Terahertz Devices Research Articles

Topological Metasurface for Spin‐Decoupled Wavefront Manipulation of Terahertz Surface Plasmons

AbstractOn‐chip wavefront manipulation of terahertz surface plasmons is crucial for the miniaturization and practical application of terahertz technology. Recently, topological metasurfaces have emerged as a promising avenue for spin‐decoupled wavefront manipulation, leveraging the unique topological properties of non‐Hermitian matrices near their singular points. Despite their innovative phase control mechanisms, topological metasurfaces have not yet been explored for surface plasmon devices. Here, spin‐decoupled wavefront manipulation of terahertz surface plasmons is experimentally achieved using freestanding topological metasurfaces. This approach promises to expand the control methods of surface plasmons and broaden the application scenarios of topological metasurfaces, providing a new paradigm for the design of integrated on‐chip terahertz devices.

Vanadium dioxide and a graphene-based ultra-wideband absorption and polarization conversion switchable terahertz device

A multifunctional terahertz functional device based on vanadium dioxide (VO2) and graphene is proposed, which can realize ultra-wideband absorption and polarization conversion. When the VO2 is in the insulating state, the device can achieve the polarization conversion function to convert the incident wave into the corresponding cross-polarized wave. Polarization conversion ratios (PCRs) can exceed 90% in the 2.1–8 THz frequency range; in the 3–7.5 THz range, the PCR can be more than 95%. When VO2 is in the metallic state and the graphene Fermi energy is 0.9 eV, the device can realize a broadband absorption function. An absorption rate of more than 90% can be achieved over a wide frequency range of 3.3–7.7 THz. In addition, the polarization conversion device can maintain high performance in broadband polarization conversion at incident angles no greater than 40°. The absorber device also exhibits insensitivity to both incident and polarization angles. These advantages which make the proposed multifunctional terahertz functional device have a wide range of applications in the fields of terahertz imaging, sensing, communication, and so on.

Measuring the radiation hardness of terahertz devices for space applications

The application of terahertz technology in space is frontier for the development of 6G technologies. Terahertz transceiver devices based on gallium arsenide Schottky barrier diodes (GaAs SBDs) have the characteristics of small size, light weight and low power consumption, making them suitable for application on spacecraft. However, there is currently a lack of experimental assessments on their space adaptability. Here, we study the radiation hardness of terahertz devices to determine their adaptability in complex space environments. We exposed GaAs SBDs and terahertz multipliers as typical terahertz devices to gamma rays and protons. The experimental results showed that the terahertz devices exhibited good tolerance to protons, but prolonged exposure to gamma rays could significantly increase the leakage current of the GaAs SBDs and alter its C-V characteristics, leading to the failure of the terahertz multiplier. Nevertheless, the terahertz devices maintained a good level of radiation hardness, making them highly suitable for use in Low Earth Orbit (LEO) satellites. The comparison between the results of proton and gamma ray tests indicated that the terahertz devices exhibited high inherent radiation hardness against displacement damage but were more sensitive to ionization damage, requiring higher shielding requirements.

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.

Reconfigurable wide-angle broadband terahertz wave antireflection using a non-volatile phase-change material.

Actively wide-angle broadband terahertz (THz) antireflection (AR) coatings with a flexible reconfigurability have a great potential for the development of next-generation versatile THz components and systems with high performance. Here, we present a reconfigurable wide-angle broadband THz AR coating using a phase change material Ge2Sb2Te5 (GST) film, which is based on the impedance matching method. The performance of GST-based AR coating can be effectively achieved by a thermal excitation, exhibiting the complete suppression of unwanted THz-wave reflections for incidence angles from 0∘ to 50∘ in the broad frequency range of 0.1-3.0 THz. Simulation and experimental results show that the GST-based AR coating can efficiently eliminate Fabry-Perot interference caused by unwanted THz-wave reflections from the substrate, thereby significantly improving the performances of THz devices. Moreover, the active AR mechanism of the GST-based coating is investigated, which elucidates the essential role of the phase transition between the amorphous and crystalline phases in changing the conductivity of the film to achieve an impedance matching condition under thermal excitation. Additionally, the non-volatile properties of GST can enable the AR coating to retain a long-term stability for optimal wave-impedance matching without power holding requirements. Our work provides a new, to the best of our knowledge, and promising way for realizing high-performance integrated THz components and systems in the future.

