THz Band Research Articles

Terahertz cancer cell sensor based on plasmonic toroidal metasurface

This paper investigates a plasmonic toroidal metasurface cancer cell sensor capable of highly sensitive, cost-effective, and label-free detection. Utilizing titanium and gold split ring resonators, the metasurface unit are formed. Numerical findings indicate a significant resonance frequency shift of the metasurface upon changes in the measured substance, enabling accurate detection of various cancer cell types, such as blood cancer cell and cervical cancer cell in the 2.5 THz-4.5 THz band, with sensitivity reaching 1.075 THz/RIU. Moreover, the metasurface sensor demonstrates superior angular stability and high sensitivity in detecting cyanide and heavy metal-contaminated water. These research outcomes lay a theoretical foundation for the development and application of highly sensitive cancer cell sensors.

Exploring effects of plasma surface engineering on cellulose nanofilms via broadband THz spectroscopy

For the extended ultra-wideband THz band up to 10 THz and beyond, materials with different functional properties are required. In addition, the THz shielding devices require the desired thickness for different shielding effects, which can be tailored by the surface functionalities. Here, a plasma-assisted soft chemistry approach was used to tailor the cellulose nanofibrils (CNFs), and the effects of plasma surface treatment were investigated using non-destructive terahertz time-domain spectroscopy (THz-TDS). CNFs with three different thicknesses were investigated as a function of mild and strong plasma treatment. The ultra-broadband THz-TDS evaluation of CNF films overcomes the challenge by surpassing the conventional low-frequency THz-TDS measurements and reveals many characteristic THz features of CNF at 2.1, 3.1, 5.1, 7.0, 10.3 and 10.9 THz. THz-TDS analysis indicates thinning of CNF after plasma treatment due to the plasma etching effect. At the same time, the microcrystalline properties of the CNF samples remain intact even after significant surface modification under plasma carbonisation conditions. Apart from this, the studies proposed the required minimum thickness for THz shielding applications based on the optical parameters and THz properties derived from the plasma-treated CNF structures, which could be used to define the thickness of sustainable shielding materials for THz devices.

Singular photon spin Hall effect in chiral-graphene heterostructures and its application in chirality detection.

In this paper, we consider the singular photonic spin Hall effect (PSHE) in a chiral-graphene-chiral (CGC) heterostructure in the THz band. We investigate the impact of a chiral medium on reflectance spectra and the modulation of the Fermi energy on the surface conductivity of graphene. Our study shows that placing a chiral medium on both sides of a monolayer of graphene results in an enhanced transverse shift (TS) compared to placing a non-chiral medium. Moreover, the direction of the TS of the PSHE can be altered by adjusting the sign of the chirality parameter and the Fermi energy of graphene. Finally, we establish a quantitative relationship between the PSHE and the chirality parameter and the Fermi energy of graphene. By dynamically modulating the PSHE in graphene, it is possible to flexibly detect chirality parameters. This work opens up new avenues for chiral molecular detection and graphene-PSHE dynamic modulation.

Performance improvement of THz MIMO antenna with graphene and prediction bandwidth through machine learning analysis for 6G application

This article provides the findings of a study that integrated simulation, an RLC equivalent circuit, and machine learning (ML) techniques to improve wireless indoor communications clusters with future 6 G applications. The antenna being presented is constructed on a polyimide substrate. It exhibits an isolation of 27 dB and has a bandwidth of 4.331 THz, ranging from 0.631 THz to 4.962 THz. Along with its small size (95.52 × 227.24) µm2, it boasts an impressive maximum gain of 13.3 dB and an efficiency rating of 95 %. The ECC value drops below 0.0002 when the DG goes over 9.99. An advanced design system (ADS) creates a model like the proposed MIMO antenna to compare the return loss caused by CST (Computer Simulation Technology). Subsequently, following extensive data sampling with CST MWS (Microwave Studio) simulation, we employed supervised regression ML techniques. Gaussian process regression demonstrates exceptional accuracy, reaching almost 99 %, as evidenced by the high R-square and var scores. Additionally, it achieves the lowest error, less than one, while predicting bandwidth. The proposed antenna demonstrates strong potential as a formidable contender for 6 G THz band applications, as evidenced by the outcomes of the CST simulations and the prognostications derived from the machine learning techniques.

