Terahertz Frequency Regime Research Articles

Broadband terahertz absorption and Q-switching behavior of 5-chloro-2-nitroaniline (5C2NA) crystals

Abstract Growth of transparent 5-chloro-2-nitroaniline (5C2NA) crystals was achieved through the slow evaporation technique. Analysis via X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy confirmed the crystalline nature of the organic crystals and revealing distinct molecular fingerprints of 5C2NA respectively. UV-VIS and Photoluminescence (PL) spectroscopy were employed to study the material's band gap and ground state absorption respectively. Thermogravimetric analysis (TGA) analysis indicated the stability of the grown crystals up to 211ºC. Dielectric measurements and Urbach plot suggested the presence of fewer defects in 5C2NA crystals. Time domain terahertz spectroscopy was utilized to observe the variation of absorption coefficient and refractive index in the terahertz frequency regime. Nonlinear optical effects such as saturable absorption (SA) and reverse saturable absorption (RSA), are pivotal in the development of all-optical logic gates. The transition between SA and RSA possesses a significant importance in optoelectronic applications. In this study, we investigate the 5C2NA crystal, revealing its ability to exhibit both SA and RSA under Z scan technique with varying pump intensities. The switching properties observed in 5C2NA can be harnessed for applications such as all-optical logic gates, rapid optical switching, optical limiting, mode storage, and more.

Bidirectional triple-band truly incident angle insensitive polarization converter using graphene-based transmissive metasurface for terahertz frequency

Abstract The design and response of a triple-band linear-to-circular polarization converter (LTCPC) in the terahertz frequency regime using a graphene-based transmission-type metasurface have been studied numerically and analytically. The unit cell of the converter is constructed using lossy silicon dioxide (SiO2) with a relative permittivity of 3.9 as the substrate. And two similar layers of graphene-based sub-wavelength structures are used at the top and bottom of it. Multiband linear to circular polarization conversion is achieved from 0.74 THz to 0.98 THz, 1.70 THz to 2.33 THz, and 4.06 THz to 5.19 THz, i.e., about 28 %, 31 %, and 24 % fractional bandwidths, respectively. As the same metasurface configuration is used at the top and bottom layers of the transmission-type unit cell, it can be used as a bidirectional polarization converter with identical responses from both sides. The geometrical optimization of the unit cell is done, and an equivalent lumped parameter circuit model is also proposed for the same, considering analogous responses. Tunability over the operating frequency is achieved by varying the chemical potential and relaxation time of graphene. Moreover, due to the ultrathin width and special types of identical configuration of the metasurfaces, the response of the system remains excellent over a wide range of incident angle variations up to 80° angle of incidence.

High-speed terahertz communication with graphene-based time-modulated low-bit phase coding metasurfaces: A novel architecture for enhanced performance

Coding metasurfaces have revolutionized electromagnetic device development by enabling precise electromagnetic wave manipulation. While spatial phase modulation alone offers significant capabilities, its limitations to comprehensive control and optimal functionality are evident. As a result of recent advancements, joint amplitude-phase coding approaches have been explored using different techniques in order to enhance performance. However, such methods often result in complex structures due to the combining of various modulation techniques within meta-atoms. Addressing this challenge, this paper proposes a novel architecture for terahertz (THz) communication utilizing low-bit coding metasurfaces featuring simplified meta-atoms. Our approach introduces a coding unit cell based on graphene, combined with time-modulated coding, to enable phase and amplitude control for different harmonics. As a proof-of-concept application, we demonstrate the effectiveness of our technique in multimode vortex communication. Moreover, we show secure hologram communication by using our proposed design within the THz frequency regime. By enhancing wavefront shaping flexibility through a straightforward approach, this work lays the foundation for next-generation devices, particularly in high-speed macroscale and nanoscale communication networks.

Far IR field generation by mixing of cosh-Gaussian and dark hollow Gaussian laser in encapsulated plasma

A scheme of laser-plasma interaction is proposed by mixing of Cosh Gaussian (ChG) and Dark Hollow Gaussian (DHG) laser in rippled density plasma when an external periodic electric field wiggler is applied because of the emission of far infrared field lying in the terahertz frequency regime. The unusual mixing of ChG and DHG laser broadens the field of THz radiation from unifocal to multifocal by controlling beam order and decentralized parameters. Moreover, the wiggler field wavenumber provides two additional resonance conditions, which govern the emission of multiple components of far IR field resonantly that may have extensive medical applications. The field amplitude of such emissions and their efficiency may be increased manifold. Moreover, the efficiency can be increased several times if parallel plates are placed with a few nanometers of separation.