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

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

Cornertronics in Two-Dimensional Second-Order Topological Insulators.

Traditional electronic devices rely on the electron's intrinsic degrees of freedom (d.o.f.) to process information. However, additional d.o.f., like the valley, can emerge in the low-energy states of certain systems. Here, we show that the quantum dots constructed from two-dimensional second-order topological insulators posses a new kind of d.o.f., namely corner freedom, related to the topological corner states that reside at different corners of the systems. Since the corner states are well separated in real space, they can be individually and intuitively manipulated, giving rise to the concept of cornertronics. Via symmetry analysis and material search, we identify the TiSiCO-family monolayers as the first prototype of cornertronics materials, where the corner states can be controlled by both electric and optical fields due to novel corner-layer coupling effect and corner-contrasted linear dichroism. Furthermore, we find that the band gap of the TiSiCO nanodisk lies in the terahertz region and is robust to size reduction. These results indicate that the TiSiCO nanodisks can be used to design terahertz devices with ultrasmall size and electric-field tunable band gap. Besides, the TiSiCO nanodisks are simultaneously sensitive to both the strength and polarization of the terahertz waves. Our findings not only pave the way for cornertronics, but also open a new direction for research in two-dimensional second-order topological insulators, quantum dots, and terahertz electronics.

Scanless Spectral Imaging of Terahertz Vortex Beams Generated by High‐Resolution 3D‐Printed Spiral Phase Plates

Terahertz technology has experienced significant advances in the past years, leading to new applications in the fields of spectroscopy, imaging, and communications. This progress requires the development of dedicated optics to effectively direct, control and manipulate terahertz radiation. In this regard, 3D printing technologies have shown great potential, offering fast prototyping, high design flexibility, and good reproducibility. While traditional 3D printing techniques allow for the preparation of terahertz optical components operating at relatively low frequencies (<0.4 THz) due to their limited resolution, two‐photon polymerization lithography (TPL) exhibits high detail resolution and low surface roughness and can thus potentially enable the fabrication of high‐frequency terahertz devices. Here, as a proof of principle, spiral phase plates operating at 1 THz are designed and fabricated by means of TPL. Moreover, these samples are characterized via a rapid and scanless terahertz imaging technique customized to obtain a coherent hyperspectral analysis of the generated vortex beams at varying distances along propagation. Numerical simulations are also conducted for comparison with experiments, revealing a good agreement. Current limitations of the technique are found to be mainly related with terahertz loss in TPL polymers, and possible solutions are discussed.

Vanadium-dioxide-assisted multifunctional switchable terahertz metamaterial devices

A multifunctional, switchable terahertz (THz) metamaterial (MM) device with wideband absorption and polarization conversion capabilities has been developed, based on the insulator-metal phase transition of vanadium dioxide (VO2). In its metallic state, the device operates as a wideband absorber within the range of 2.56–6.74 THz, achieving a bandwidth of 4.18 THz and an absorption rate of ≥90%. The wideband absorption is insensitive to both oblique incident angles and polarization. When VO2 is in its insulating state, the device switches to a polarization converter, facilitating linear-to-cross polarization (LTX) conversion between 1.04 and 4 THz, and linear-to-circular polarization (LTC) conversion between 1 and 1.04 THz, with a conversion efficiency exceeding 90%. Additionally, the effects of incident angle and polarization angle on the device’s performance were analyzed. This THz device offers advantages of wide angle, wide bandwidth, and high efficiency, making it a valuable reference for research into new multifunctional THz devices. It has great potential applications in short-range wireless THz communication, ultrafast optical switches, high-temperature resistant switches, transient spectroscopy, and optical polarization control devices. In specific application scenarios, particularly in fields requiring efficient detection, transmission, and analysis—such as security and non-destructive testing, secure communication systems, imaging and sensing, multidimensional spectral analysis, pollutant detection, smart stealth coatings, dynamic optical control devices, and integrated optical systems—these devices offer multifunctional capabilities. They enhance system performance and flexibility, meeting diverse application needs.

