THz Range Research Articles

Inducing exceptional points, enhancing plasmon quality and creating correlated plasmon states with modulated Floquet parametric driving.

The control of low frequency collective modes in solids by light presents important challenges and opportunities for condensed matter physics. We propose a method to parametricallydrive low THz range collective modes in an interacting many body system using Floquet driving at optical frequencies with a modulated amplitude. We demonstrate that it can be used to design plasmonic time-varying media with singular dispersions. Plasmons near resonance with half the modulation frequency exhibit two lines of exceptional points connected by dispersionless states. Above a critical driving strength, resonant plasmon modes become unstable and undergo a continuous transition towards a crystal-like structure stabilized by interactions and nonlinearities. This new state breaks the discrete time translational symmetry of the drive as well as the translational and rotational spatial symmetries of the system and exhibits soft, Goldstone-like phononic excitations. Below the instability threshold, our method can be used to enhance the quality of plasmon resonances.

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.

Absorption processes of the first and second order in GaAs in the THz-IR range

Absorption of THz radiation in gallium arsenide (GaAs) substrates of photoconductive antennas determines the generation and detection efficiency, as well as the dynamic and frequency ranges, of THz spectrometers. Based on the THz and IR experimental data and their calculations using classical dispersion models, absorption processes in the THz–IR regions were analyzed, and broadband data on the temperature evolution of the optical parameters of GaAs substrates were obtained. The contribution of anharmonicity effects in the formation of the dielectric response function of GaAs in the THz range is demonstrated.

High efficient bi-functional metasurface with linear polarization conversion and asymmetric transmission for terahertz region

Abstract Polarization control is crucial in contemporary photonics, but achieving broad bandwidths and high efficiencies remains a significant challenge. Here, we propose a wideband and high-efficiency metasurface designed for linear polarization conversion and asymmetric transmission (AT) in the terahertz region based on a three-layer chiral structure. By integrating a double-split ring and strip resonator, the design can improve both wide bandwidth and high-efficiency cross-polarization conversion and AT capabilities for a linearly polarized wave. The operating band for AT is 0.73-3.01 THz with an efficiency above 0.8, while the polarization conversion ratio exceeds 0.99 across the 0.56-3.94 THz range. The underlying physical mechanism behind wideband transmission polarization conversion was fully elucidated through theoretical analysis and the characterization of surface current distribution. This designed structure can be used in terahertz imaging, sensing, and communication.

Circularly polarized twin port tunable silicon-graphene antenna with beam tilting characteristics for THz based 6G communication system

Abstract This communication explains the design and analyse of twin port silicon-graphene aerial. The intended aerial has three distinct features: (i) stimulation of ceramic through uneven cross slot produces the circularly polarised waves from 3.45 THz to 3.65 THz; (ii) a mirror arrangement of uneven cross shaped slot increases separation between ports by almost 25 dB; and (iii) suspension of partial reflecting surface over twin port antenna slants the radiation pattern by ±450. A sheet of graphene over silicon creates the tunablity in terms of working band and circularly polarized band by altering its chemical potential. The optimized result from HFSS is compared to the CST EM tool, and it is determined that the suggested aerial functions well in the 3.25-3.75 THz range with a peak gain of 3.5 dBi. By including pattern diversity characteristics, the suggested antenna's multi-port parameters are improved, and it becomes suitable for THz-built 6G wireless applications.

Compact Metasurface Terahertz Spectrometer

AbstractThe terahertz frequency region of the electromagnetic spectrum is crucial for understanding the formation and evolution of galaxies and stars throughout the universe's history, as well as the process of planet formation. Detecting the unique spectral signatures of molecules and atoms requires terahertz spectrometers, which must be operated in space observatories due to water vapor absorption in the Earth's atmosphere. However, current terahertz spectrometers face challenges such as low resolution, limited bandwidth, large volume, and complexity. In this paper, the issues of size and weight are addressed by demonstrating a concept for a centimeter‐sized, low‐weight terahertz spectrometer using a metasurface. The design of the metasurface spectrometer is first discussed for the 1.85 to 2.4 THz range, followed by its fabrication. Next, an array of quantum cascade lasers operating at slightly different frequencies around 2.1 THz is utilized to characterize the spectrometer. Finally, a spectrum inversion method is applied to analyze the measured data, confirming a resolution R (λ/Δλ) of at least 273. This concept can be extended to other application areas, such as planetary observations and various wavelengths in the far‐infrared (FIR) and near‐infrared (NIR) ranges.

