although significant progress has been made in the past two decades, operating it at room temperature still remains a great challenge. Here, we demonstrate, for the first time, a room temperature THz detector at single-photon levels based on nonlinear wave mixing in thermal Rydberg atomic vapor. The low-energy THz photons are coherently upconverted to high-energy optical photons via a nondegenerate Rydberg state involved in a six-wave mixing process, and therefore, single-photon THz detection is achieved by a conventional optical single-photon counting module. The noise equivalent power of such a detector reaches 9.5 × 10−19 W/Hz1/2, which is more than four orders of magnitude lower than the state-of-the-art room temperature THz detectors. The optimum quantum efficiency of the whole-wave mixing process is about 4.3%, with 40.6 dB dynamic range, and the maximum conversion bandwidth is 172 MHz, which is all-optically controllable. The developed fast and continuous-wave single-photon THz detector at room temperature operation has a great potential for portability and chip-scale integration, and could be revolutionary for a wide range of applications in remote sensing, wireless communication, biomedical diagnostics, and quantum optics.

Microfabrication Technologies for Interaction Circuits of THz Vacuum Electronic Devices

Advances in manufacturing technology are allowing for the realization of interaction circuit with microstructures. The capability to produce small circuit structures is allowing new opportunities for vacuum electronic devices producing terahertz (THz) frequency radiation, which is impractical with traditional machining technology. This publication reviews recent progress on advanced microfabrication technologies applicable to interaction circuits of THz vacuum electronic devices, including LIGA/UV-LIGA (Ultraviolet Lithographic, Galvonoformung and Abformung), deep reactive ion etching (DRIE), micro/nano computer numerical control (CNC) milling, three-dimension (3D) printing, etc., and describes the current State-of-the-Art of their applications.

Terahertz Wave Absorption of a Rubidium Manganese‐Iron Prussian Blue Phase Transition Material

AbstractRubidium manganese hexacyanidoferrate, Rb0.97Mn[Fe(CN)6]0.99 ⋅ 0.5H2O, which has a three‐dimensional cyanido‐bridged Mn−Fe framework encapsulating Rb+ ions, shows a terahertz (THz) wave absorption property. This compound undergoes a charge‐transfer phase transition between FeIII‐CN‐MnII [high temperature (HT) phase] and FeII‐CN‐MnIII [low temperature (LT) phase] near room temperature. The HT phase exhibits a THz wave absorption at 1.06 THz due to the slow vibration of the heavy Rb+ ions encapsulated in the three‐dimensional framework. By contrast, the LT phase displays a higher resonance frequency at 1.13 THz. This shift to a higher frequency is attributed to the framework shrinkage, which decreases the space available for the trapped Rb+ ion. Indeed, the void volume accompanying the phase transition decreases by 16.4 % from 41.4 Å3 (HT phase) to 34.6 Å3 (LT phase). Tuning the THz wave absorption frequency is necessary for THz devices such as absorbers, switches, filters, and modulators, especially for THz technology with THz waves in the sub‐THz region or near 1 THz.

Point-to-multipoint beam-steering terahertz communications using a photonics-based leaky-wave transmit antenna

Abstract We report on the successful implementation of a photonic-based beam steering approach for point-to-multipoint terahertz (THz) communications. The frequency-agile radiation properties of a THz leaky-wave antenna connected to a photodiode are utilized in the remote transmitter for generating multiple individually steerable THz beams. The approach benefits from the ultra-wide frequency tunability of optical heterodyne systems for frequency-agile THz beam steering. Moreover, the approach allows to exploit high performance and mature optical modulation techniques for generating THz beams with a high data capacity. In the THz receiver, a carrier frequency insensitive envelope THz detector based upon a single Schottky-barrier diode is used. In detail, THz communications in the 300 GHz band (WR3.4) with a maximum steering angle up to 90° is reported. Experimentally, for single steerable THz beam operation, a record high data rate of 35 Gbit/s at a wireless distance of 30 cm is achieved using two-level pulse amplitude modulation. Also, longer wireless distances up to 110 cm with 5 Gbit/s are demonstrated. Furthermore, point-to-multipoint THz communications is reported using two individually steerable THz beams carrying 10 Gbit/s and 5 Gbit/s over 70 cm and 50 cm, respectively.

