Terahertz Frequency Regime Research Articles

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.

Terahertz Spin-Current Pulses from an Off-Resonant Antiferromagnet

The generation of terahertz (THz) spin current is highly desired for enhancing the speed and efficiency of information manipulation by ultrafast spintronics. We develop an analytical model to investigate the coherent and pure spin-current pulses generated by spin pumping in a canted insulating antiferromagnet (AFM) system with Dzyaloshinskii-Moriya interaction (DMI). As expected, the spin pumping signal is significant at resonance. However, we also predict efficient THz spin-current transient with a magnitude comparable with that of its resonant counterparts in the much higher off-resonance THz frequency regime. Its ultrafast temporal change compensates for the suppression of the dynamic magnetization amplitudes at off-resonance. In addition, a stronger DMI field enhances the amplitude of the spin-current transient. Our results represent an efficient approach to generate coherent THz spin-current pulses based on canted AFM heterostructures without requiring a static magnetic field.

Strong coupling between a topological insulator and a III-V heterostructure at terahertz frequency

We theoretically probe the emergence of strong coupling in a system consisting of a topological insulator (TI) and a III-V heterostructure using a numerical approach based on the scattering matrix formalism. Specifically, we investigate the interactions between terahertz excitations in a structure composed of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and GaAs materials. We find that the interaction between the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ layer and AlGaAs/GaAs quantum wells with intersubband transitions (ISBTs) in the terahertz frequency regime creates new hybrid modes, namely Dirac plasmon-phonon-ISBT polaritons. The formation of these hybrid modes results in anticrossings (spectral mode splitting) whose magnitude is an indication of the strength of the coupling. By varying the structural parameters of the constituent materials, our numerical calculations reveal that the magnitude of splitting depends strongly on the doping level and the scattering rate in the AlGaAs/GaAs quantum wells, as well as on the thickness of the GaAs spacer layer that separates the quantum-well structure from the TI layer. Our results reveal the material and device parameters required to obtain experimentally observable signatures of strong coupling. Our model includes the contribution of an extra two-dimensional hole gas (2DHG) that is predicted to arise at the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$/GaAs interface, based on density functional theory (DFT) calculations that explicitly account for details of the atomic terminations at the interface. The presence of this massive 2DHG at the TI/III-V interface shifts the dispersion of the Dirac plasmon-ISBT polaritons to higher frequencies. The damping rate at this interface, in contrast, compensates the effect of the 2DHG. Finally, we observe that the phonon resonances in the TI layer are crucial to the coupling between the THz excitations in the TI and III-V materials.

Review of Techniques for Particle Spectroscopy From DC to Terahertz Frequency

Non-destructive, contact-less, and label-free characterization of small particles plays an important role in laboratory research and industrial manufacturing. Also, the analysis of small things, especially particulate bio-matter, using electromagnetic waves has a long history in optical microscopy. Light, however, cannot reveal much information about the material property of the particle. Radio waves, on the other hand, apart from being more sensitive and featuring compact instrumentation, interact strongly with a dielectric particle. Thus, in principle, it can provide information about the particle in both shape and material. Therefore, particle spectroscopy using electrical signals plays a vital role in research and development. Although not firmly established as its optical counterpart, it is still reasonable to analyze the nascent stages of its development till now. In this review article, different state-of-art techniques for particle spectroscopy spanning from DC to terahertz frequency regime are categorized into two board domains: electromagnetic perturbation and electromagnetic scattering based approaches. A critical analysis of the uniqueness of terahertz frequency in context to particle spectroscopy is outlined. Finally, the paper discusses a realistic scaling approach from generic optical sensors to an electronic lab-on-chip design for sensing and characterization of particle-in-flow in microfluidics. It also addresses the feasibility of modern high-frequency circuit design technology for potential electromagnetic scattering-based particle spectroscopy innovations.

Active Multiple Access Secure Communication Enabled by Graphene-Based Time-Modulated Metasurfaces

This article presents a technique for establishing active secure communication links over single and multiple orthogonal frequency channels via a reflective graphene-based time-modulated metasurface operating in the low terahertz frequency regime. For this purpose, the Fermi energy level of unit cells is modulated via pseudorandom radio frequency biasing signals whose dc offsets are set to steer the reflected beam toward the desired direction by imposing a spatial phase gradient profile across the surface, while their waveforms are engineered to spread the incident wave spectrum into a noise-like spectrum with a near-zero power spectral density via random modulation of the reflection phase of each element between 0 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> with respect to its offset phase. This allows for addressing a legitimate user in real-time who can retrieve the signal via synchronous demodulation with the pseudorandom key of the metasurface while camouflaging the signal from the adversary by lowering the probability of detection. The approach is extended to enable multichannel secure communication by dividing the metasurface into interleaved subarrays modulated with orthogonal pseudorandom keys, which provides simultaneous and independent control over multiple beams with nonoverlapping spread spectra which can be retrieved by independent legitimate users while rejecting unwarranted access by eavesdroppers.

