Potential Terahertz Research Articles

Focused THz wave from a spintronic terahertz Fresnel Zone Plate emitter

Spintronic terahertz emitters (STEs) with the feature of low cost and high performance are considered as one of the most potential terahertz (THz) sources for the future applications of THz waves. At present, the demand for a STE with some additional functions apart from the generation of a THz wave only has become more and more urgent. THz lenses, as a wavefront control device, which is one of the indispensable elements in a THz system, are widely used in THz focus and image systems. However, the common THz lenses are bulky and costly and not applicable for some miniaturization and integration THz systems. Based on two nonferromagnetic film with comparable magnitude of spin Hall angle, but with opposite sign, a binary-phase spintronic terahertz Fresnel Zone Plate emitter (ST-FZPE) is proposed in this paper. Compared with reported THz emitters with a FZP structure, the ST-FZPE is low cost, easy to be fabricated and has a higher efficiency of directing emission focused THz wave. As a planar optical element, the ST-FZPE has tremendous significance to realize integration and miniaturization of THz devices and systems, and it has potential applications in the field of THz bio-sensing, THz imaging and THz communication in the future.

Switchable terahertz orbital angular momentum Bessel beams based on spin-decoupled multifunctional reflective metasurfaces.

In this study, switchable terahertz (THz) multi-orbital angular momentum (OAM) Bessel beams (BBs) were developed based on a spin-decoupled reflective multifunctional metasurface (MTS). Switchability was achieved by switching the feed between left-hand circular polarization (LCP), right-hand circular polarization (RCP), and linear polarization (LP) incidences. A switchable physical model was established for calculating the beam direction, OAM mode, polarization, and non-diffractive distance of the outgoing BBs. As an example, a spin-decoupled MTS was designed to generate dual BBs under LCP incidence, which was subsequently switched to RCP or LP for switchability. The outgoing BBs could be switched among three types of beams: Type-1 under LCP incidence (LCP, θL = 40°, φL = 0°, lL = 1, dL = 18 cm) and (RCP, θR = -40°, φR = 0°, lR = -1, dR = 20 cm); Type-2 under RCP incidence (RCP, θR = 40°, φR = 0°, lR = 1, dR = 18 cm) and (LCP, θL = -19°, φL = 0°, lL = 3, dL = 16.4 cm); and Type-3 under LP incidence (LP, θ = 40°, φ = 0°, l = 1, d = 18 cm), (RCP, θR = -40°, φR = 0°, lR = -1, dR = 20 cm) and (LCP, θL = -19°, φL = 0°, lL = 3, dL = 16.4 cm). Compared with previous MTSs, the proposed spin-decoupled MTS has the advantages of switchability among BBs, high non-diffractive distance/aperture size ratio of 15, large beam deflection angle of up to 40°, and high BB conversion efficiency of up to 96%. The simulated results were consistent with those calculated using the physical model, thus validating the physical model. The designed switchable BBs have potential THz near-field applications, such as high-capacity near-field wireless communications, wireless power transfer, high-resolution imaging, non-destructive testing, and speed detection of high-speed rotating objects.

Numerical study on THz radiation of two-dimensional plasmon resonance of GaN HEMT array

The GaN high electron mobility transistor (HEMT) has been considered as a potential terahertz (THz) radiation source, yet the low radiation power level restricts their applications. The HEMT array is thought to improve the coupling efficiency between two-dimensional (2D) plasmons and THz radiation. In this work, we investigate the plasma oscillation, electromagnetic radiation, and the integration characteristics of GaN HEMT targeting at a high THz radiation power source. The quantitative radiation power and directivity are obtained for integrated GaN HEMT array with different array periods and element numbers. With the same initial plasma oscillation phase among the HEMT units, the radiation power of the two-element HEMT array can achieve 4 times as the single HEMT radiation power when the array period is shorter than 1/8 electromagnetic wavelength. In addition, the radiation power of the HEMT array varies almost linearly with the element number, the smaller array period can lead to the greater radiation power. It shows that increasing the array period could narrow the main radiated lobe width while weaken the radiation power. Increasing the element number can improve both the radiation directivity and power. We also synchronize the plasma wave phases in the HEMT array by adopting an external Gaussian plane wave with central frequency the same as the plasmon resonant frequency, which solves the problem of the radiation power reduction caused by the asynchronous plasma oscillation phases among the elements. The study of the radiation power amplification of the one-dimensional (1D) GaN HEMT array provides useful guidance for the research of compact high-power solid-state terahertz sources.