Analyzing Performance of THz Band Graphene-Based MIMO Antenna for 6G Applications

In this paper, a compact 2×2 hexagon ring-shaped MIMO antenna operating at the terahertz band is proposed for future 6G wireless communication applications. The antenna is designed using graphene, due to its unique high-speed transmission capabilities. DGS and NL decoupling approaches are applied to enhance isolation between the two radiating elements. A parametric study is performed to investigate the significance of using these methods. Performance in terms of different metrics is studied using the CST Microwave Studio simulator. The final outcomes show that the proposed MIMO antenna achieves 23 dB of isolation, 0.004859 of ECC, 0.004 bits/sec/Hz of CCL, and efficiency of 98%.

Designing a Novel THz Band 2-D Wide-Angle Scanning Phased-Array Antenna Based on a Decoupling Surface

This paper proposes a novel THz band 2-D wide-angle scanning phased-array antenna (PAA) based on a decoupling surface. The simulated S11 bandwidth under periodic boundary conditions is 106–119 GHz, with stable gain within the bandwidth. The designed decoupling surface effectively reduces the coupling between elements, and the simulated active VSWR performance and ground surface current distribution under periodic boundary conditions confirm this. An 8 × 8 (64-element) planar PAA is modeled and simulated in CST2022 to verify the beam-scanning performance of the PAA. According to the simulation results, a 2-D wide-angle scanning of ±48° is achieved in the 106–114 GHz range, while in the 115–119 GHz range, a wide-angle scanning of ±48° is achieved on the E-plane, and the beam-scanning range on the H-plane reaches ±40°. Moreover, the normal peak gain is stably maintained at 21.9–22.8 dBi, with a normalized radiation efficiency as high as 95%, and the scanning radiation efficiency is higher than 81%. Due to its stable gain and 2-D wide-angle scanning performance, the proposed PAA has a broad application prospect in terahertz wireless communication equipment.

Innovative design to achieve a multi-band electromagnetic wave stealth.

Metamaterials have opened up a new field of electromagnetic wave stealth that can achieve cross-band electromagnetic wave stealth through high electromagnetic wave absorption and low infrared emission. However, traditional cross-band stealth metamaterials make covering the terahertz band challenging and have certain design flaws. This Letter introduces an innovative cross-band electromagnetic wave stealth metasurface design that can achieve cross-band stealth in the infrared, microwave, and THz bands. We use phase change materials and the gradient principle to achieve GHz and THz cross-band absorption. We also design surface height-covered low infrared emitting materials, which give them lower infrared emissivity. These functions give it enormous potential in military applications, and using phase change materials for cross-band absorption also provides new, to our knowledge, ideas for multifunctional stealth materials.

Terahertz wide-band wide-angle absorber and polarization converter based on VO2 metamaterial

In this study, a Terahertz wide-band wide-angle absorber and polarization converter based on VO2 metamaterial is proposed. The device is simulated by using full-wave electromagnetic computing software CST. The device functions as a wide-band absorber while VO2 is in the metallic mode. In the 1.82 THz to 4.27 THz band, the absorption efficiency exceeds 90% and the relative bandwidth reaches 81.4%. The absorption mechanism is analyzed bying equivalent circuit theory. Additionally, the absorber can work at large polarization angle and incident angle. While VO2 is in the insulated mode, the design can convert the incident y-polarized terahertz wave into the x-polarized state and realize the transformation of linear polarization. Polarization conversion efficiency (PCR) exceeds 90% in the band of 1.57 THz to 3.82 THz, with a relative bandwidth of 83.5%. The device also supports large incidence angle tolerances. These results show that the disigned metamaterial device has potential applications in absorption, polarization conversion and imaging.