FSS-embedded substrates: a facile method of augmenting functions of metasurfaces

Metasurfaces, usually composed of 2D arrays of sub-wavelength metallic patterns (also called meta-atoms), have provided unprecedented freedoms for manipulating electromagnetic (EM) waves. In practice, meta-atoms are usually patterned on a certain substrate, so the EM properties of substrates directly affect the performances of metasurfaces. Therefore, it is desirable that the substrates can also be engineered with a high degree of freedom (DOF) so as to augment the functions of the metasurfaces. With this in mind, in this paper, we propose a method of designing functional metasurfaces by engineering the EM properties of substrates. By embedding frequency selective surface (FSS) within dielectric substrates, the electrical thickness of the substrates can be tailored according to the specific operating frequencies. In this way, functions of the metasurfaces can be augmented, either by better performances or by additional functions. As an example, we designed a multi-functional metasurface and applied it to a low-RCS reflector antenna. A circular patch was inserted into the dielectric substrate beneath the meta-atom. Due to the band-stop property of the metallic patch, two polarization conversion (PC) bands, together with a polarization-keeping band in between them, were created. Using this meta-atom, the metasurface is further designed, which can simultaneously achieve in-band focusing and out-of-band PC. A prototype was fabricated and measured. Both the simulated and measured results verified the good performance. Empowered by the metasurface, the reflector antenna achieved high radiation gain in band and significant RCS reduction out of band. This work provides a facile method of designing multi-functional metasurfaces and can be readily extended to terahertz, optical and other frequency regimes.

Cold Characteristics of Overmoded Ridge- and Groove-Loaded Folded Waveguides for High-Power Traveling Wave Tubes

The TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> high-order mode operation characteristics of the ridge-loaded (RL) and groove-loaded (GL) folded waveguide slow wave structures (FWG SWSs) with two electron beam channels are studied for high-power traveling wave tubes (TWTs) at the terahertz (THz) frequency regime. Operating in a higher order mode means that the SWSs can have larger structure dimensions to reduce the fabrication difficulty at the THz frequency band and use higher current to increase the output power. For FWG SWSs operating in the high-order mode, it is difficult to suppress backward wave oscillation (BWO) of fundamental mode. In this article, we proposed the RL and GL FWG SWSs to suppress BWO and studied the dispersion, coupling impedance, and transmission loss of them. The results show that the RL structure can substantially increase the start oscillation length of the interaction circuit and avoid using many circuit sections for spurious mode depression which ease the assembly process of the tube. The GL structure can broaden the bandwidth with high-order mode operation but with the ease of BWO of fundamental mode. The straight RL and GL waveguides are used to explain the above results, and the effect of electron beam channel position on coupling impedance is also studied.

Tunable Extraordinary Optical Transmission with Graphene in Terahertz

Tunable extraordinary optical transmission (EOT) with graphene is realized using a novel metallic ring–rod nested structure in the terahertz frequency regime. The generated double-enhanced transmission peaks primarily originate from the excitation of localized surface plasmon resonances (LSPRs). On using graphene, the resonating surface plasmon distribution changes in the reaction plane, which disturbs the generation of LSPRs. By regulating the Fermi energy (Ef) of the graphene to reach a certain level, an adjustment from bimodal EOT to unimodal EOT is obtained. As the Ef of the graphene integrated beneath the rod increases to 0.5 eV, the transmittance of the peak at 2.42 THz decreases to 6%. Moreover, the transmission peak at 1.77 THz virtually disappears due to the Ef increasing to 0.7 eV when the graphene is placed beneath the ring. The significant tuning capabilities of the bimodal EOT indicate its promising application prospects in frequency-selective surfaces, communication, filtering, and radar.

Magnetic field-induced emissivity tuning of InSb-based metamaterials in the terahertz frequency regime

This work demonstrates the magnetic field-induced spectral properties of metamaterials incorporating both indium antimonide (InSb) and tungsten (W) in the terahertz (THz) frequency regime. Nanostructure materials, layer thicknesses and surface grating fill factors are modified, impacting light-matter interactions and consequently modifying thermal emission. We describe and validate a method for determining spectral properties of InSb under an applied direct current (DC) magnetic field, and employ this method to analyze how these properties can be tuned by modulating the field magnitude. Notably, an InSb-W metamaterial exhibiting unity narrowband emission is designed, suitable as an emitter for wavelengths around 55 µm (approximately 5.5 THz), which is magnetically tunable in bandwidth and peak wavelength.