Emergence of zero-field non-synthetic single and interchained antiferromagnetic skyrmions in thin films

Antiferromagnetic (AFM) skyrmions are envisioned as ideal localized topological magnetic bits in future information technologies. In contrast to ferromagnetic (FM) skyrmions, they are immune to the skyrmion Hall effect, might offer potential terahertz dynamics while being insensitive to external magnetic fields and dipolar interactions. Although observed in synthetic AFM structures and as complex meronic textures in intrinsic AFM bulk materials, their realization in non-synthetic AFM films, of crucial importance in racetrack concepts, has been elusive. Here, we unveil their presence in a row-wise AFM Cr film deposited on PdFe bilayer grown on fcc Ir(111) surface. Using first principles, we demonstrate the emergence of single and strikingly interpenetrating chains of AFM skyrmions, which can co-exist with the rich inhomogeneous exchange field, including that of FM skyrmions, hosted by PdFe. Besides the identification of an ideal platform of materials for intrinsic AFM skyrmions, we anticipate the uncovered knotted solitons to be promising building blocks in AFM spintronics.

Anomalous low-energy carrier dynamics of stripe-type charge ordered Pr0.5Sr0.5MnO3 thin film revealed by magneto-terahertz spectroscopy

The perovskite manganites host a range of charge- and orbital-ordered phases wherein the intrinsic and extrinsic controls of magnetic field, doping, epitaxial strain, chemical pressure, etc., induce subtleties of phase transition. The Pr0.5Sr0.5MnO3 (PSMO) is one such system that possesses a unique stripe-type charge-order manifesting in anisotropic transport. It exhibits a Drude-type and charge-density-wave (CDW) type of low-energy phases; the magnetic field is contemplated as a key control for both these phases; however, it has not yet been explored. Here, we have investigated the magnetic-field dependence of Drude-type terahertz (THz) carriers dynamics and the CDW type of low-energy excitations along two non-identical orthogonal in-plane axes of the (110) epitaxial thin film in the energy range of 1–7 meV using magneto-THz time-domain spectroscopy in Faraday geometry. While THz Drude conductivity expectedly increases with the increasing magnetic field, the CDW type of resonance absorption peak anomalously strengthens with the field. The origin of this unusual field-induced stabilization of the CDW-type mode is expected to lie in the d(x2–y2) orbital ordering in the A-type antiferromagnetic ordered state and anisotropic charge ordering. This study brings out a unique facet of the effect of magnetic field on orbital ordering of anisotropic stripe-type charge-ordered PSMO, which has potential THz modulation and switching applications.

Tamm plasmon polaritons induced active terahertz ultra-narrowband absorbing with MoS2

Tamm cavity resonant properties and singular phase behaviors of the one-dimensional photonic crystal stacked system with doped MoS2 have been comprehensively studied. Active multichannel terahertz ultra-narrowband wave absorption originated from the coupling effect of Tamm plasmon polaritons and Fabry-Perot modes. To reveal the critical absorption criterion, absorption amplitude changes in the same coupling mode order are discussed. We specifically discovered that this system has a pair of phase singularities (i.e., points of maximum absorptivity) corresponding to opposite abrupt phase transitions. It means an additional monitoring indicator used for high-sensitive label-free detection of media that outperforms metasurface absorption systems with one singularity is valuable, and no etching is required. More importantly, the phase transition phenomenon can be switched by electric-driven MoS2 via changing conductivity. These results demonstrated the proposed structure is suitable for potential terahertz applications including multifunctional phase-sensitive detectors and high-performance sensors.