Tunable mid-infrared absorber based on Dirac metamaterials

Abstract In this paper, a tunable metamaterial absorber based on Dirac semimetal is proposed, which consists of three different structures, from top to bottom, namely a double semicircular Dirac semimetal resonator, a silicon dioxide (SiO2) substrate, and a continuous vanadium dioxide (VO2) reflector layer. When the Fermi energy level of the Dirac semimetal is 10\ meV, the absorber absorbs more than 90% in the 39.06-84.76\ THz range. Firstly, taking advantage of the tunability of the conductivity of the Dirac semimetal, the dynamic tuning of the absorption frequency can be achieved by changing the Fermi energy level of the Dirac semimetal without the need to optimise the geometry and remanufacture the structure. Secondly, the structure has been improved by the addition of the phase change material VO2, resulting in a much higher absorption performance of the absorber. Since vanadium dioxide is a temperature-sensitive metal oxide with an insulating phase below the phase transition temperature (about 68℃) and a metallic phase above the phase transition temperature, this paper also analyses the effect of vanadium dioxide on the absorptive performance at different temperatures, with the aim of further improving the absorber performance.

Single layer vanadium oxide assisted switchable metasurfaces for transmissive and reflective polarization conversion

Polarization control using metasurfaces is highly advantageous; however, most metasurfaces operate exclusively in either transmission or reflection mode. Additionally, achieving both wide bandwidth and high efficiency simultaneously in their design remains a significant challenge. Here, we design a high-efficiency and wideband switchable metasurface that can switch between transmissive and reflective polarization conversion modes by utilizing a single layer of I-shaped resonators and gold-vanadium dioxide (VO2) alternating gratings printed on a quartz substrate. When VO2 transitions to its metallic phase, the continuous metal layers of gold-VO2 eliminate transmission and the designed metasurface operates as a reflective cross-polarization converter with a polarization conversion ratio (PCR) exceeding 0.9 over a broad frequency range of 1.4–3.3 THz. Conversely, when VO2 is in its insulator phase, the THz incoming wave can transmit through the gratings, and the design behaves as a transmissive cross-polarization converter with a PCR above 0.99 in the 0.75-4.0 THz range for a forward y- polarized wave. The working mechanism for both transmissive and reflective polarization conversion modes is explained through theoretical analysis and surface current distributions. With the advantages of facile design, high performance, and dual functionality, our design is expected to be applied in the fields of imaging, sensing, and communication.

Suppression of Kerr-induced satellites in multi-pulse CPA

Amplification of bursts of ultrashort pulses is very challenging when the intraburst repetition frequency reaches the THz range, corresponding to (sub)-ps intervals between consecutive pulses. Periodic interference significantly modifies conditions for chirped pulse amplification (CPA), leading to temporal and spectral distortions during CPA due to optical Kerr nonlinearity. Multi-pulse chirped amplification to mJ energies may lead to a pronounced degradation of burst fidelity and the appearance of periodic temporal satellites after de-chirping the amplified waveform. We study, experimentally and numerically, the limitations of THz burst-mode CPA caused by self- and cross-phase modulation. A number of practical recipes to suppress nonlinear distortions and improve energy scaling by optimizing burst parameters and applying modulation techniques are presented.