Terahertz binary computing in a coupled toroidal metasurface

The applications of terahertz metamaterials are being actively explored in recent times for applications in high-speed communication devices, miniature photonic circuits, and bio-chemical devices because of their wide advantages. The toroidal resonance, a new type of metasurface resonance, has been examined with great interest to utilize its properties in terahertz metasurface applications. This study reports a proof of concept design of a toroidal metasurface that experimentally demonstrates binary computing operations in the terahertz frequency regime. The analog computing of binary operations is achieved by the passive tuning of distance between the split ring resonators comprising the meta-molecule. The amplitude modulation is utilized as a method of determining the Boolean logic outputs of the system. The proposed metasurface could be further optimized for high amplitude modulations and active logic gate operations using tunable materials including graphene and ITO. The proposed metasurface consists of three split-ring resonators, and the near-field coupling between the adjacent resonators dictates the Boolean operations. A multipole analysis of the scattered powers of terahertz radiation determines the toroidal excitation in the metasurface. The proposed metasurfaces experimentally define AND Boolean logic operation at 0.89 terahertz, and OR Boolean logic operation at 0.97 terahertz. Numerical simulations support the experimentally obtained results. Additionally, we numerically report the excitation of NAND operation at 0.87 THz. Such toroidal analog computing metasurfaces could find applications in digitized terahertz circuits and integrated photonic devices.

Controlled circular dichroism with graphene-based metamaterial for terahertz wave

This article explores the design and analysis of a metal-graphene hybrid metamaterial structure tailored for tunable circular dichroism (CD) effects in the terahertz (THz) frequency regime. Chiral metamaterials have garnered considerable interest in photonics due to their versatile applications, including sensing, polarization manipulation, and chiral imaging. The proposed metamaterial unit cell features four meta-atoms with C4 rotational symmetry, composed of gold on a polyimide substrate. By strategically integrating the graphene sheets above the gold patterns, selective control over the absorption efficiency for the incident wave of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light is achieved. The study demonstrates that adjusting graphene chemical potential enables precise modulation of CD from 0.80 to 0.10 across a wide THz frequency spectrum. Furthermore, the article investigates the structure optical response for incident angles ranging up to 75°, revealing stable CD behavior up to 30° and intriguing dual-band effects beyond 50°. These findings underscore the potential of the proposed metamaterial for practical applications in photonics, sensing, and chiral imaging, offering tunable control over the CD effects in the THz regime.

Dual-band polarization insensitive graphene-based reconfigurable absorber in a terahertz regime.

In this paper, a dual-band graphene-based absorber is structured and investigated in a terahertz frequency domain. The designed absorber contains a plus-shaped graphene sheet, as well as four dual concentric graphene-based rings. It is designed according to the SiO2-based substrate. A dual concentric graphene ring provides more than 90% absorption from 2.36-3.24THz, while a plus-shaped graphene sheet is accountable for absorption from 5.0-5.32THz. The maximum absorption is found at 2.56, 3.08, and 5.16THz with 99.81%, 97.34%, and 99.74%, respectively. The variation in the chemical potential of the graphene material offers re-configurability in the dual absorption bands. Due to the four-fold symmetry of the designed structure, the absorptivity value is approximately constant for various polarizations with a wide incident angle (more than 60°). The aforementioned features of the proposed graphene-based absorber make it appropriate in a terahertz frequency regime.

Graphene based filter design using triangular patch resonator for THz applications

In this paper, graphene based tunable bandpass and bandstop filters are designed in terahertz frequency regime using a triangular microstrip patch resonator. Initially, a bandpass filter is designed at an operating frequency of 1.65 THz with a bandwidth of 65 GHz. Further a dual-band bandstop filter is designed with resonance frequencies of 1.25 THz and 2.14 THz. In both designs, the triangular patch is coupled to the transmission line to achieve bandpass and bandstop characteristics. A graphene layer is deposited between the dielectric and the conductor layer to enhance the propagation of plasmonic waves. The simulation results reveal that the designed filters are capable of achieving the desired frequency response. By varying the graphene’s chemical potential, a shift in the transmission response’s resonance frequency is observed. The proposed filters have the potential to be used as key components for future terahertz band communications systems.

Emulating the Deutsch-Josza algorithm with an inverse-designed terahertz gradient-index lens.