High power terahertz radiation generated by beam-plasma system in multi-filament regime

In the particle-in-cell simulation study of this paper, it is found that the high-power terahertz radiation at twice the plasma frequency can be generated by the interaction between a weakly relativistic, large-radius, focused electron beam and an overdense plasma. The terahertz radiation is emitted from a multi-filament structure in the electron beam, which is caused by the coupling of the longitudinal two-stream instability and the transverse filamentation instability. The analysis of the k space of the plasma waves indicates that the radiation can be explained by the mode coupling among a forward oblique mode, a backward oblique mode, and a second harmonic radiation mode. The influences of the beam Lorentz factor and plasma density on the radiation power, efficiency, and coherence are also investigated. The present results not only contribute to the development of novel megawatt-level narrowband terahertz radiation sources, but also propose a potential terahertz radiation mechanism for the fundamental research of the beam-plasma system.

Plasmonics-based gas sensor with photonic spin hall effect in broad terahertz frequency range under variable chemical potential of graphene.

Graphene monolayer of sub-nanometer thickness possesses strong metallic and plasmonic behavior in a broad terahertz (THz) frequency range. This plasmonic effect can be considerably manipulated when graphene layer is subjected to a variable chemical potential (Ef) via chemical doping or electrical gating. The strong adsorption characteristics of graphene layer is another important advantage. In this work, a photonic spin Hall effect (PSHE) based plasmonic sensor consisting of germanium prism, organic dielectric layer, and graphene monolayer is simulated and analyzed in THz range aiming at highly sensitive and reliable gas sensing. Modified Otto configuration and Kubo formulation for graphene at room temperature are considered. The sensor’s performance is examined in terms of figure of merit (FOM). The analysis indicates that under angular interrogation scheme of sensor operation, the FOM improves for smaller chemical potential (moderate doping) and higher THz frequency. Moreover, the influence of temperature on gas sensor’s performance (FOM) is negligible, which suggests that the sensor is capable of providing stable sensing performance against temperature variation. The sensor design is highly flexible in terms of selection of THz frequency as an alternative interrogation scheme (i.e., measuring the variation in spin-dependent shift peak value of PSHE spectrum upon change in gas medium refractive index) can also be implemented. It is found that there is no need to change the moderate doping of graphene monolayer (i.e., Ef remains around its normal value ~ 0.1 eV) as the sensitivity achievable with this alternative method has considerably greater magnitude at smaller THz frequency (e.g., 2 THz). The magnitudes of FOM (with angular interrogation method) and sensitivity (with alternative method) are found to be significantly greater for rarer gaseous media, which might possibly assist in early detection of airborne viruses such as SARS-Cov-2 (while using appropriate specificity method) and to measure the concentration of a particular gas in a given gaseous mixture.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11082-022-03626-7.

Multi-functional high-efficient reflective polarization converter based on all-metal stereostructured anisotropic metamaterials at terahertz frequency

In this paper, a high-efficient reflective polarization converter based on all-metal stereostructured anisotropic metamaterials (AMMs) in terahertz (THz) region was proposed and investigated numerically. The unit-cell structure of the converteris assembled by vertical metallic double L-shaped stereostructures (DLSSs) adhered on the ground plane. The simulated results show that the proposed stereostructured AMMs can convert the incident x/y-polarized (x/y-pol.) wave into the right/left-handed circular polarization (RCP/LCP) wave, y/x-polarized (y/x-pol.) wave, and left/right-handed circular polarization (LCP/RCP) wave at 1.37 THz, 1.45 THz and 1.54 THz, respectively. Furthermore, the high-efficient polarization conversion of the proposed AMMs can be sustained within a wide incident angle from 0° to 75° for both transverse electric (TE) and transverse magnetic (TM) modes. The physical mechanisms of the observed polarization conversion are elucidated by taking advantage of surface current distributions. The further simulation results indicate that the operation frequency can be adjusted by changing the geometric parameters of the DLSSs. The proposed stereostructured AMMs-based reflective polarization converter can find potential THz applications, such as remote sensors, imaging systems, and reflector antennas.

Tunable and coherent terahertz source based on CdSiP2 crystal via collinear difference frequency generation.