Polarization insensitive and wideband terahertz absorber using high-impedance resistive material of RuO2

This paper introduces a novel, cost-effective solution designed to achieve large absorption bandwidth within the THz spectrum employing a miniaturized, single-layer metamaterial structure. The designed structure features a single circular ring composed of an ohmic resistive sheet with notably higher sheet resistance than traditional metallic resonators. This distinctive design is implemented on a lossy dielectric polyimide substrate with a backing of metallic gold. Our developed absorbing structure demonstrates the capability to achieve a substantial absorption bandwidth ranging from 3.78 to 4.25 THz, maintaining a consistent absorption rate of over 90%. Moreover, we conducted an analysis to assess its absorption performance under various sheet resistance values within the top layer. Additionally, we characterized its angular stability and polarization insensitivity through oblique incident and polarization angle analysis. Finally, an RLC circuital and interference theory approach is adopted to justify its simulated results. The proposed absorber shows potential for a broad spectrum of applications, encompassing communication, imaging, and diverse integrated circuits operating within the THz band.

Graphene-Based Tunable Polarization Conversion Metasurface for Array Antenna Radar Cross-Section Reduction.

A graphene-based tunable polarization conversion metasurface (PCM) was designed and analyzed for the purpose of reducing the radar cross-section (RCS) of array antennas. The metasurface comprises periodic shuttle-shaped metal patches, square-patterned graphene, and inclined grating-patterned graphene. By adjusting the Fermi energy levels of the upper (μ1) and lower (μ2) graphene layers, different states were achieved. In State 1, with μ1 = 0 eV and μ2 = 0.5 eV, the polarization conversion ratio (PCR) exceeded 0.9 in the bandwidths of 1.65-2.19 THz and 2.29-2.45 THz. In State 2, with μ1 = μ2 = 0.5 eV, the PCR was greater than 0.9 in the 1.23-1.85 THz and 2.24-2.60 THz bands. In State 3, with μ1 = μ2 = 1 eV, the PCR exceeded 0.9 in the 2.56-2.75 THz and 3.73-4.05 THz bands. By integrating the PCM with the array antenna, tunable RCS reduction was obtained without affecting the basic radiation functionality of the antenna. In State 1, RCS reduction was greater than 10 dB in the 1.60-2.43 THz and 3.63-3.72 THz frequency ranges. In State 2, the RCS reduction exceeded 10 dB in the 2.07-2.53 THz, 2.78-2.98 THz, and 3.70-3.81 THz bands. In State 3, RCS reduction was greater than 10 dB in the 1.32-1.43 THz, 2.51-2.76 THz, and 3.76-4.13 THz frequency ranges. This polarization conversion metasurface shows significant potential for applications in switchable and tunable antenna RCS reduction.

The Influence of Structural Irregularity in Dielectric-tube-supported Metal Rod Transmission Line at THz Band

The Influence of Structural Irregularity in Dielectric-tube-supported Metal Rod Transmission Line at THz Band

4096-QAM Signal Transmission by an IM/DD System at THz Band Using Delta-Sigma Modulation

4096-QAM Signal Transmission by an IM/DD System at THz Band Using Delta-Sigma Modulation

Versatile design of tunable Faraday rotator for terahertz band using graphene-InSb-graphene and 1D photonic crystal

A Faraday rotator, composed of graphene-InSb-graphene as magnetic floor and Si- SiO2 materials as the non-magnetic floor is designed to operate at 10 THz frequency, promising for realizing future communication devices. The height of the dielectric layers significantly influences the selection of the resonance frequency. Additionally, the structure can be tuned to operate at a desired frequency thanks to the adjustable surface conductivity of graphene. The findings reveal that variations in the strength of the applied magnetic field intricately modulate the Faraday rotation (FR) and subsequently alter the transmission spectrum. Specifically, values exceeding 40 degrees of FR and a transmission rate of 50% were attainedwhen the magnetic field strength (B) was set at 2 Tesla. Finally, we have proposed a design process for a Faraday rotator that can operate at any arbitrary frequency in the THz band. This innovative design has the potential to introduce a novel approach in ultrathin magneto-optical nanophotonic devices, enabling the realization of polarization rotators characterized by elevated transmittance and reduced reliance on applied static magnetic fields within the THz band.