All-dielectric double-layer honeycomb tunable metamaterial absorber with integrated gold nanoparticles

The optical regulation strategy of gold nanoparticles can significantly improve the performance of terahertz devices. We designed an all-dielectric double-layer honeycomb metamaterial absorber (MA) to demonstrate the broadband terahertz absorption characteristics in the presence or absence of gold nanoparticles. When it does not contain gold nanoparticles, MA exhibits a peak absorption efficiency of over 99% within the bandwidth range of ∼486 GHz. In particular, gold nanospheres (AuNPs), gold nanobipyramids (AuNBPs), and gold nanorods (AuNRs) are used to modulate the optical coupling effect of metamaterial absorbers, which improves their modulation performance. In the simulation, the effective medium theory (EMT) was applied to quantitatively calculate the optical response of a metamaterial absorber with an integrated gold nanoparticle equivalent gold layer. The integrated gold nanoparticle equivalent gold layer can achieve modulation enhancement of one order of magnitude. In the experiment, our process is compatible with CMOS technology, which may contribute to the development of terahertz detectors. In addition, the tunability and modulation enhancement characteristics demonstrated are beneficial for creating dynamic functional terahertz devices, such as THz modulators and switches.

A dynamically switchable and tunable metamaterial device based on vanadium dioxide and Dirac semimetal

In this paper, a dual-function metamaterial device based on vanadium dioxide (VO2) and Dirac semimetal (DS) operating in terahertz (THz) frequency range is designed. The reversible phase transition properties of VO2 are exploited to achieve the dynamic switch between the function of broadband absorption and broadband polarization conversion. It is demonstrated that when VO2 is in the metallic state and the Fermi energy level () of DS is set to 30 the broadband absorption in the operating frequency range of 1.42-3.40 THz can be achieved, and its absorbance exceeds 90% with a relative bandwidth of 82.16%. When VO2 is in the insulating state and the of DS is set at 180 , the high efficient broadband polarization conversion can be obtained in the operating frequency range of 2.08-4.88 THz and the polarization conversion ratio (PCR) is greater than 90% with a relative bandwidth of up to 80.46%. Therefore, the proposed switchable dual-functional THz device can be extensively utilized in absorption, polarization conversion and other potential fields.

Active control of terahertz quasi-BIC and asymmetric transmission in a liquid-crystal-integrated metasurface

Quasi-bound states in the continuum (quasi-BICs) offer an excellent platform for the flexible and efficient control of light-matter interactions by breaking the structural symmetry. The active quasi-BIC device has great application potential in fields such as optical sensing, nonlinear optics, and filters. Herein, we experimentally demonstrate an active terahertz (THz) quasi-BIC device induced by the polarization conversion in a liquid crystal (LC)-integrated metasurface, which consists of a symmetrically broken double-gap split ring resonator (DSRR), an LC layer, and double graphite electrodes. In the process of LC orientation control under the external field, the device realizes the active control from the OFF state to the ON state. In the OFF state, the LC has no polarization conversion effect, and the device behaves in a non-resonant state; but for the ON state, the device exhibits obvious quasi-BIC resonance. Furthermore, we achieve asymmetric transmission based on polarization-induced quasi-BIC modulation precisely at the quasi-BIC resonance position, and its isolation can be controlled by the external field. The study on dynamic quasi-BIC by the LC-integrated metasurface introduces a very promising route for active THz devices, which guarantees potential applications for THz communications, switching, and sensing systems.