Theoretical and numerical investigation of an ultra-broadband near-perfect absorption in auxetic metamaterials

In this paper, we present and numerically investigate an auxetic metamaterial absorber based on vanadium dioxide (VO2), achieving more than 90% absorption of the incident terahertz (THz) waves between 4.15 THz to 8.43 THz with an average absorption of 98.4%. To our knowledge, the absorption bandwidth is higher than previously reported VO2-based absorbers. The proposed absorber contains two VO2 based resonator rings placed diagonally at the top of the dielectric substrate in such a way as to create an auxetic shape and provide more design space than the existing absorbers. Considering the vector nature of electromagnetic fields in three-dimensional space, numerical analysis is performed while keeping the phase-changing material VO2 in the metallic state. According to the full wave simulation, it is shown that under normal incidence, the proposed absorber provides almost flat and near-unity absorption, which covers from 4.82 THz to 7.53 THz. We describe the physical mechanism of the absorber through impedance matching theory, and the absorber performance is evaluated by observing the electric field distributions at various frequencies. The proposed structure also exhibits a satisfactory tunable range from 2% to 100%, which may satisfy the requirements of re-configurable metamaterial absorbers. We also show that the absorber provides better wide-angle absorption performance than the existing VO2-based models for both transverse polarization i.e., transverse electric (TE) and transverse magnetic (TM) modes. Owing to the higher absorption bandwidth and better tunable range, the proposed auxetic metamaterial has great potential applications in THz imaging, detectors, and sensing.

Ultra-Low Threshold Resonance Switching by Terahertz Field Enhancement-Induced Nanobridge.

Ongoing efforts spanning decades aim to enhance the efficiency of optical devices, highlighting the need for a pioneering approach in the development of next-generation components over a broad range of electromagnetic wave spectra. The nonlinear transport of photoexcited carriers in semiconductors at low photon energies is crucial to advancements in semiconductor technology, communication, sensing, and various other fields. In this study, ultra-low threshold resonance mode switching by strong nonlinear carrier transport beyond the semi-classical Boltzmann transport regime using terahertz (THz) electromagnetic waves are demonstrated, whose energy is thousands of times smaller than the bandgap. This is achieved by employing elaborately fabricated 3D tip structures at the nanoscale, and nonlinear effects are directly observed with the THz resonance mode switching. The nanotip structure intensively localizes the THz field and amplifies it by more than ten thousand times, leading to the first observation of carrier multiplication phenomena in these low-intensity THz fields. This experimental findings, confirmed by concrete calculations, shed light on the newly discovered nonlinear behavior of THz fields and their strong interactions with nanoscale structures, with potential implications and insights for advanced THz technologies beyond the quantum regime.

A 2DEG‐Based GaN‐on‐Si Terahertz Modulator with Multi‐Mode Switchable Control

AbstractAs terahertz (THz) technology has been widely considered as a key candidate for future sixth‐generation wireless communication networks, THz modulators show profound significance in wireless communication, data storage, and imaging. In special, dynamic tuning of THz waves through 2D electron gas (2DEG) incorporated with a hybrid design of metasurface has attracted keen interest due to the combined merits of deliberate structural design, rapid switching speed and the process compatibility. Current meta‐modulator enables very high modulation depth but encounter limited bandwidth. In this paper, by taking into account the co‐functional effects of temperature and voltage‐dependent dynamic control on transmission amplitude, a 2DEG‐based GaN‐on‐Si modulator with two switchable operational states (or four modes) of active wave control is proposed. Under cryogenic temperature conditions, the proposed device exhibits prominent 2D plasmons characteristics with switchable transitions between the gated mode and ungated mode for active control. Under room temperature conditions, the proposed device exhibits non‐resonance broadband spectra characteristics with tunable transitions between the linearity mode and depletion mode for transmission control. The scheme provides an option for the development of the actively tunable THz meta‐modulator and paves a way for the robust multifunctionality of electrically controllable THz switching, and biosensors.