Terahertz RCS reduction employing reconfigurable graphene-based AMC structures

Recently, modern technology has towards stealth technology, especially in military applications so, this paper presents a terahertz radar cross-section (RCS) reduction utilizing reconfigurable graphene-based artificial magnetic conductor (AMC) arrays. The AMC unit cell has a Vivaldi shape with circular slots etched in the radiator to reduce the RCS from metallic surfaces at THz bands. The AMC cells affect the surface impedance of the metallic objects which reduces the reflected EM waves from them. The RCS reduction bandwidth is achieved and controlled by varying the voltage applied to graphene cells which varies its chemical potential (µc). The effect of changing the graphene conductivity on the RCS reduction is investigated. Different arrangements to obtain maximum RCS reduction are presented. A 12 × 12 hybrid arrangement of the graphene-based AMC structures achieved maximum RCS reduction from 1.5 to 4 THz with 22 dB greater than the unloaded metallic surface. The CST simulator is employed in the simulation.

Ultra-thin wide-band split-ring resonator-based compact mu-negative metamaterial for terahertz applications

This paper focuses on designing and investigating a new type of metamaterial called an ultra-thin wide-band mu-negative (MNG) metamaterial. The targeted frequency range is 0.1–10 THz. At the resonance frequency of 6.37 THz, the permeability (μ) of the structure is negative, allowing it to function as an MNG metamaterial. Computer Simulation Technology (CST) and High-Frequency Structure Simulator (HFSS) 3D software simulations, examining the scattering and medium parameters of the MNG metamaterial. The findings demonstrate that the proposed MNG metamaterial exhibits resonance peaks in the mu-negative region at 6.37 THz and wide frequency bands of 8.79 THz at −10 dB in the CST simulator. The structure is adaptable based on design parameters. This MNG metamaterial is unique due to its ultra-thin, a form mu-negative wide bandwidth of 3.20 THz, and suitable resonance frequencies. The simulation results from Ansys HFSS software almost align with CST results. The structure, resonating in the THz region with wide bandwidth, shows potential applications in spectroscopy and biomedical engineering. The study encountered difficulties in achieving precise fabrication at the nanoscale and ensuring the reproducibility of metamaterial properties across various samples. Despite these challenges, the proposed metamaterial shows significant advancements in terahertz technology.

Terahertz vortex beam generation based on reflective and transmissive graphene metasurfaces

Graphene metasurfaces have substantial potential for generating vortex electromagnetic waves carrying orbital angular momentum (OAM) at terahertz frequencies. In this paper, a deformed H-shaped graphene unit cell is utilized to design a half-space reflective metasurface (with a metal backplane, operating in the reflection mode) and a full-space metasurface (operating in both the reflection and transmission modes) to generate OAM beams. The two structures are intertransformable at the level of physical composition, where one configuration enables efficient anomalous reflection of electromagnetic waves within the 4.6–8.9 THz band, while the other enables uniform modulation efficiency of reflective and transmissive waves within the 5.5–7.0 THz band. Equivalent circuit models have been provided to establish a convenient closed-form mathematical description of the metasurface’s electromagnetic behavior. By performing the near- and far-field simulations, a series of THz vortex generators based on the proposed graphene metasurface are demonstrated. In particular, high-purity reflective and transmissive OAM waves with distinct topological charges are observed. Our study provides a path towards the practical realization of active THz OAM metadevices.