CdSiP2 (CSP) crystals have attracted increasing attention as efficient optical conversion media. Herein, the optical properties of a CSP crystal grown with the vertical Bridgman method are measured by a terahertz time-domain spectrometer (THz-TDS) at 0.2-3 THz. For the first time, to the best of our knowledge, the broadband, tunable, coherent, monochromatic THz radiation from 0.08 to 1.68 THz (3775-178 µm) is generated experimentally via this crystal, which is pumped by a nanosecond Q-switched Nd:YAG laser and an optical parametric oscillator (OPO) and based on difference frequency generation (DFG) technology. The output power and its corresponding conversion efficiency at 0.74 THz are 26.6 mW and 1.4 × 10-7, respectively. Our work demonstrates that the CSP crystal is a potential efficient terahertz DFG candidate for out-of-door applications.

Direct observation of terahertz topological valley transport.

Topological photonics offers the possibility of robust transport and efficiency enhancement of information processing. Terahertz (THz) devices, such as waveguides and beam splitters, are prone to reflection loss owing to their sensitivity to defects and lack of robustness against sharp corners. Thus, it is a challenge to reduce backscattering loss at THz frequencies. In this work, we constructed THz photonic topological insulators and experimentally demonstrated robust, topologically protected valley transport in THz photonic crystals. The THz valley photonic crystal (VPC) was composed of metallic cylinders situated in a triangular lattice. By tuning the relevant location of metallic cylinders in the unit cell, mirror symmetry was broken, and the degenerated states were lifted at the K and K' valleys in the band structure. Consequently, a bandgap of THz VPC was opened, and a nontrivial band structure was created. Based on the calculated band structure, THz field distributions, and valley Berry curvature, we verified the topological phase transition in such type of THz photonic crystals. Further, we showed the emergence of valley-polarized topological edge states between the topologically distinct VPCs. The angle-resolved transmittance measurements identified the bulk bandgap in the band structure of the VPC. The measured time-domain spectra demonstrated the topological transport of valley edge states between distinct VPCs and their robustness against bending and defects. Furthermore, experiments conducted on a topological multi-channel intersectional device revealed the valley-polarized characteristic of the topological edge states. This work provides a unique approach to reduce backscattering loss at the THz regime. It also demonstrates potential high-efficiency THz functional devices such as topologically protected beam splitters, low-loss waveguides, and robust delay lines.

Broadband terahertz tunable multi-film absorber based on phase-change material.

Based on the impedance matching method, we have numerically demonstrated a broadband tunable multilayer structure in a terahertz (THz) regime. The switchable functional characteristics of the absorber can be achieved by utilizing the phase transition property of vanadium dioxide (VO2). When VO2 is in the metallic state, the designed device behaves as a broadband absorber with an absorbance greater than 90% under normal incident from a 4.5 to 10 THz range. When VO2 is in the insulating state, the absorption in this band is down to near 0%. Moreover, this high absorption band shows a good polarization insensitive property and can be maintained over a range of incident angles up to 45°. Our proposed device exhibits the merits of wideband reconfigure absorbance in THz, and the absorber can be easily fabricated without involving any lithographic process, both of which are very attractive to potential THz applications such as sensing, camouflaging, and modulation of THz waves.

A Tutorial on Terahertz-Band Localization for 6G Communication Systems

Terahertz (THz) communications are celebrated as key enablers for converged localization and sensing in future sixth-generation (6G) wireless communication systems and beyond. Instead of being a byproduct of the communication system, localization in 6G is indispensable for location-aware communications. Towards this end, we aim to identify the prospects, challenges, and requirements of THz localization techniques. We first review the history and trends of localization methods and discuss their objectives, constraints, and applications in contemporary communication systems. We then detail the latest advances in THz communications and introduce THz-specific channel and system models. Afterward, we formulate THz-band localization as a 3D position/orientation estimation problem, detailing geometry-based localization techniques and describing potential THz localization and sensing extensions. We further formulate the offline design and online optimization of THz localization systems, provide numerical simulation results, and conclude by providing lessons learned and future research directions. Preliminary results illustrate that under the same transmission power and array footprint, THz-based localization outperforms millimeter wave-based localization. In other words, the same level of localization performance can be achieved at THz-band with less transmission power or a smaller footprint.