A THz Metamaterial Absorber with Multiple Polarization - Insensitive, Sensitive, and Tunable

Terahertz (THz) absorbers are gaining interest in many applications. In this paper, we present the design and simulation of a multiband metamaterial absorber (MMA) with combined polarization properties and prominent absorption at 2.2 THz and 3.9 THz. The MMA comprises two square split-ring resonators and one square ring resonator placed on top of a polyimide dielectric spacer, offering multiband absorption characteristics with maximum absorptivity of 93.18% and 96.09%, respectively. The most protruding feature of this design is that it displays multiple polarization characteristics, including insensitivity, sensitivity, and tunability, even though the structure is similar to those of conventional absorbers. Firstly, the distinctly visible absorption spectra at 1.8 THz, gradually diminishes with an increase in polarization angle and then completely vanishes for TM polarization. Secondly, the prominent band at 2.2 THz is insensitive to changes in polarization of the incident wave, whereas, at 3.9 THz, the absorption band displays polarization tunability characteristics. Due to the multiple characteristics displayed by the structure, this MMA can be simultaneously used for several applications in the terahertz frequency regime such as imaging, terahertz spectroscopy, sensing, and stealth technology.

Second harmonic generation in the tilted type-I Dirac metals under terahertz frequency regime

Abstract In this work, we theoretically demonstrate that the second order response can survive in a two-dimensional (2D) tilted type-I Dirac metal due to the time reversal symmetry (TRS) breaking. We find that, in the terahertz regime, the second harmonic generation (SHG) is appreciably strong and is widely depending on the tilting parameter, temperature, excitation frequency and the applied electric field, respectively. By comparing with the first and third order optical response, we also note that each of the three modes of optical response dominates in different frequency intervals in accordance with the strength of the applied electric field. With the help of the ω − E c diagram (with ω and E c being the excitation frequency and the critical electric field, respectively), we are able to engineer the desired nonlinear response by varying the excitation frequency and the strength of the applied electric field. Finally, we briefly discuss on the influences of relaxation time on SHG by considering four types of k-dependent scattering mechanisms.

Performance analysis of octagonal photonic crystal fiber (O-PCF) for various communication applications

In this research work, a novel design of octagonal cladding with two elliptical cores in photonic crystal fiber (O-PCF) is studied for application in the terahertz (THz) frequency regime. There exists five layers octagonal figure in circular air holes (CAH) of cladding region with two elliptical shapes of core area are proposed in this research work. The O-PCF features of the fiber have been explored using full vector finite element method (FEM) with perfectly match layer (PML) boundary condition that have been used to confine the light inside the core region properly. Some supplementary significant fiber features for THz signal spread with low confinement loss, high core power fraction, scattering loss and single-mode PCF circumstances of the fiber have also been considered. Simulated results exhibition compares to the low effective material loss (EML) of 0.0153 cm−1, larger effective area of 5.95 × 10–8 m2, core power fraction of 79%, a low confinement loss and scattering loss such as 3.36 × 10–14 dB m−1 and 1.25 × 10–10 dB km−1 respectively at 1 THz frequency regime. To shorten design and smooth fabrication, only circular shaped air holes have been employed. Due to its promising characteristics, the proposed SM-OPCF waveguide provides efficient transmission of broadband terahertz signals. The suggested excellent designed of O-PCF can be used mostly for in nano-optics, biomedical signal processing, optical amplification, SONET and others communication areas.

Hybrid energy surface plasmon modes supported by graphene-coated circular chirowaveguide

In-depth analysis of hybrid SPP wave propagation at the Chiral–Graphene–Dielectric (CGD) interface is carried out. The single-layer graphene is modeled using Kubo formulism and the impedance matching boundary conditions are utilized to derive the dispersion relation. TM-TE decoupled modes at the CGD interface are shown. The effective mode index and propagation length of hybrid plasmon modes are found to be sensitive to chiral strength (ξ), chemical potential (μc), radius of waveguide, and refractive index of chiral (nc). It is observed that the cutoff values of chiral strength and radius promote Goos–Henchen effect at CGD interface. The present work may have potential applications for on-chip optical, chiral sensing and enantiomeric detection and renewable energy technologies in the terahertz frequency regime.