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

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

Graphene metamaterial (GMM) based dual mode, tunable, and broadband THz absorber with triple circular split ring structure

In this study we investigated a novel approach to designing a graphene metamaterial (GMM) based terahertz (THz) absorber with dual-mode functionality, tunability, and broadband absorption capabilities. The study leverages the unique properties of graphene, a material known for its exceptional electronic and optical characteristics, combined with metamaterials to achieve efficient THz absorption. Here we performed extensive simulation on four different types of configurations and found the optimized structure has the highest bandwidth of 3.8 THz and absorption over 90 %. The absorber is designed to operate in two distinct modes, enhancing its versatility for different applications in the THz spectrum. Moreover, the tunability of the absorber is a significant feature, allowing for dynamic adjustment of the absorption frequency, which is crucial for applications in THz imaging, sensing, and communication systems. The broadband nature of the absorber ensures effective performance over a wide range of frequencies, addressing the need for flexible and high-performance devices in emerging THz technologies. This work represents a significant advancement in the field of THz metamaterials, with potential implications for the development of next-generation THz devices.

Computational study of the structural, mechanical, electronic, optical and thermal properties of BaLiX (X =P, As, Sb) perovskites

This study explores the structural, optical, mechanical, electronic, and thermal characteristics of ionic semiconductor compounds BaLiX (X = P, As, Sb) using Density Functional Theory (DFT). A comprehensive analysis of BaLiX (X = P, As, Sb) is conducted, proving that the estimated and observed lattice parameters coincide well and confirming mechanical stability through elastic stiffness constants. Electronic band structures reveal direct bandgap semiconductor properties with values of 0.70 eV, 0.30 eV, and 0.95 eV for BaLiP, BaLiAs, and BaLiSb, respectively, suggesting specific applications such as mid-infrared photodetectors, terahertz devices, and near-infrared sensors. Optical property analyses, including energy loss function, reflectivity, refractive index, and absorption coefficient, highlight the potential of these materials for optoelectronic and photovoltaic applications. Despite elastic anisotropy, optical anisotropy remains minimal. The materials exhibit potential for use as thermal barrier coatings (TBC) due to their comparatively lower Debye temperature (D), minimum thermal conductivity (Kmin) and lattice thermal conductivity (kph). Moreover, heat capacity calculations and thermal coefficient of expansion are also calculated.

A bidirectional broadband multifunctional terahertz device based on vanadium dioxide

We propose a switchable and bidirectional metamaterial terahertz absorber/reflector based on vanadium dioxide. Simulation results show that the device can achieve broadband absorption and reflection by adjusting the phase state of vanadium dioxide. When the vanadium dioxide is in the insulating phase, the device can be a perfect bidirectional broadband absorber with more than 90% absorptivity under normal forward/backward TE/TM-polarized incident wave in the frequency range from 1.9 THz to 10 THz. This excellent absorption characteristic is insensitive to the polarization angle of the incident wave. When the vanadium dioxide is in the metal phase, the device achieves more than 90% reflectivity in the low-frequency range under forward/backward TE-polarized incident wave. In the high-frequency range, the device has both reflection and absorption effects on the TE-polarized wave. When vanadium dioxide is in two different phase states, the performance of device is not sensitive to the electromagnetic wave incidence angle. The change in vanadium dioxide conductivity does not affect the stable performance of the device at high (low) frequencies when used as an absorber (reflector). The proposed device has potential applications in terahertz energy harvesting, detection, sensing, optical switching, and shielding.

Graphene Terahertz Devices for Sensing and Communication.

Graphene-based terahertz (THz) devices have emerged as promising platforms for a variety of applications, leveraging graphene's unique optoelectronic properties. This review explores recent advancements in utilizing graphene in THz technology, focusing on two main aspects: THz molecular sensing and THz wave modulation. In molecular sensing, the environment-sensitive THz transmission and emission properties of graphene are utilized for enabling molecular adsorption detection and biomolecular sensing. This capability holds significant potential, from the detection of pesticides to DNA at high sensitivity and selectivity. In THz wave modulation, crucial for next-generation wireless communication systems, graphene demonstrates remarkable potential in absorption modulation when gated. Novel device structures, spectroscopic systems, and metasurface architectures have enabled enhanced absorption and wave modulation. Furthermore, techniques such as spatial phase modulation and polarization manipulation have been explored. From sensing to communication, graphene-based THz devices present a wide array of opportunities for future research and development. Finally, advancements in sensing techniques not only enhance biomolecular analysis but also contribute to optimizing graphene's properties for communication by enabling efficient modulation of electromagnetic waves. Conversely, developments in communication strategies inform and enhance sensing capabilities, establishing a mutually beneficialrelationship.

