Terahertz Frequency Research Articles

Tunable dynamic metamaterial for negative refraction

The effective ability to manipulate the negative refraction behaviors of electromagnetic waves in the terahertz (THz) frequency range is of great interest for various practical applications in communication, imaging, and sensing. In this study, we propose a robust graphene-integrated metamaterial design for the dynamic manipulation of negative refraction at terahertz frequencies. Using the distinctive characteristics of graphene combined with the specially designed metamaterial structure, we demonstrated the active control of negative refraction by modulating the graphene's chemical potential from 0.0 to 1.0 eV. By dynamically tuning the carrier density of graphene, we create the switchable states of the negative index for the transmission or the reflection modes, tailoring the terahertz wave behavior in real time. In particular, in the case of different applied temperatures (through the bias applied voltages) of the proposed metamaterial, these two modes are maintained over 60% for both reflection and transmission while preserving the negative refractive index. Our results reveal the tunability of negative refraction, paving the way for developing reconfigurable THz meta-devices with enhanced functionalities in the near future.

Impulse ultrawideband wireless communication system of the terahertz frequency band

Impulse ultrawideband wireless communication system of the terahertz frequency band

High-performance THz Metallic Axial Mode Helix Antenna with Optimised Truncated Hollow Cone Ground Plane for 6G Wireless Communication System

The Terahertz (THz) band antenna configuration operates in the 0.1–10 THz frequency range and offers a stable performance for future 6th Generation (6G) wireless communication systems. However, the available metallic axial mode helix antenna designs exhibit a peak directivity of lower than 18 dBi within 0.5–1 THz, making it inappropriate to be applied in wireless communication systems. Therefore, this study proposed a high-performance THz metallic five-turn axial mode helix antenna with an optimised truncated hollow cone ground plane for 6G wireless communication systems. Following the creation of the proposed antenna design using cost-effective copper (annealed), the truncated hollow cone ground plane of the THz axial mode helix antenna was optimised via simulation in a Computer Simulation Technology Microwave Studio (CST MWS) software and a verification of the proposed THz antenna design in Analysis System High-Frequency Structure Simulator (Ansys HFSS) software for a fair comparison. Based on the results, the proposed THz metallic axial mode helix antenna with optimised truncated hollow cone ground plane recorded an impedance bandwidth of 0.46 THz, Fractional Bandwidth (FBW) of 61.33% for |S11| ≤ -10 dB, and a maximum directivity and realised gain of 21.8 dBi and 21.5 dBi at 0.85 THz, respectively. Within the 0.5–1 THz, the proposed optimised THz antenna design achieved an outstanding performance, including an FBW of more than 50%, excellent directivity of higher than 15.8 dBi, radiation efficiency of greater than 87%, circular polarisation, and low-profile helix turns. In short, the proposed THz metallic axial mode helix antenna with optimised truncated hollow cone ground plane design is appropriate for various THz 6G wireless applications.

Adjustable broadband absorber based on vanadium dioxide multiple coupled diagonally sliced square ring shaped structure for THz frequency

A broadband absorber with multiple coupled diagonally sliced square rings at terahertz frequency using vanadium dioxide is proposed. The proposed structure exhibits more than 90 % absorption in the frequency range of 2.85–7.51 THz, with a relative bandwidth of 89.96 % and an absorption bandwidth of 4.66 THz. The absorptivity curve increases as vanadium dioxide conductivity rises from 200 S/m to 200,000 S/m, giving a wide range of tunability from 1.62 % to 100 % at 3.4 THz. Due to its geometrical symmetry, the proposed structure is independent of the polarization angle under normal incident plane waves. The proposed structure works for different incident angles for transverse electric (TE) mode and transverse magnetic (TM) mode with oblique incidence plane waves. The results demonstrate the broad bandwidth compared to the state-of-the-art designs within the same frequency band with potential applications in sensors, switches, tuning, and modulation in the terahertz range.