Aluminium nanoparticle-based ultra-wideband high-performance polarizer

The polarizer-based device industry is expanding quickly, requiring high-quality research on nanoscale wideband polarizers. Here, we investigated the possibility of utilizing Al dimer nanostructures on broad-band polarizers. Metals are always considered promising candidates for reflection-based polarizer development because of their high extinction ratio. This study proposes a novel nanoparticle polarizer comprised of semi-immersed Al nano-dimers with a 200 nm radius on a CaF2 substrate. Our proposed nano-dimer-based design demonstrated impressive polarization anisotropy in the near-infrared (NIR) and THz ranges than conventional wire-grid-based ones. This study includes calculating performance parameters for the extraction of the proposed polarizer, including insertion loss, extinction ratio (ER), Mueller matrix values, and polarization ellipse diagram. The finite-difference time-domain (FDTD) simulation-based results suggested obtaining more than 55 dB extinction ratio for the 0.2 to 9 THz range. In the THz region, simulation results suggest impressively better performance than conventional wire-grid polarizers. In THz frequency range, our proposed polarizer demonstrated its extinction ratio of up to 60 dB. The average extinction ratio and insertion loss over the 1–1665 μm wavelength were 29.01 dB and ∼1 dB, respectively. The idea of Al dimer and the insight gained from the results extracted from the rigorous simulation report suggested a great opportunity for developing micro-scale metallic wideband polarizers.

Application of terahertz time-domain spectroscopy and chemometrics-based crested porcupine algorithm in identification of different medicinal parts of Angelica sinensis

ObjectiveAngelica sinensis is one of the commonly used Chinese herbal medicine in traditional Chinese medicine clinic, exhibits different pharmacological characteristics due to variations in the content of active ingredients in its head, body, and tail. Therefore, research on the identification methods of different medicinal parts of Angelica sinensis is of great practical significance. Terahertz Time-Domain Spectroscopy (THz-TDS) technology is widely used in the field of nondestructive testing because of its unique electromagnetic wave characteristics. This study explores the feasibility of combining THz-TDS with chemometrics to identify different medicinal parts of Angelica sinensis. MethodsBy comparing the spectral response characteristics of different parts of Angelica sinensis to various optical parameters, the absorption coefficient spectrum in the 0.6–3.0 THz range was selected, and three types of feature extraction algorithms, namely, joint Competitive Adaptive Reweighted Sampling (CARS), Uninformative Variable Elimination (UVE), and Successive Projections Algorithm (SPA), were used to establish the classification models of Extreme Learning Machine (ELM), Random Forest (RF), and Support Vector Machine (SVM) in turn, and optimize the models by using the crown porcupine algorithm (Crested Porcupine Optimizer (CPO) to optimize the model. ResultsThe research results indicate that the CPO optimizer significantly improved the classification accuracy of the models, with the accuracy of the ELM, RF, and SVM models increasing by 4.36%, 1.11%, and 12.22%, respectively. The SPA-CPO-SVM model exhibited the best overall performance, achieving accuracies of 96.11% and 97.96% on the prediction and training sets, respectively, while the number of input features was only 5% of the total feature set. ConclusionThe results show that the fully joint feature extraction strategy and optimization algorithm can play a powerful synergistic effect in model construction, confirming the feasibility of THz-TDS technology to correctly identify different medicinal parts of Angelica sinensis, and providing an important reference for the application of terahertz technology in the identification of Chinese herbal medicines.

Optical parameters extraction of soil and its microplastics contamination using terahertz spectroscopy

The primary focus of this study investigates the potential of continuous-wave terahertz frequency-domain spectroscopy(CW-THz-FDS) as a rapid, non-destructive technique for detecting microplastic contamination in soil. Focusing on low-density polyethylene (LDPE) as a model microplastic, we prepared soil mixtures with varying LDPE concentrations (1 %,5 %,10 %,15 % by weight). We used a THz-FDS system operating between 0.1 and 0.7 THz to analyze the optical properties (transmission coefficient, attenuation and refractive index) of these mixtures and LDPE samples separately. Notably both soil and LDPE particles were of micrometer scale. Our results revealed distinct, concentration-dependent variations in these properties. The refractive index for the higher concentration (15 %) ranged from 2.2 to 2.3, while the attenuation coefficient varies between 10 and 14 cm−1. Crucially, we identified polylactic acid [PLA] as a suitable sample holder material due to its high transmission in the THz range. Consequently, this study suggests that terahertz spectroscopy can be a promising technique for detecting microplastic contamination in soil, offering a non-destructive and sensitive method for environmental monitoring.

