Terahertz Frequency Research Articles

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.

Scaled-Model Radar Cross-Section Measurement: The Influence of the Scattered Field under Gaussian Beam Illumination-A Theoretical Analysis.

At microwave frequencies, radar cross-section (RCS) measurements are usually performed by placing the target in the far-field region of the antenna. The wavefront of the radiating field from the antenna can be approximated as planar, ensuring that the incident field and the power interact with the target independently of the antenna. However, for electrically large targets, the required distance becomes significant, posing challenges for implementation. Scaled-model RCS measurements offer an alternative solution. RCS measurements at terahertz and optical frequencies typically require a collimated beam as the source, where the intercepted power and RCS become dependent on the excitation. To address this dependency, researchers have proposed modifying the RCS definition to account for the intercepted power and to analytically formulate the scattering problem. However, such modifications require prior knowledge of the target's geometry and material properties, which are often not readily available in practice. This also limits the study to only canonical targets. In this paper, we propose an alternative approach for modelling the intercepted power. The Gaussian beam is decomposed into a number of plane waves travelling to different directions using the theory of plane wave spectrum. The scattering problem is solved using the full-wave method of moment. Through theoretical proofs and numerical examples involving spheres and a non-canonical target, with a scaled-model aircraft, we demonstrate that the original RCS definition can serve as a good approximation for scaled measurements, provided that the beam waist is approximately four times the target's dimensions. These findings provide valuable guidelines for radar engineers when performing scaled measurements using collimated beams. The results, which match those obtained from full-model measurements, enable us to predict the RCS of full-scale targets. This capability facilitates various target-related applications, such as target characterization, classification, detection, and even recognition.

Electrically Tunable Nonlinearity at 3.2 Terahertz in Single-Layer Graphene.

Graphene is a nonlinear material in the terahertz (THz) frequency range, with χ(3) ∼ 10-9 m2/V2 ∼ 15 orders of magnitude higher than that of other materials used in the THz range, such as GaAs or lithium niobate. This nonlinear behavior, combined with ultrafast dynamic for excited carriers, proved to be essential for third harmonic generation in the sub-THz and low (<2.5 THz) THz range, using moderate (60 kV/cm) fields and at room temperature. Here, we show that, for monochromatic high peak power (1.8 W) input THz signals, emitted by a quantum cascade laser, the nonlinearity can be controlled using an ionic liquid gate that tunes the graphene Fermi energy up to >1.2 eV. Pump and probe experiments reveal an intense absorption nonlinearity at 3.2 THz, with a dominant 3rd-order contribution at EF > 0.7 eV, hence opening intriguing perspectives per engineering novel architectures for light generation at frequencies > 9 THz.

Black Phosphorus–Based Metamaterial Double-Band Anisotropic Absorber for Sensing Application in Terahertz Frequency

Black Phosphorus–Based Metamaterial Double-Band Anisotropic Absorber for Sensing Application in Terahertz Frequency

A perspective on recent developments of structure‐terahertz property relationships in glasses

AbstractWe present our perspective of recent terahertz (THz) time‐domain spectroscopy (TDS, THz‐TDS) studies leading to structure‐THz property relationships for three distinct glass families, that is, borosilicate, tellurite, and chalcogenide glasses. We have carried out comprehensive structural studies using spectroscopy techniques, for example, nuclear magnetic resonance, infrared, and Raman spectroscopy, with THz‐TDS used to record the refractive indices and absorption coefficients at THz frequencies. These results have culminated in structure‐THz property relationships, which are critical for targeting, selecting, and processing glasses for a wide range of THz optical and photonic applications.

Graphene-based metasurface for real-time control of three electromagnetic wave modes and polarization state

In various practical applications, the demand for switchable metasurfaces with versatile functionalities has grown significantly. This paper proposes a graphene-based metasurface that offers independent control over all three electromagnetic (EM) wave modes (i.e., reflection, transmission, and absorption) as well as the polarization state for both reflection and transmission modes, all achieved in real-time at terahertz frequency. The metasurface is designed with a multilayer structure, comprising quartz, gold, and graphene. Two graphene sheets are placed at the beginning and end of the structure to control specific functionalities. By dynamically adjusting the chemical potential of each graphene sheet, the proposed metasurface can manipulate all three EM wave modes and convert linear to circular polarization independently for both reflection and transmission modes. Additionally, the metasurface can switch from polarization conversion to without polarization conversion of the reflected wave by adjusting the chemical potential of graphene. Due to its numerous switchable functions and a high degree of freedom, the presented metasurface has the potential to be used in wireless communication, biomedicine, and optoelectronic devices.