An all-dielectric photonic metastructure is investigated for application as a quantum algorithm emulator (QAE) in the terahertz frequency regime; specifically, we show implementation of the Deustsh-Josza algorithm. The design for the QAE consists of a gradient-index (GRIN) lens as the Fourier transform subblock and patterned silicon as the oracle subblock. First, we detail optimization of the GRIN lens through numerical analysis. Then, we employed inverse design through a machine learning approach to further optimize the structural geometry. Through this optimization, we enhance the interaction of the incident light with the metamaterial via spectral improvements of the outgoing wave.

Design of graphene-based multi-input multi-output antenna for 6G/IoT applications

In the future, 6G wireless communications will be integrated into various applications. It is expected that it will handle all Internet of Things services as well as satellite communications. Additionally, it is predicted to support machine learning (ML) and artificial intelligence (AI). So this work investigated four orthogonal ports graphene-based multiple-input and multiple-output (MIMO) antenna in the terahertz frequency regime. With wide-band (7.1-13) THz, minimum return loss close to -30 dB and a good isolation value. That was designed over silicon dioxide (Sio2) 100×100 substrate with a thickness of 10 µm and a maximum gain reaching 8.3 dB at 11 THz. By adjusting the provided DC voltage, the chemical potential of graphene can be tuned, which in turn allows for the MIMO antenna characteristics to be changed. Envelop correlation coefficient (ECC) and diversity gain (DG) were investigated for the presented design. In addition, these values were determined to assess compatibility and difficulties connected with communication over a short distance. The geometry of the MIMO antenna is adaptable to various applications in the THz band with highperformance requirements, such as sensing, scanning for security threats, biomedical applications, wireless communication systems with high speed built on the 6G standard, and the internet of things (IoT).

Tunable reflective tri-band polarization converter based on graphene composite metasurface

A graphene composite metasurface-based reflective tri-band polarization converter working in the terahertz frequency regime is proposed and investigated by CST Microwave Studio software. The polarization converter achieves high polarization conversion ratios (above 99%) for linear-to-linear polarization conversion at 6.21, 9.11, and 11.66 THz, and for linear-to-circular polarization conversion at 5.14 THz, with a fixed Fermi energy level (Ef) of 1.0 ev due to the excitation of localized surface plasmons. Subsequently, the reason for cross-polarization conversion at three resonant frequencies and the incident angle sensitive property of the converter were discussed. Based on the graphene component, a tunable tri-band polarization converter was obtained and investigated; this converter could be applied in the active modulation area of wireless communication.

New insights into APCVD grown monolayer MoS2 using time-domain terahertz spectroscopy

In modern era, wireless communications at ultrafast speed are need of the hour and search for its solution through cutting edge sciences is a new perspective. To address this issue, the data rates in order of terabits per second (TBPS) could be a key step for the realization of emerging sixth generation (6G) networks utilizing terahertz (THz) frequency regime. In this context, new class of transition metal dichalcogenides (TMDs) have been introduced as potential candidates for future generation wireless THz technology. Herein, a strategy has been adopted to synthesize high-quality monolayer of molybdenum di-sulfide (MoS2) using indigenously developed atmospheric pressure chemical vapor deposition (APCVD) set-up. Further, the time-domain transmission and sheet conductivity were studied as well as a plausible mechanism of terahertz response for monolayer MoS2 has been proposed and compared with bulk MoS2. Hence, the obtained results set a stepping stone to employ the monolayer MoS2 as potential quantum materials benefitting the next generation terahertz communication devices.

Highly sensitive tunable terahertz absorber for biosensing applications

A multicontrollable terahertz perfect absorber has been proposed and demonstrated numerically through an L-shaped metasurface tri-layer structure. The proposed tri-layer structure comprises a top L-shaped InAs layer and gold ground plane separated by a dielectric polyimide layer which shows a perfect absorption of more than 99.8% with the application of magnetic field from 0.5–0.8 T along with a higher tunability rate of 0.76 THz/T in the frequency spectrum. The multicontrollability and tunability in the frequency response are achieved by the application of a magnetic field as well as the geometrical rotation of meta-atom. Thus, the proposed structure act as a bicontrollable metasurface providing an efficient approach for control over the maximum absorptance frequency. Furthermore, the performance of the proposed absorber shows greater applicability for the bio-chemical sensors with a high value of sensitivity of around 1109 GHz/RIU for the detection of different chemicals, 1012 GHz/RIU for different pathogens, and 1066 GHz/RIU for different hemoglobin concentrations. Thus, the proposed metasurface provides a bicontrollable approach for efficient absorption and prevents transmission as well as acts as multifunctional metasurface which provides potential applications in designing bio-chemical sensors at terahertz frequency regimes.