Stretchable MWCNTs-OH/PDMS composite elastomer with hierarchical porous structure for wideband THz absorption

It is important to develop a wideband THz absorber for the prevention of terahertz electromagnetic pollution and information leakage. Some commonly used methods, such as metamaterials or dynamic modulation technology, have played important roles in the study of wideband THz absorbers. However, most of these absorbers are rigid or non-stretchable, which limits the practical applications in large mechanical deformation and non-plane scenarios. In the paper, we proposed a stretchable MWCNTs-OH/PDMS composite elastomer using the sugar template method. A series of characterization methods were carried out to verify the excellent compatibility of PDMS and MWCNTs-OH. Benefiting from its hierarchical porous structure, the draw-length ratio of sample and tensile strength reaches ∼ 190% and ∼ 0.4 MPa, respectively. More THz waves could be trapped inside the sample for conductive loss because of better impedance matching and conductivity loss (the smallest square value is ∼0.4 kΩ/□), which leads to wideband and high absorptivity (over 98% in 0.2∼2.0 THz range). Furthermore, the tested value of EMI SE was over 10 dB in 0.22∼1.82 THz frequency range at transmission mode. Combined with inherent hydrophobicity characteristics, the material also could be used in humid and rainy environments. The systemic study of stretchable MWCNTs-OH/PDMS composite elastomer will provide theoretical and technical support for subsequent THz absorption in practical scenarios.

A wavefront processing coding metasurface with independent dynamic control of circularly and linearly polarized

To address the current issue of complex coding metasurface structure and deficiency of independent phase control for distinct polarized light within the broadband range, we present a wavefront processing coding metasurface with independent dynamic control of circularly and linearly polarized. The results demonstrate that when VO2 is in the metallic state and circularly polarized (CP) is incident, the metasurface can achieve efficient broadband circular polarization conversion in the 0.48–1.19 THz range, with a relative bandwidth of 85.02 %. It can achieve full 0-2π phase coverage by leveraging the P-B phase principle. When VO2 is in the dielectric state and linearly polarized (LP) is incident, x-LP can attain full 0-2π phase coverage in the range of 1.54–1.60 THz and y-LP can reach full 0-2π phase coverage in the range of 0.65–1.05 THz, which can be adjusted independently. Through encoding and arranging the metasurface, abnormal reflection and vortex beam generation of different polarized light in the same device are realized.

Broadband THz Wave Generation and Detection in Organic Crystal PNPA at MHz Repetition Rates

AbstractOrganic nonlinear optical (NLO) crystals have emerged as efficient room‐temperature emitters of broadband THz radiation with high electric field strengths. Although initially confined to high‐pulse‐energy excitation in the millijoule range with kHz rates, recent efforts have focused on tailoring the physical properties of organic NLO materials for compatibility with popular MHz‐rate telecommunication wavelength lasers emitting nanojoule energy pulses. This is motivated by the large potential of such crystals for more portable and user‐safe spectroscopic systems, additionally complemented by their room‐temperature field‐sensitive THz detection capabilities. In this work, the MHz‐rate operation of the organic crystal PNPA ((E)‐4‐((4‐nitrobenzylidene)amino)‐N‐phenylaniline), which was recently discovered through data mining algorithms and reported to surpass the conversion efficiency of rivaling NLO crystals at 1 kHz repetition rate is demonstrated. Using a compact 200 mW Er:fiber laser producing 18 fs pulses at 50 MHz repetition rate, the THz field generation and detection capabilities of PNPA via performing gas phase spectroscopy in the 1.3–8.5 THz range are demonstrated. A dynamic range of 40 dB over 3.6 s and 70 dB over 2 h is obtained. The work extends the family of organic crystals compatible with telecommunication‐wavelength excitation using nJ pulses for spectroscopy beyond 5 THz.

An energy efficient leaky integrate and fire neuron using Ge-source TFET for spiking neural network: simulation analysis

The basic building block of neural network is a device, which can mimic the neural behavior. The spiking neural network (SNN) is an efficient methodology in terms of power and area. Due to the excess energy consumption and larger area, various spintronic neural devices are unfit for neuron applications. In this article, we have implemented Ge source based Tunnel FET (TFET) for ultralow energy spike generation using TCAD simulator. It is seen that Ge source TFET has signature spiking frequency in THz range versus input voltage curve of an artificial biological neuron. The simulated device deploy the leaky integrate and fire (LIF) technique for generation of neurons. The simulation result highlights that the energy of device is 1.08 aJ/spike, which is several order less than existing neural based FET devices in literature.