Numerical investigation of polarization-insensitive multiband metamaterial for terahertz solar absorber

This study presents a nanostructured multiband metamaterial absorber for the optical regime application. The proposed structure is exhibited 0° to 90° polarization insensitivity and up to 45° angular stability with more than 90% peak absorption in between 300 THz800 THz frequency range. Proposed MMA structure is acquired perfect absorption of 99.99% at 571THz, 99.50% at 488.26 THz, 99.32% at 598 THz frequency by adjusting geometrical parameters. Due to perfect impedance matching with plasmonic resonance characteristics, these structures achieved an average absorption of 95.30%, 91.96%, 97.25%, 97.65%, 91%,90%,90.23% in between (337.5 THz-346.5 THz), (471 THz-478 THz), (497.5 THz-505 THz), (519 THz-530.5 THz), 564.5 THz-577 THz), (604 THz-673 THz), (686 THz-708 THz), respectively. The near field pattern of proposed MMA is used to explain the absorption mechanism at resonance frequency point and the geometric parameters are explored and analyzed to demonstrate the performance of the proposed structure. Moreover, CST-HFSS interference is validated the simulation data with the help of the finite element method (FEM). Polarization insensitivity and wide angular stability in terahertz (THz) frequency regime make this structure suitable for the application of magnetic resonance imaging (MRI), color images, thermal imaging and solar cell applications like exploitation of solar energy.

Vivaldi dipole nano‐rectenna for IR energy harvesting at 28.3 THz

AbstractA Vivaldi dipole rectenna system for infrared (IR) energy harvesting is investigated. First, a parametric study on the Vivaldi dipole antenna is performed to collect the maximum electric field between the Vivaldi poles. The antenna arms were optimized to achieve a high‐efficiency rectenna system. The two arms of the antenna were formed using two different metals, that is, gold and titanium. These two metals have different work functions, which facilitate the diode operation through tunneling at zero bias. The two arms of the Vivaldi dipole are overlapped, and a suitable insulator layer is injected in the overlapped area to form the metal‐insulator‐metal (MIM) diode. The MIM diode is an ideal candidate for this operation as it works without any bias, provided the two metals have different work functions. For rectenna operation, it is crucial that the rectifying diodes should work without any aid of external bias. The Al2O3 is the insulator layer of the MIM diode. We have chosen Al2O3 because it has a low dielectric constant at terahertz frequency regime, which allows us to match the operational cut‐off frequency, that is, 28.3 THz. A parametric study of the Al2O3 insulator layer is performed to increase the captured received intensity. At the end, the nano‐antenna operates at a frequency band of [26 … 30 THz] to harvest IR energy from the environment with good efficiency and demonstrate its capacity to capture incident waves and obtain high‐intensity values within its gap. It is a development that could eventually boost electricity generation.

Electrically and magnetically switchable nonlinear photocurrent in \u0420\u0422-symmetric magnetic topological quantum materials

Nonlinear photocurrent in time-reversal invariant noncentrosymmetric systems such as ferroelectric semimetals sparked tremendous interest of utilizing nonlinear optics to characterize condensed matter with exotic phases. Here we provide a microscopic theory of two types of second-order nonlinear direct photocurrents, magnetic shift photocurrent (MSC) and magnetic injection photocurrent (MIC), as the counterparts of normal shift current (NSC) and normal injection current (NIC) in time-reversal symmetry and inversion symmetry broken systems. We show that MSC is mainly governed by shift vector and interband Berry curvature, and MIC is dominated by absorption strength and asymmetry of the group velocity difference at time-reversed ±k points. Taking {cal{P}}{cal{T}}-symmetric magnetic topological quantum material bilayer antiferromagnetic (AFM) MnBi2Te4 as an example, we predict the presence of large MIC in the terahertz (THz) frequency regime which can be switched between two AFM states with time-reversed spin orderings upon magnetic transition. In addition, external electric field breaks {cal{P}}{cal{T}} symmetry and enables large NSC response in bilayer AFM MnBi2Te4, which can be switched by external electric field. Remarkably, both MIC and NSC are highly tunable under varying electric field due to the field-induced large Rashba and Zeeman splitting, resulting in large nonlinear photocurrent response down to a few THz regime, suggesting bilayer AFM-z MnBi2Te4 as a tunable platform with rich THz and magneto-optoelectronic applications. Our results reveal that nonlinear photocurrent responses governed by NSC, NIC, MSC, and MIC provide a powerful tool for deciphering magnetic structures and interactions which could be particularly fruitful for probing and understanding magnetic topological quantum materials.