High sensitivity of semimetal photodetection via Bose–Einstein condensation

AbstractThe discovery of semiconductor has witnessed remarkable strides toward high performance of photodetectors attributed to its excellent carrier properties. However, semimetal, owning to the high carrier concentration and low carrier mobility compared to those of semiconductor, is generally considered unsuitable for photodetection. Herein, we demonstrate an outstanding photodetection in a layered semimetal titanium diselenide (TiSe2) in Bose–Einstein condensation (BEC) state. High sensitivity of semimetal photodetector is realized in the range of visible, infrared and terahertz bands. The noise equivalent power (NEP) has threefold improvement at the visible and infrared wavebands, and significant decrease by one order of magnitude in the terahertz frequencies via BEC phenomenon, attributed to the electrical parameter variation after condensation. The best NEP value in the terahertz frequency is comparable to that of commercial Si photodetector. Our results show another recipe to fabricate high performance of photodetection via semimetal except for semiconductor and pave the way to exploit macroscopic quantum phenomena for optoelectronics.image

Ultrafast spin-to-charge conversion in antiferromagnetic (111)-oriented L12-Mn3Ir

Antiferromagnetic L12-Mn3Ir combines outstanding spin-transport properties with magnons in the terahertz (THz) frequency range. However, the THz radiation emitted by ultrafast spin-to-charge conversion via the inverse spin Hall effect remains unexplored. In this study, we measured the THz emission and transmission of a Permalloy/(111)-oriented L12-Mn3Ir multilayer by THz time-domain spectroscopy. The spin Hall angle was determined to be approximately constant at 0.035 within a frequency range of 0.3–2.2 THz, in comparison with the THz spectroscopy of a Permalloy/Pt multilayer. Our results not only demonstrate the potential of L12-Mn3Ir as a spintronic THz emitter but also provide insights into the THz spin transport properties of L12-Mn3Ir.

Zinc oxide nanorods electronically controlled terahertz modulator

AbstractMetal oxides are commonly employed in terahertz (THz) modulator devices operating in the THz frequency band. These metal oxides, which rely on electronic control, exhibit a clear response to THz waves. The modulation of THz waves is achieved by applying an electric field directly onto the surface of ZnO nanorods and their metal dopants. This external electric field has the capability to modulate both the phase and transmittance of the THz wave. The devices based on Au‐doped ZnO nanorods demonstrate a significant enhancement in modulation depth due to the incorporation of the localized surface plasmon effect. In the realm of active THz modulation, the velocity of electric modulation is enhanced while the required driving power is reduced. The primary focus of this research article is to present an analysis of the active modulation properties and the underlying principles of ZnO nanorods and their metallic dopants when subjected to an applied electric field. The modulation bandwidth range from 0.1 to 1.0 THz for our devices. The phase modulation of 0.866 rad and the transmittance modulation with the modulation depth of 0.4 was realized.

Direct ink writing printed flexible double-layer staggered woodpile structure for multi-band compatible absorption of gigahertz and terahertz waves

Soft multi-band compatible electromagnetic wave absorbers have wide applications in 6G communication, radar stealth, detection instruments, and radio astronomy. In this context, a kind of double-layer staggered woodpile absorber is designed and manufactured by the direct ink writing (DIW) technology with a composite ink system composed of reduced graphene oxide (RGO)/spherical carbonyl iron (SCI) and polydimethylsiloxane (PDMS). The as-prepared absorber shows compatible absorption in both microwave and terahertz frequency bands. The effective absorption bandwidth (EAB) and minimum reflection loss (RLmin) reach 8.24 GHz and −52.2 dB at a thickness of 3.2 mm, respectively. In the terahertz band (0.5–3.5 THz), it shows effective absorption (absorption rate greater than 99 %) and achieves RL < -20 dB throughout the entire tested band. In addition, the double-layer staggered woodpile absorber has good flexibility and can be conveniently used for curved substrates. This work enables the potential for rapid manufacturing of flexible and compatible absorbing devices through DIW 3D printing for applications in 5/6G communication, military stealth and flexible wearable intelligent devices.