A New Design of a Terahertz Metamaterial Absorber for Gas Sensing Applications

Metamaterial absorbers are used in the terahertz frequency regime as photo-detectors, as sensing elements, in imaging applications, etc. Narrowband absorbers, on account of their ultra-slender bandwidth within the terahertz frequency spectrum, show a significant shift in the absorption peak when an extrinsic entity relative to the absorber, like refractive index or temperature of the encircling medium, is altered. This property paves the path for the narrowband absorbers to be used as potential sensors to detect any alterations in the encircling medium. In this paper, a novel design of a terahertz metamaterial (MTM) absorber is proposed, which can sense the variations in the refractive index (RI) of the surrounding medium. The effective permeability of the structure is negative, while its permittivity is positive; thus, it is a μ-negative metamaterial. The layout involves a swastika-shaped design made of gold on top of a dielectric gallium arsenide (GaAs) substrate. The proposed absorber achieved a nearly perfect absorption of 99.65% at 2.905 terahertz (THz), resulting in a quality factor (Q-factor) of 145.25. The proposed design has a sensitivity of 2.12 THz/RIU over a range of varied refractive index from n = 1.00 to n = 1.05 with a step size of 0.005, thereby achieving a Figure of Merit (FoM) of 106. Furthermore, the sensor was found to have a polarization-insensitive characteristic. Considering its high sensitivity (S), the proposed sensor was further tested for gas sensing applications of harmful gases. As a case study, the sensor was used to detect chloroform. The proposed work can be the foundation for developing highly sensitive gas sensors.

Tunable Reflection Properties and Photonic Bandgap Behavior of a One-Dimensional Metamaterial-Superconductor-Based Ternary Photonic Crystal

In the present communication, we have investigated the tunable reflection properties and photonic bandgap variation of a one-dimensional metamaterial and superconductor based ternary photonic crystal. To design the 1D ternary photonic crystal, we have taken three alternate layers of metamaterial, superconductor (BSSCO), and silica (SiO2). Transfer matrix method (TMM) is employed to determine the optical reflection properties of the considered 1DPC for TE-mode by varying five parameters, viz. incident angle, superconductor temperature, thickness and refractive index of dielectric layer, and number of unit cells. It is noted that the ternary PC exhibits the features of tunable narrow band reflector, whose width increases with increase in the incident angle with a blue shift, and decreases with operating temperature of superconductor with red shift. It also enhances with increase in the dielectric layer thickness in low frequency region, and decreases in high frequency regime with red shifts. We obtain only one PBG at n3=1.46 in low frequency regime, while with increase in the refractive index multiple gaps are obtained in different frequency regions having distinct widths at n3=2.4 and 3.4. An increase in the number of unit cells causes increase in the reflectance, wherein no PBG is found below N=8. This analysis gives some insights to design tunable devices based on narrowband reflectivity in the terahertz frequency regime, including THz-metadevices, emitters, thermo-optical devices, and security sensors.

The Coming Age of Pnictide and Chalcogenide Ternary Crystals in the Terahertz Frequency Regime

The last decade has witnessed explosive growth in terahertz radiation fields, ranging from fundamental research to applications. This progress has been marked by the introduction of various terahertz radiation sources and detectors. Here, we review the progress of nonlinear pnictide and chalcogenide ternary crystals for use in the terahertz frequency regime. At the center of this topical review are the various physical and optical properties of numerous emerging pnictide and chalcogenide nonlinear crystals, which are anchored to state-of-the-art coherent terahertz radiation sources based on optical rectification and difference frequency generation, as well as terahertz radiation phase-resolved detectors based on electro-optic sampling. This review aims to provide the foundation necessary to continue advancing pnictide and chalcogenide ternary crystals for the generation, detection, and manipulation of terahertz radiation.

Delineation of profoundly birefringent nonlinear photonic crystal fiber in terahertz frequency regime

Abstract The photonic aspects of semiconducting hexagon-shaped photonic crystal fiber including effective mode area, effective mode index, dispersion, and confinement loss, have already been investigated. The finite element method has been used to compute the maximum distribution of the studied photonic crystal fiber by COMSOL software. The linear modifications from both the effective mode index and an effective mode area have been investigated. Dispersion and confinement loss are examined in terms of air hole ring number and wavelength. For every wavelength, the effective-index model implies that the studied fiber can indeed be single mode. Even though its practical single-mode range inside the opacity aperture of silica appears large, it is eventually confined by a bend-loss edge at both brief & medium wavelengths. Moreover, the reported fiber offers minimal confinement loss of almost 10−8 dB/cm, birefringence 0.0012, and dispersion around 10−11 ps/km nm.