Switchable Vanadium Dioxide Metasurface for Terahertz Ultra-Broadband Absorption and Reflective Polarization Conversion.

Based on the unique insulator-metal phase transition property of vanadium dioxide (VO2), we propose an integrated metasurface with a switchable mechanism between ultra-broadband absorption and polarization conversion, operating in the terahertz (THz) frequency range. The designed metasurface device is constructed using a stacked structure composed of VO2 quadruple rings, a dielectric layer, copper stripes, VO2 film, a dielectric layer, and a copper reflection layer. Our numerical simulations demonstrate that our proposed design, at high temperatures (above 358 K), exhibits an ultra-broadband absorption ranging from 4.95 to 18.39 THz, maintaining an absorptivity greater than 90%, and achieves a relative absorption bandwidth of up to 115%, significantly exceeding previous research records. At room temperature (298 K), leveraging VO2's insulating state, our proposed structure transitions into an effective polarization converter, without any alteration to its geometry. It enables efficient conversion between orthogonal linear polarizations across 3.51 to 10.26 THz, with cross-polarized reflection exceeding 90% and a polarization conversion ratio over 97%. More importantly, its relative bandwidth reaches up to 98%. These features highlight its wide-angle, extensive bandwidth, and high-efficiency advantages for both switching functionalities. Such an ultra-broadband convertible design offers potential applications in optical switching, temperature dependent optical sensors, and other tunable THz devices in various fields.

Advancement in Carbon Nanotubes Optoelectronic Devices for Terahertz and Infrared Applications

AbstractBenefiting from the outstanding optical, thermal, electrical, and mechanical properties, carbon nanotubes (CNTs) hold significant potential in the fields of materials, physics, chemistry, biology, as well as emerging disciplines such as electronics and optoelectronics. This paper provides a comprehensive review of the latest developments of CNTs optoelectronic devices operating in the terahertz (THz) and infrared (IR) wave range. At the beginning of this review, the unique structural characteristics of CNTs are introduced, overviewing their fundamental electronic structure, and emphasizing their impact on optoelectronics behavior. Further, on various synthesis techniques are discussion employed for the growth of single nanotube and the preparation of CNTs films. The penultimate section, the optical properties of CNTs are analyzed within the THz and IR spectral region and overview the application of THz time‐domain spectroscopy in extracting key material parameters of CNTs, and further discuss the theoretical models describing THz conductivity in CNTs. In the end, the most promising CNTs‐based device concepts are highlighted for sources, detectors, modulators, absorbers, and sensors within in THz and IR frequency band. This comprehensive review provides a valuable insight into the utilization of CNTs across various aspects of THz and IR optoelectronic devices.

A high-performance sub-THz planar antenna array for THz sensing and imaging applications

Terahertz (THz) spectral region from 0.1 to 3 THz is envisaged to hold immense potential in the next generation of wireless technologies. Recently, research has focused on this terahertz gap, because of its unprecedented channel capacities. At the physical layer, the design complexities and fabrication of THz devices, especially antennas are the prime bottlenecks to realize its full potential. This article introduces a cost-effective, easy-to-fabricate, and reproducible sub-THz antenna design based on a single-layer planar printed circuit board technology. The antenna incorporates carefully designed quasi-cross slots and applied machine learning-assisted global optimization techniques to achieve the desired performance metrics. The antenna performance is elucidated through numerical simulations and verified through a rigorous in-house THz experimental framework around 100–110 GHz. The proposed antenna offers a peak gain of 13.90 dBi with less than 1 dB variation within the entire band of 100–110 GHz. The antenna holds the potential to achieve terabits per second data rates and futuristic high-resolution short-range THz imaging applications.