Optical bistability modulation based on graphene sandwich structure with topological interface modes.

In this paper, we have investigated optical bistability modulation of transmitted beam that can be achieved by graphene sandwich structure with topological interface modes at terahertz frequency. Graphene with strong nonlinear optical effect was combined with sandwich photonic crystal to form a new sandwich structure with topological interface modes. The light-limiting properties of the topological interface modes, as well as its high unidirectionality and high transmission efficiency, all contribute positively to the reduction of the optical bistability threshold. In addition, the topological interface modes can effectively ensure the stability of the two steady state switching in the case of external interference. Moreover, optical bistability is closely related to the incident angle, the Fermi energy, the relaxation time, and the number of layers of graphene. Through parameter optimization, optical bistability with threshold of 105 V/m can be obtained, which has reached or is close to the range of the weak field.

Arbitrary manipulations of broadband terahertz focused Poincaré beams enabled by anisotropic silicon metasurfaces

Focused Poincaré beams (FPBs) carrying inhomogeneous polarization properties describe the paraxial solution of the vector Helmholz equation in cylindrical coordinate systems, with potential for classical and quantum applications. Here, we theoretically demonstrate a generalized strategy for generating FPBs based on the spin decoupling mechanism. Silicon metasurfaces with high degrees of freedom allow the proposed design to generate vector beams carrying arbitrary topological charges in a broadband terahertz (THz) frequency range. By adopting a purely phase-based modulation approach, the single-layer polarization-multiplexed metasurface can act as a bridge between the conventional Poincaré sphere (PS) and the higher-order Poincaré sphere (HOPS), preserving the corresponding mapping relations. Benefiting from the independence of the on-demand tailored helical phases, we further evaluate the coaxial evolutionary behavior of FPBs, generating structured THz fields with incremental topological charges. It is foreseeable that this work will provide an efficient meta-platform for the generation of broadband THz structured fields and facilitate their application in information encryption and quantum scientific information.

Graphene Oxides as Epsilon‐Near‐Zero Platforms Operating in Terahertz Frequency Range

AbstractEpsilon‐near‐zero (ENZ) materials, with their unique light manipulation capabilities, have fascinating applications such as cloaking, super‐resolution imaging, energy harvesting, and sensing. Herein, free‐standing graphene oxide films are fabricated whose dielectric constant can be engineered by thermal reduction, resulting in ENZ characteristics at the transition between insulating and metallic phases. The ENZ frequency is found to be 0.4–0.5 THz under an annealing temperature of 200 °C, whereas another ENZ phase appeared at 360 °C. ENZ film can be transferred to curved metal surfaces, serving as superabsorbers that suppress wave reflection from underneath metal surfaces. Hyper‐resolution in nondestructive THz imaging is also possible, owing to superior beaming. Spatial resolution is improved two‐fold by adding the ENZ film, even when the metal pattern is enclosed by a paint layer. Lastly, hybrid sensors are developed by incorporating the meta‐pattern on the free‐standing ENZ substrate. The energy splitting of metamaterial resonance originating from the coupling with ENZ mode is observed. Hybridized devices have proven effective in detecting microorganisms with a low refractive index, owing to the unique dielectric configuration offered by ENZ material. Using the novel ENZ platform, a 15‐fold improvement in microbial sensitivity is achieved compared with the conventional sensors.

Recent Developments in Terahertz Nanosensors

Terahertz (THz) waves occupy the electromagnetic spectrum between microwave and infrared radiation, with frequencies typically ranging from 0.1 to 10 THz. Compared with other optic and electronic tools, this frequency range allows for unique sensing applications such as nondestructive, label‐free, and fast detection. Despite the promising features of THz sensing applications, the dimensional mismatch between THz wavelength and nanoscale agents hinders practical applications, especially in biosensing and chemical sensing. Several recent studies propose that engineered THz resonators, such as split ring resonators, linear dipole and slot antennas, and nanogap loop antennas, enhance the sensitivity for detecting trace amounts of target molecules, such as viruses and explosives. When combined with near‐field imaging techniques in the THz range, these THz nanosensors may revolutionize our understanding of complex nanoscale systems, including 2D materials, as researchers can observe quantum dynamics directly in molecules, mobile carriers in semiconductors, THz quantum nonlocal effects, and dynamics of excitons and polaritons at THz frequencies. Additionally, THz biomolecular sensors are also discussed, where the sensor platforms will lead to a great impact in the advancement of ultrasmall‐quantity characterization of proteins, label‐free diagnosis of Alzheimer's disease, and conformational dynamics of biomolecules in their aqueous environment.