Design Considerations of Photonic THz Communications for 6G Networks

Large bandwidths at terahertz (THz) frequencies higher than 100 GHz have been garnering significant attention as key enablers for future wireless networks to provide data rates in the range of terabits per second, which is approximately one hundred times higher than the transmission rate of millimeter wireless systems. To fully utilize the merits of THz frequencies, devices with large operating bandwidths are highly desired. From this perspective, promising choices are photonic devices developed for fiber-optic communications, which have been commonly used since the early stages of THz communications research with different architectures and parameters. However, the factors limiting the performance of photonics-based THz systems have not yet been investigated thoroughly in terms of general system aspects. Here, we comprehensively analyze and compare various potential THz communications systems with a photonics-based THz transmitter. Based on modulation and optical light sources, we characterize the three most typical architectures and theoretically and experimentally analyze the impact of optical and phase noise on system performance.

Switchable and tunable terahertz metamaterial absorber with broadband and multi-band absorption.

In this paper, we propose and demonstrate a switchable terahertz metamaterial absorber with broadband and multi-band absorption based on a simple configuration of graphene and vanadium dioxide (VO2). The switchable functional characteristics of the absorber can be achieved by changing the phase transition property of VO2. When VO2 is insulating, the device acts as a broadband absorber with absorbance greater than 90% under normal incidence from 1.06 THz to 2.58 THz. The broadband absorber exhibits excellent absorption performance under a wide range of incident and polarization angles for TE and TM polarizations. Moreover, the absorption bandwidth and intensity of the absorber can be dynamically adjusted by changing the Fermi energy level of graphene. When VO2 is in the conducting state, the designed metamaterial device acts as a multi-band absorber with absorption frequencies at 1 THz, 2.45 THz, and 2.82 THz. The multi-band absorption is achieved owing to the fundamental resonant modes of the graphene ring sheet, VO2 hollow ring patch, and coupling interaction between them. Moreover, the multi-band absorber is insensitive to polarization and incident angles for TE and TM polarizations, and the three resonance frequencies can be reconfigured by changing the Fermi energy level of graphene. Our designed device exhibits the merits of bi-functionality and a simple configuration, which is very attractive for potential terahertz applications such as intelligent attenuators, reflectors, and spatial modulators.

Growth, spectral, laser damage and hirshfeld surface studies on configurationally locked 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene) malononitrile (OH1) single crystal- A potential terahertz emitter

We report on the growth, spectral, optical, laser damage and Terahertz (THz) wave generation studies and Hirshfeld analysis of the pyrrolidine-based, configurationally locked, polyene single crystals 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene) malononitrile (OH1), a potential organic material for THz generation. The laboratory synthesized OH1 was verified for its composition and crystalline phase using Carbon–Hydrogen–Nitrogen (CHN) analysis and powder X-ray diffraction method respectively. The functional groups of OH1 were confirmed through Fourier transformed infrared analysis whereas its band gap energy was calculated using the UV–Vis spectrum recorded on (100) plate of thickness of about 1 mm. The laser damage studies performed by employing Q-switched Nd:YAG laser revealed that the OH1 crystals could withstand laser fluences of the order of about 2.21 GW/ cm2.Further, the OH1 single crystals were effectively used for the generation of intense THz waves using indigenously developed setup which comprises of a Ti: Sapphire laser (with 140 fs pulses) and detection using a pyroelectric sensor. Furthermore, 3D Hirshfeld surfaces and 2D finger print plots of OH1 were supportive in decoding the intermolecular interaction behaviour and the packing structure of the crystals.

Enabling switchable and multifunctional terahertz metasurfaces with phase-change material

Achieving switchable and diversified functionalities in a single metasurface has garnered great research interest for potential terahertz applications. Here, we propose and demonstrate a phase-change metasurface that simultaneously supports broadband electromagnetically induced transparency (EIT) and broadband nearly perfect absorption, depending on the phase state of a phase change material-vanadium dioxide (VO2). The phase-change metasurface is composed of a VO2 nanofilm, a quartz spacer and gold split-square-ring resonators with VO2 nanopads embedded into the splits. When VO2 is in its insulating phase at room temperature, a broadband EIT window (maximum transmittance reaching 83%) with a bandwidth of 0.27 THz (relative bandwidth 30%) can be observed. Alternatively, when VO2 transforms into its fully metallic phase, the EIT functionality will be switched off and instead, the metasurface operates as a broadband absorber with the total absorption exceeding 93% and a bandwidth of 0.5 THz (relative bandwidth 74%). The electric and magnetic field distributions indicate that the broadband EIT stems from the bright-bright mode coupling and the broadband absorption arises from the excitation and superposition of two resonances within a metal-insulator-metal cavity. The design scheme is scalable from terahertz to infrared and optical frequencies, enabling new avenues towards switchable and multifunctional meta-devices.