Ultra-wideband efficient polarization conversion of terahertz wave in a planar metamaterial

We have investigated numerically and experimentally an ultra-wideband efficient polarization conversion of a linearly polarized terahertz wave in a planar metamaterial (MM) geometry comprising an array of C-shaped resonators. Both the co- and cross-polarization transmission spectra of the proposed MM configuration have been studied for numerous polarizations of incident terahertz starting from 0∘ to 90∘ in steps of 15∘. Our design promises a cross-polarization conversion of 40% from 1.22 to 2.75 THz with a maximum of 45% when the MM design is rotated by 45∘ with respect to the incident light. The change in rotation angle excites an electric dipole in the C resonator, which radiates an electric field in the co-and cross-polarization. The polarization conversion has been explained with induced electric field profiles as well as surface current profiles. It is shown that the change in dimension of the C-shaped resonator causes a shift in the transmission spectrum. Numerically simulated results are in good agreement with the experimental results. Our design eliminates the complexity of the fabrication of multilayer structures and opens avenues to design ultra-wideband and efficient polarization rotation devices in the terahertz frequency regime.

Landau levels and magneto-optical transport properties of a semi-Dirac system

We study magneto-optical (MO) properties of a semi-Dirac system in presence of a perpendicular magnetic field using kernel polynomial method (KPM) based on the Keldysh formalism in few hundreds of the terahertz (THz) frequency regime. For the semi-Dirac case, the band structure demonstrates a linear (Dirac-like) dispersion along the $y$ direction, while it has a quadratic (nonrelativistic) behavior along the $x$ direction in the Brillouin zone. For comparison, we have also included results for the Dirac systems as well, so that the interplay of the band structure deformation and MO conductivities can be studied. We have found that the MO conductivity shows features in the semi-Dirac system that are quite distinct from the Dirac case in the above mentioned frequency regime. The real parts of the longitudinal MO conductivities, namely $\mathrm{Re}({\ensuremath{\sigma}}_{xx})$ and $\mathrm{Re}({\ensuremath{\sigma}}_{yy})$ (which are different in the semi-Dirac case, as opposed to a Dirac one), present a series of resonance peaks as a function of the incident photon energy. We have also found that the absorption peaks corresponding to the $y$ direction are larger (roughly one order of magnitude) than those corresponding to the $x$ direction for the semi-Dirac case. In the case of the MO Hall conductivity, that is, $\mathrm{Re}({\ensuremath{\sigma}}_{xy})$, there are extra peaks in the spectra compared to the Dirac case, which originate from the distinct optical transitions of the carriers from one Landau level to another. These peaks are otherwise absent in a Dirac system, where some of these peaks result from transitions between pairs of Landau levels that differ by the same energy and hence those peaks can not be resolved. We have also explored how the carrier concentration influences the MO conductivities. In the semi-Dirac case, there is the emergence of additional peaks yet again in the absorption spectrum underscoring the presence of an asymmetric dispersion compared to the Dirac case. Further, we have explored the interplay between the polarization of the incident beam and the features of the absorption spectra, which can be probed in experiments. Finally, we evaluate the MO activity of the medium by computing the Faraday rotation angle ${\ensuremath{\theta}}_{F}$. The semi-Dirac case shows two maxima and two minima of ${\ensuremath{\theta}}_{F}$ at different photon energies, which can be captured in experiments.

Polar magneto-optical Kerr effect in antiferromagnetic M2As (M=Cr,Mn,Fe) under an external magnetic field

Antiferromagnetic (AFM) metals have attracted tremendous interest for memory applications due to their expected fast response dynamics in the terahertz frequency regime. Spin dynamics such as AFM resonance and the magnon relaxation rate have been measured using the linear magneto-optical Kerr effect (MOKE) with coherent spin precession induced by laser pumping. Polarized electromagnetic radiation is a promising alternative for probing the response in canted AFM systems. Hence, in this paper, we use first-principles simulations to study the magneto-optical response of AFM ${M}_{2}\mathrm{As} (M=\mathrm{Cr},\mathrm{Mn},\text{and} \mathrm{Fe})$ under external magnetic fields. We devise a computational scheme to compute the magnetic susceptibility from total-energy changes using constraints on magnetic-moment tilting. Our predictions of the spectral dependence of polar magneto-optical Kerr rotation and ellipticity allow us to attribute these effects to breaking of the magnetic symmetry. We show that tilting of magnetic moments affects the exchange interaction, while the spin-orbit interaction remains unaffected. In this paper, we provide an understanding of the polar MOKE on a band structure level and underscore the importance of the magnetic susceptibility when searching for materials with large magneto-optical response.