Topologically originated optical analog of electromagnetically induced reflectance in Dirac semi-metal based photonic structure

The optical analog of the electromagnetically induced reflectance (EIR) effect was theoretically studied in an active topological photonic structure comprising Dirac semi-metal and topological photonic crystal. The destructive interference between the optical Tamm state and topological edge state results in an induced reflection. It was known that the EIR-like effect occurs in a system having a radiative state and a metastable state. Topological protection is used here to achieve a metastable state, so an effective design of the EIR-like effect was possible. The observed EIR-like effect was modeled as a coupled oscillator system. The use of bulk Dirac semi-metal makes this an active photonic system at terahertz frequencies where the Fermi energy can act as a tunable and controlling parameter through which the induced transparency can be varied.

Terahertz Plasmon Polaritons in Large Area Bi2Se3 Topological Insulators

AbstractAssessing the nature of topological quantum materials, and in particular probing the existence of topological surface states, is a very challenging task. Terahertz (THz) frequency scattering near‐field optical microscopy has emerged as an effective technique to investigate the presence of massless surface carriers by locally probing collective surface excitations, i.e., plasmon polaritons, whose dispersion critically depends on the density and nature of surface carriers. Here, thin (14–19 nm) films of Bi2Se3 are experimentally investigated through a combination of x‐ray diffraction, Hall‐bar magneto‐transport, and near‐field detectorless optical holography at THz frequencies, from 2 to 4.3 THz. The dispersion of surface plasmon polaritons are determined for different Bi2Se3 film thicknesses, proving the presence of massless surface carriers. The results open intriguing opportunities in THz nano‐plasmonics and topological nano‐photonics including the development of superlenses and metasurfaces, making use of plasmon polaritons.

Strong coupling of plasmonic bright and dark modes with two eigenmodes of a photonic crystal cavity.

Dark modes represent a class of forbidden transitions or transitions with weak dipole moments between energy states. Due to their low transition probability, it is difficult to realize their interaction with light, let alone achieve the strong interaction of the modes with the photons in a cavity. However, by mutual coupling with a bright mode, the strong interaction of dark modes with photons is possible. This type of mediated interaction is widely investigated in the metamaterials community and is known under the term electromagnetically induced transparency (EIT). Here, we report strong coupling between a plasmonic dark mode of an EIT-like metamaterial with the photons of a 1D photonic crystal cavity in the terahertz frequency range. The coupling between the dark mode and the cavity photons is mediated by a plasmonic bright mode, which is proven by the observation of a frequency splitting which depends on the strength of the inductive interaction between the plasmon bright and dark modes of the EIT-like metamaterial. In addition, since the plasmonic dark mode strongly couples with the cavity dark mode, we observes four polariton modes. The frequency splitting by interaction of the four modes (plasmonic bright and dark mode and the two eigenmodes of the photonic cavity) can be reproduced in the framework of a model of four coupled harmonic oscillators.

Discovery of high-Q Fabry–Pérot supercavity modes

We report on a high-quality Fabry–Pérot supercavity mode observed in the terahertz frequency range. The experiment is carried out on a silicon chip with metallic gratings of equal period lithographically fabricated on both sides of the substrate. We show that the supercavity mode arises from interference between the Fabry–Pérot and substrate waveguide modes. As a result, Q factors as high as 880 are achieved at the terahertz frequency band. Possible applications of surface-enhanced electromagnetic field amplification are discussed and demonstrated experimentally.