Multiple designs with broad applicability for enhancing birefringence in low-loss terahertz HC-ARF

The use of hollow-core anti-resonant fiber (HC-ARF) as a transmission medium for the terahertz (THz) frequency range is attracting increasing attention from researchers. This paper introduces a low-loss, high birefringence anti-resonant fiber. The proposed structure achieves a maximum birefringence of 2.72 × 10−3 around 1 THz and maintains a birefringence above the 10−3 level over a wide frequency band (0.7∼1.12 THz). It has a minimum loss of 0.2 dB/m and a minimum bend radius of 0.3 m, and the paper validates that this structure has good manufacturing error tolerance. Furthermore, three methods for introducing high birefringence into low-loss ARFs are proposed and three structures are selected for verification. These findings contribute to the development of more efficient and reliable THz transmission systems.

Near-Field Spectroscopy of Individual Asymmetric Split-Ring Terahertz Resonators.

Metamaterial resonators have become an efficient and versatile platform in the terahertz frequency range, finding applications in integrated optical devices, such as active modulators and detectors, and in fundamental research, e.g., ultrastrong light-matter investigations. Despite their growing use, characterization of modes supported by these subwavelength elements has proven to be challenging and it still relies on indirect observation of the collective far-field transmission/reflection properties of resonator arrays. Here, we present a broadband time-domain spectroscopic investigation of individual metamaterial resonators via a THz aperture scanning near-field microscope (a-SNOM). The time-domain a-SNOM allows the mapping and quantitative analysis of strongly confined modes supported by the resonators. In particular, a cross-polarized configuration presented here allows an investigation of weakly radiative modes. These results hold great potential to advance future metamaterial-based optoelectronic platforms for fundamental research in THz photonics.

Electrically controllable broadband reflective linear cross-polarization conversion based on liquid crystals.

This paper presents an electrically controllable reflective broadband linear polarization (LP) converter based on liquid crystals (LCs) for cross-polarization conversion (CPC) in the terahertz frequency range. The proposed structure achieves a high polarization conversion ratio (PCR) exceeding 0.9 within the frequency range of 236.8 - 269.6 GHz. A vital feature of this design is the dynamic control of polarization conversion by re-orienting the nematic liquid crystal molecules through voltage bias switching between 'on' and 'off' states, allowing for precise manipulation of cross-polarized and co-polarized reflected waves. Experimental results validate the simulation outcomes, demonstrating excellent agreement. In contrast to conventional reflective polarization converters with fixed frequency responses, the proposed electrically controllable polarization conversion offers significant potential in imaging and optical communications.

Wide-Aperture MIMO via Reflection off a Smooth Surface

This paper provides a deterministic channel model for a scenario where wireless connectivity is established through a reflection off a smooth planar surface of an infinite extent. The developed model is rigorously built upon the physics of wave propagation and is as precise as tight are the unboundedness and smoothness assumptions on the surface. This model allows establishing how line-of-sight multiantenna communication is altered by a reflection off an electrically large surface, a situation of high interest for mmWave and terahertz frequencies.

The Far-/Near-Field Beam Squint and Solutions for THz Intelligent Reflecting Surface Communications

Terahertz (THz) and intelligent reflecting surface (IRS) have been regarded as two promising technologies to improve the capacity and coverage for future 6G networks. Generally, IRS is usually equipped with large-scale elements when implemented at THz frequency. In this case, the near-field model and beam squint should be considered. Therefore, in this paper, we investigate the far-field and near-field beam squint problems in THz IRS communications for the first time. The far-field and near-field channel models are constructed based on the different electromagnetic radiation characteristics. Next, we first analyze the far-field beam squint and its effect for the beam gain based on the cascaded base station (BS)-IRS-user channel model, and then the near-field case is studied. To overcome the far-field and near-field beam squint effects, we propose to apply delay adjustable metasurface to IRS, and develop a scheme of optimizing the reflecting phase shifts and time delays of IRS elements, which effectively eliminates the beam gain loss caused by beam squint. Finally, simulations are conducted to demonstrate the effectiveness of our proposed schemes in combating the far-field and near-field beam squint.