Intrinsic dipole Hall effect in twisted MoTe2: magnetoelectricity and contact-free signatures of topological transitions

We discover an intrinsic dipole Hall effect in a variety of magnetic insulating states at integer fillings of twisted MoTe2 moiré superlattice, including topologically trivial and nontrivial ferro-, antiferro-, and ferri-magnetic configurations. The dipole Hall current, in linear response to in-plane electric field, generates an in-plane orbital magnetization M∥ along the field, through which an AC field can drive magnetization oscillation up to THz range. Upon the continuous topological phase transitions from trivial to quantum anomalous Hall states in both ferromagnetic and antiferromagnetic configurations, the dipole Hall current and M∥ have an abrupt sign change, enabling contact-free detection of the transitions through the magnetic stray field. In configurations where the linear response is forbidden by symmetry, the dipole Hall current and M∥ appear as a crossed nonlinear response to both in-plane and out-of-plane electric fields. These magnetoelectric phenomena showcase fascinating functionalities of insulators from the interplay between magnetism, topology, and electrical polarization.

Graphene-based metamaterial ultrawideband absorber with enhanced terahertz performance

In this paper, we present a graphene-based metamaterial ultrawideband absorber operating in the terahertz regime. The absorber is composed of two patterned and continuous graphene layers, a bottom gold layer, and a SiO2 substrate that separates the continuous graphene layer from the gold layer. The absorption spectrum is enhanced by integrating a graphene resonator with water, which has frequency-dependent permittivity. The water is encapsulated in polytetrafluoroethylene (PTFE) in the form of a circular ring sandwiched between the two graphene layers. Simulation results show that the absorber attains a 90 % absorption bandwidth of about 14.55 THz. The proposed absorber is polarization-insensitive under normal incidence. For TE polarization, the absorption level remains stable for incident angles θ from 0° to 55° across the entire frequency band (14.55 THz), except for the 5–8 THz range. For TM polarization, the absorption level remains stable for θ ranging from 0° to 60° over the entire bandwidth. Additionally, the absorption level is consistent for both TM and TE polarizations across the entire bandwidth for incident angles φ from 0° to 90°.

Germanium Single Crystals for Photonics

Germanium (Ge) is a system-forming material of IR photonics for the atmospheric transparency window of 8–14 µm. For optics of the 3–5 µm range, more widespread silicon (Si), which has phonon absorption bands in the long-wave region, is predominantly used. A technology for growing Ge single crystals has been developed, allowing the production of precision optical parts up to 500 mm in diameter. Ge is used primarily for the production of transparent optical parts for thermal imaging devices in the 8–14 µm range. In addition, germanium components are widely used in a large number of optical devices where such properties as mechanical strength, good thermal properties, and climatic resistance are required. A very important area of application of germanium is nonlinear optics, primarily acousto-optics. The influence of doping impurities and temperature on the absorption of IR radiation in germanium is considered in detail. The properties of germanium photodetectors are reported, primarily on the effect of photon drag of holes. Optical properties in the THz range are considered. The features of optical properties for all five stable isotopes of germanium are studied. The isotopic shift of absorption bands in the IR region, caused by phonon phenomena, which was discovered by the authors for the first time, is considered.

Terahertz tomography for testing wrapped scintillating crystals

Terahertz time-domain spectroscopy (THz-TDS) is demonstrated to be effective in novel application, for 3D characterization of wrapped scintillator crystals in search for defects affecting the light-yield of these crystals. This is of special importance for fast scintillators currently in demand to be exploited in high-energy physics experiments and medical imaging devices. Typical inorganic (GAGG:Ce) and organic (BC-408) scintillators wrapped in polytetrafluoroethylene (PTFE) and enhanced specular reflector (ESR) tapes have been studied. Time-of-flight information extracted from THz-TDS data reveals positions of the interfaces between the materials, wrapping thickness variations, and allows for the wrapping inspection with resolution better than a single layer, typically ∼200 μm-thick, of PTFE, the currently most common wrapping material. The results are supported by the study of the properties of the wrapping materials in the THz range and evidence of the prospectiveness of THz-TDS technique as a novel tool for nondestructive inspection of wrapped scintillators.