Anisotropic honeycomb stack metamaterials of graphene for ultrawideband terahertz absorption

Abstract Graphene aerogels have implied great potential for electromagnetic wave absorption. However, the investigation of their design for broadband absorption in the terahertz (THz) range remains insufficient. Here, we propose an anisotropic honeycomb stack metamaterial (AHSM) based on graphene to achieve ultrawideband THz absorption. The absorption mechanism is elucidated using the effective medium method, offering deeper physics insights. At low THz frequencies, the impedance matching from the air to the AHSM can be improved by reducing the chemical potential of graphene for high absorption. There is a suppression of absorption at the intermediate frequencies due to constructive interference, which can be avoided by shortening the sizes of honeycomb edges. With the aim to elevate absorption at high frequencies, one can increase the stack layer number to enhance multiple reflections and destructive interference within the metastructure. Based on the above principles, we design an AHSM that achieves a broadband absorbance of over 90 % from 1 THz to 10 THz. This absorption can tolerate a wide range of incident angles for both TE and TM wave excitations. Our research will provide a theoretical guide to future experimental exploration of graphene aerogels for THz metamaterial absorber applications.

Enhancement of graphene absorption: a numerical formula to design a suitable Fabry-Perot cavity in a terahertz spectral window

In this article, we investigate the absorption characteristics of a graphene-embedded FP cavity in a terahertz spectral window. The optical attributes were determined by a 4 × 4 transfer matrix procedure. The findings demonstrate that perfect absorption is completely reliant on the structural characteristics of the FP cavity throughout a broad range of terahertz frequencies. From the obtained dataset, numerical formulae are generated for structural parameters (N FD , N BD ) using a linear regression machine learning algorithm to achieve higher than 90% absorption. The artificial neural network trained using our dataset provided a coefficient of determination (R2)=1, opening up new pathways to design perfect terahertz absorbers. Furthermore, we explored the influence of magnetic biasing on absorption traits, and our findings show that fine absorption improvement is conceivable. The formulated numerical relations have greater importance in the design of perfect terahertz absorbers.

Efficient ultrafast field-driven spin current generation for spintronic terahertz frequency conversion

Efficient generation and control of spin currents launched by terahertz (THz) radiation with subsequent ultrafast spin-to-charge conversion is the current challenge for the next generation of high-speed communication and data processing units. Here, we demonstrate that THz light can efficiently drive coherent angular momentum transfer in nanometer-thick ferromagnet/heavy-metal heterostructures. This process is non-resonant and does neither require external magnetic fields nor cryogenics. The efficiency of this process is more than one order of magnitude higher as compared to the recently observed THz-induced spin pumping in MnF2 antiferromagnet. The coherently driven spin currents originate from the ultrafast spin Seebeck effect, caused by a THz-induced temperature imbalance in electronic and magnonic temperatures and fast relaxation of the electron-phonon system. Owing to the fact that the electron-phonon relaxation time is comparable with the period of a THz wave, the induced spin current results in THz second harmonic generation and THz optical rectification, providing a spintronic basis for THz frequency mixing and rectifying components.

A novel method for the measurement of superconducting transmission lines at terahertz frequencies.

Characterizing the properties (e.g., effective dielectric constant εeff, attenuation constant α, and characteristic impedance Z0) of terahertz (THz) superconducting transmission lines is of particular interest in designing on-chip integrated THz bandpass filters, which are a critical component for THz astronomical instruments, such as multi-color camera and broadband imaging spectrometers. Here, we propose a novel method for the characterization of three parameters (εeff, α, and Z0) of THz superconducting transmission lines. This method measures the ratio of the THz signal powers through two different-length branches of the superconducting transmission line to be measured. In addition, only one measurement is required for an all-in-one device chip, including an antenna, a half-power divider, the superconducting transmission line to be measured, and two detectors. The key point is that the superconducting transmission line to be measured is impedance-mismatched with the two integrated detectors. The method is validated through simulation and measurement for superconducting coplanar waveguide transmission lines around 400GHz.