Terahertz broadband near-perfect absorber with a single-layer coating on doped semiconductor

We demonstrate that a single-layer coating on a doped GaAs or Si substrate enables broadband antireflection and, hence, broadband perfect absorption in the terahertz frequency range. This broadband behavior can be generally expected when the substrate material has a Drude-type dispersion. Our mathematical analyses show that the reflection from Drude-type material may have an anomalously dispersive phase shift. The anomalous dispersion of the reflection phase is used to compensate for the normal dispersion of the accumulation phase in the single-layer coating film. Consequently, the antireflection conditions are satisfied in a wide frequency range, and broadband antireflection is achieved. Thus, broadband perfect absorption is realized with only a single-layer coating film on the substrate. Our method provides a simple and efficient approach to achieving broadband perfect absorption, which is critical in many applications such as radar stealth techniques and solar cells.

High-performance terahertz polarization filter based on the anti-resonant fiber

A novel terahertz polarization filter based on the anti-resonant fiber is designed and analyzed. The polarization extinction ratio (PER) is improved by using high-resistivity silicon (HRS) as the bulk material in conjunction with a double conjoined tube cladding structure. The effects of the structure and material of the anti-resonant fiber on the polarization filtering characteristics are analyzed by the finite element method. The PER of the anti-resonant fiber reaches 100 dB and the insertion loss of y-polarization fundamental mode (YPFM) is less than 0.66 dB at 1 THz when the anti-resonant fiber length is 5.58 cm. The bandwidth can be expanded by changing the fiber length and for a fiber length of 8 cm, the frequency bandwidth is 0.92–1.14 THz and the insertion loss is less than 3 dB. The polarization filter is demonstrated to deliver excellent performance in the terahertz frequency range and has large commercial potential in terahertz communication and biomedical spectroscopy.

Multifunctional metasurface for broadband absorption and polarization conversion based on graphene-VO2

A multifunctional metasurface based on graphene and vanadium dioxide (VO2), which can function as a broadband absorber, half-wave plate (HWP), and quarter-wave plate (QWP) in the terahertz frequency band, is designed in this study. When VO2 is in the metal phase, the multifunctional metasurface operated as a broadband absorber in the range of 0.89–2.36 THz. The designed multifunctional metasurface can switch between HWP and QWP by adjusting the Fermi energy of graphene through voltage biasing when VO2 is in the insulated phase. When the Fermi level of graphene is 0 eV, the designed structure acts as an HWP to achieve linear-to-linear (LTL) polarization conversion and preserve the reflection chirality of circular polarization at 0.68–2.64 THz. When Ef = 0.3 eV, the designed structure functions as a QWP in 0.89–2.51 THz to realize the interconversion of linear polarization and circular polarization. Furthermore, the effects of the polarization and incidence angles on the operating performance are discussed. This study demonstrates that the multifunctional metasurface has promising applications in terahertz optical switches, electromagnetic stealth, modulators, and communication systems.

THz-Enabled UAV Communications Using Non-Orthogonal Multiple Access

The fact that integrating the unmanned aerial vehicles (UAV) newfangled applications in the envisioned next generation (nextG) necessitates a considerable high throughput, makes terahertz (THz) band an emerging key enabler to support stringent requirements in the nextG wireless ecosystem. To fully utilize the potential of a synergy of UAV with THz frequency band, a THz-enabled UAV communication is scrutinized in this paper, where a single antenna UAV is serving the ground users by incorporating non-orthogonal multiple access (NOMA) scheme. Our objective is to maximize the total rate of all ground users by jointly optimizing bandwidth allocation, power allocation, and UAV deployment in THz communication systems. The formulated max-sum rate optimization problem is non-convex, which makes it hard to solve optimally. Therefore, to cope with the aforementioned puzzle, we adopt an inner convex approximation-based path-following approach that considers a Joint Optimization of Power, Bandwidth allocation, and UAV's Deployment (JOPBD) by utilizing NOMA scheme. Furthermore, a comparison of our proposed JOPBD algorithm, with the orthogonal multiple access, and other baseline schemes is conducted. Then, the numerical simulations are carried out to reveal the impact of our proposed approach, and also to confirm its significance for the UAV operating in THz band.