Terahertz emission from nonlinear interaction of laser beat wave with nanoparticles

Terahertz radiation is investigated using nonlinear interaction of a laser beat wave with a density-modulated medium of graphite nanoparticles. A beam-decentering parameter, b, is used to modify the polarizing field’s profile to produce different-shaped laser pulse envelopes, e.g. Gaussian, top head, ring-shaped and cosh-Gaussian. The normal vectors corresponding to the basal planes of graphite nanoparticles are considered to be aligned parallel and perpendicular to the polarization of the propagating laser pulse. The electronic cloud of the graphite nanoparticles acquires a nonlinear oscillatory velocity under the influence of a nonlinear force that produces a strong nonlinear current at the beat-wave frequency . The strong nonlinear current allows the emission of radiation in the terahertz frequency regime. The terahertz radiation intensifies and attains a peak value when the laser beat-wave frequency () equals the plasmon frequency of the nanoparticles. The terahertz radiation’s amplitude is enhanced by up to the fourth order of magnitude in the case of a cosh-Gaussian laser pulse, when the beam-decentering parameter,b, equals 5. The present numerical results reveal that by changing the shape of a laser pulse and the properties of nanoparticles, one can control and tune the THz emission.

Modulators for mid-infrared and terahertz light

The rise of mid-infrared and terahertz wave technology over the past two decades has led to incredible insights and potential applications for next-generation optoelectronics. Modulators, which control amplitude, phase, and/or polarization of incident light, are widely used in communications, imaging, and sensing and are crucial for further development of technology functioning in the mid-infrared and terahertz frequency regimes. The lack of natural materials with optical responses in these frequency regimes has led to a surge in engineering efforts to create novel devices and architectures for achieving control over the properties of mid-infrared and terahertz radiation. Major efforts in the field have been devoted to studying carrier concentration modulation, liquid crystals, phase-change materials, and micro-electromechanical systems for controlling the light–matter interaction. Although there has been considerable progress in realizing mid-infrared and terahertz modulators, novel approaches are seeking higher modulation speed, more functionality, and miniaturized size. In this perspective, we review the recent advancements of modulators for mid-infrared and terahertz wavelengths. We discuss various modulation mechanisms, along with their relative performance, and consider future architectures to improve upon the current technology for mid-infrared and terahertz modulation.

Design and Performance Improvement of Optical Chemical Sensor Based Photonic Crystal Fiber (PCF) in the Terahertz (THz) Wave Propagation

A heptagonal cladding (HC) with the rotated-hexacore (RH) in photonic crystal fiber (H-PCF) has been made for the field of chemical sensing in the terahertz (THz) area. There are five layers of circular air holes (CAH) in the heptagonal cladding region and two layers of rotated-hexa of CAH in the area of the core that have been second hand to enterprise this PCF. This suggested H-PCF, a full-vectorial finite element method (FEM) and perfectly matched layers (PML) boundary condition has been utilized based on a software instrument. After simulation results, the proposed sensor displays the relative sensitivity (RS) 68.48%, 69.20%, 66.78% and the confinement losses (CL) are 2.13 × 10−09 dB/m, 1.92 × 10−09 dB/m and 2.70 × 10−06 dB/m for focused analytics Ethanol (n = 1.354), Benzene (n = 1.366) and Water (n = 1.330) independently at 1 THz frequency regime. We have also discussed total power fraction, effective mode index, and effective area elaborately here. However, our proposed H-PCF is suitable for the user such as in chemical sensors as well as in many diverse industrial and medical areas.

Terahertz refractive index sensor with high sensitivity based on two‐core photonic crystal fiber

AbstractThis article contributes a highly sensitive novel refractive index (RI) sensor based on two‐core photonic crystal fiber (PCF) is proposed in the terahertz (THz) frequency regime. The sensing performance of the proposed RI sensor is based on the mode‐coupling theory. Two vertical cores of the PCF divided by the central air hole, act as two separate waveguides. The analyte whose refractive index being sensed is filled in the central air hole of the PCF. A full‐vector finite element method (FEM) approach is used to investigate the sensor. Numerical data reveal that the sensitivities of the proposed PCF are 672 600, 940480, 1035300, 1332200, and 1 827 300 nm/RIU for the RI sensing range of analyte from 1.30 to 1.33, 1.34 to 1.37, 1.38 to 1.40, 1.41 to 1.43, and 1.44 to 1.46, respectively. This indicates that the proposed RI sensor is highly sensitive to the refractive index of the analyte.