Near-field Terahertz Sensing of HeLa Cells and Pseudomonas Based on Monolithic Integrated Metamaterials with a Spintronic Terahertz Emitter.

Label-free biosensors operating within the terahertz (THz) spectra have helped to unlock a myriad of potential THz applications, ranging from biomaterial detection to point-of-care diagnostics. However, the THz wave diffraction limit and the lack of emitter-integrated THz biosensors hinder the proliferation of high-resolution near-field label-free THz biosensing. Here, a monolithic THz emission biosensor (TEB) is achieved for the first time by integrating asymmetric double-split ring resonator metamaterials with a ferromagnetic heterojunction spintronic THz emitter. This device exhibits an electromagnetically induced transparency window with a resonance frequency of 1.02 THz and a spintronic THz radiation source with a bandwidth of 900 GHz, which are integrated on a fused silica substrate monolithically for the first time. It was observed that the resonance frequency experienced a red-shift behavior with increasing concentration of HeLa cells and Pseudomonas because of the strong interaction between the spintronic THz radiation and the biological samples on the metamaterials. The spatial frequency red-shift resolution is ∼0.01 THz with a Pseudomonas concentration increase from ∼0.5 × 104 to ∼1 × 104/mL. The monolithic THz biosensor is also sensitive to the sample concentration distribution with a 15.68 sensitivity under a spatial resolution of 500 μm, which is determined by the infrared pump light diffraction limit. This TEB shows great potential for high-resolution near-field biosensing applications of trace biological samples.

A 400-GHz Low Cost Planar Leaky-Wave Antenna With Low Sidelobe Level and Low Cross-Polarization Level

This letter proposes a 400-GHz leaky-wave antenna with low sidelobe level (SLL) and low cross-polarization level (CPL). Tapered radiation slots are designed for achieving a low SLL, virtual electrical walls are deployed to remove the loss from metal sidewalls by using a high-order mode feeding network, and a symmetrical layout with alternative-phase excitations is adopted for the array to suppress the CPL. The conventional computer numerical controlled metal-milling and printed-circuit-board processes are used to fabricate the designed prototype. Experiments are carried out to verify the design. The measured peak gain and radiation efficiency of the fabricated prototype are 21.32 dBi and 34.65% at 395 GHz, respectively. In addition, its measured SLL and CPL are about -25 and -30 dB, respectively. The proposed antenna shows merits of the low cost, low SLL and CPL performance for potential terahertz applications.

Excitation of graphene magneto-plasmons in terahertz range and giant Kerr rotation

We propose three classical coupling methods (Otto, Kretschmann, and grating configurations) to excite graphene magneto-plasmons (MPs). After MP excitation, extraordinary Kerr rotations are obtained. As a demonstration, we discuss the Otto configuration in detail, but the other two methods have similar mechanisms and are only given limited descriptions. The dispersion relations of the MPs are discussed and found to well match previous predictions. It is found that the MP excitation and critical coupling are directly responsible for the large Kerr rotations. Combining the graphene MPs and critical coupling, much larger Kerr rotations (easily above 20°) away from the cyclotron resonance are achieved, as compared to previously described Faraday rotations (below 10°) using graphene MPs. By optimizing the graphene properties and structure parameters, the Kerr rotations are further enhanced. Moreover, using the grating coupling method, we easily find the largest Kerr rotation over 50° at 3 THz. The results are calculated using the anisotropic scattering-matrix method, and the finite element method is used as a comparison. The calculation methods are discussed in detail as a basis for future studies. These results are helpful not only to better understand graphene MPs but also for their potential terahertz applications.