Ultrafast and terahertz spintronics: Guest editorial

Spin-based electronics (spintronics) aims at extending electronic functionalities, which rely on the electron charge as information carrier, by the spin of the electron. To make spintronics competitive and compatible with other information carriers like photons and electrons, their speed needs to be pushed to femtosecond time scales and, thus, terahertz frequencies. In ultrafast and terahertz spintronics, femtosecond optical and terahertz electromagnetic pulses are used to induce spin torque and spin transport and to monitor the subsequent time evolution. The two approaches, sometimes referred to as femto-magnetism and terahertz magnetism, have provided new, surprising, and relevant insight as well as applications for spintronics. Examples include the ultrafast optical switching of magnetic order and the generation of broadband terahertz electromagnetic fields. This APL Special Topic Collection is dedicated to provide a platform for the newest developments and future trends in the very active, dynamic, and exciting research field of ultrafast and terahertz spintronics.

Thermally tunable Dyakonov surface waves in semiconductor nanowire metamaterials

The development of engineered metamaterials has enabled the fabrication of tunable photonic devices capable of manipulating the characteristics of electromagnetic surface waves. Integration of semiconductors in metamaterials is a proven approach for creating thermally tunable metamaterials through temperature control of the semiconductor carrier density. In this paper, an interface consisting of an isotropic dielectric material as a cover and an indium antimonide (InSb) nanowire metamaterial as a substrate, is theoretically introduced to investigate the propagation conditions of Dyakonov surface waves in terahertz (THz) frequencies. Various temperature-dependent properties of Dyakonov surface waves in such a geometry is studied, including allowed THz regions, angular existence domain, dispersion relation, directionality, localization degree and figure of merit. The proposed configuration due to the presence of significant birefringence in InSb nanowire metamaterial, has potential applications in THz sensing, imaging and spectroscopy.

Demonstration of the plasmonic THz phase shifter at room temperature

We experimentally demonstrate that above 300 GHz, the plasmonic phase shifter can operate at up to room temperature. We investigate the temperature-dependent behavior of the phase shift introduced by a two-dimensional electron system in a GaAs/AlGaAs heterostructure. We find that the temperature-effected changes in the relaxation time and effective mass contribute most to the phase shifter performance. The physical model developed in the study shows good agreement with the experimental data. The results open up the prospects for the practical applications of plasmonics in the terahertz frequency gap.

Micro-fabricated Si subwavelength grating for frequency-domain THz beam steering covering the 0.3-0.5 THz frequency band.

We designed and fabricated beam steering subwavelength grating (BS-SWG) with high efficiency, wide angles, and broadband beam steering in the terahertz (THz) range. Beam steering technology in the THz range by a fixed structure and frequency sweep has to date lacked a device combining high efficiency and a wide beam steering angle. A subwavelength structure using float zone Si, a low-loss dielectric, could combine both of these aspects, but no experimental demonstration in the THz range has been performed to our knowledge. The BS-SWG was designed with an efficiency of 0.708 at 0.4 THz and beam steering angles of -72.1°--34.8° by sweeping the incident frequency from 0.3 THz to 0.5 THz including the Beyond 5 G/6 G communication bands. An efficiency of 0.354 at 0.400 THz and beam steering angles of -74°--34° were experimentally achieved, demonstrating the potential of high-efficiency, wide-angle beam steering for THz communications, imaging, and radar applications.

Review of Bioplastics Characterisation by Terahertz Techniques in the View of Ensuring a Circular Economy

The increasing scarcity of natural resources, worsening global climate change, environmental degradation, and rising demand for food are forcing the biotechnology and plastics industries to seek and apply circular economy models that would lead to a sustainable transition in the production and use of bioplastics. Circular economy models can improve the economic productivity of bio-based plastics and have a positive impact on the environment by reducing conventional plastic waste and the consumption of petrochemical feedstocks for plastic production. In addition, some agricultural wastes that have the potential to be used as bioplastics can be reused. Terahertz (THz) systems are already used in the plastics and rubber industries for non-destructive testing, detection, imaging, and quality control. Several reports have highlighted the potential applications of THz spectroscopy and imaging in polymer analysis and plastics characterisation. This potential is even greater with chemometric methods and artificial intelligence algorithms. In this review, we focus on applications that support the transformation of the biotechnology sector to the circular economy, particularly via the transition from conventional plastics to bioplastics. In this review, we discuss the potential of THz systems for the characterisation and analysis of bioplastics and biopolymers. The results of previous studies on biopolymers in the THz frequency range are summarised. Furthermore, the potential of using artificial intelligence approaches such as machine learning as advanced analytical methods in THz spectroscopy and imaging, in addition to the conventionally used chemometric methods, is discussed. The results of this review highlight that THz technology can contribute to closed technological circles in important areas of biotechnology and the related plastics and rubber industries.