Compressive sensing imaging with periodic perturbation induced caustic lens masks in a ripple tank

Terahertz imaging presents immense potential across many fields but the affordability of multiple-pixel imaging equipment remains a challenge for many researchers. To address this, the adoption of single-pixel imaging emerges as a lower-cost option, however, the data acquisition process necessary for reconstructing images is time-intensive. Compressive Sensing, which allows for generation of images using a reduced number of measurements than Nyquist's theorem demands, presents a promising solution but long processing times are still issue particularly large-sized images. Our proposed solution to this issue involves using caustic lens effect induced by perturbations in a ripple tank as a sampling mask. The dynamic nature of the ripple tank introduces randomness into the sampling process and this reduces measurement time by exploiting the inherent sparsity of THz band signals. This work employed Convolutional Neural Network to perform target classification based on the distinct signal patterns acquired through the caustic lens mask. The proposed classifier achieved 99.22% accuracy rate in distinguishing targets shaped like Latin letters. The controlled randomness introduced by the caustic lens mask is believed to play a crucial role in achieving this high accuracy by mitigating overfitting, a common challenge in machine learning.

Self-Powered Terahertz Modulators Based on Metamaterials, Liquid Crystals, and Triboelectric Nanogenerators.

6G communication mainly occurs in the THz band (0.1-10 THz), which can achieve excellent performance. Self-powered THz modulators are essential for achieving better conduction, modulation, and manipulation of THz waves. Herein, a self-powered terahertz modulator, which is based on metamaterials, liquid crystals (LCs), and rotary triboelectric nanogenerators (R-TENGs), is proposed to realize the driving of different array elements. The corresponding designs can achieve an integrated design and preparation method for dynamic spectrum-reconfigurable liquid crystal metamaterials. In addition, for the type of cross-structure metamaterial liquid crystal box, a phase modulation of 1 GHz is achieved at frequencies of 0.117 and 0.161 THz with modulation depths of 13 and 11%, respectively. Because the R-TENG with a multifan blade and circular electrodes can generate 18 peaks of electric output in every rotation, it can successfully provide sufficient frequency alternating-current electric energy to drive the terahertz modulator and achieve a self-powered function. Our findings lay a solid theoretical foundation for further building self-powered THz communication systems and promote the development of a theoretical system for LC-driving spectrum-reconfigurable devices in the THz domain.

The maximal frequency of cosmic gravitons

We show that the maximal frequency of cosmic gravitons must not exceed the THz domain. From a classical viewpoint, both in conventional inflationary scenarios and in bouncing models the largest frequency of the spectrum overshoots the MHz band even if its specific signature is model dependent. According to a quantum mechanical perspective the maximal frequency is instead associated with the range of energies where a single pair of gravitons with opposite (comoving) three-momenta is produced. The upper limit on the largest frequency determines the minimal chirp amplitude [typically O(10−32)] required for a direct detection of a cosmic signal in the THz band. Below this limiting frequency the minimal chirp amplitude can be enhanced so that the optimal range ultimately depends on the physical properties of the diffuse backgrounds. In case a hypothetical instrument (at present just a figment of a hopeful imagination) would reach chirp amplitudes down to O(10−30) in the MHz or GHz bands, the Bose-Einstein correlations could be used to probe the properties of cosmic gravitons and their super-Poissonian statistics.

Dielectric resonator antenna with graphene layer and NMP structure for THz applications

Antennas with high performance and technological materials are relevant for several application on THz band. In this work, we investigate the cylindrical dielectric resonator antenna (CDRA) with non-resonant microstrip patch (NMP) feed combined with a thin layer of graphene on the top of the Dielectric Resonator (DR) designed for 3.30 THz. The NMP structure provides high order HEM12δ mode with great advantages in gain values and radiation efficiency. The graphene layer associated with the NMP structure provided improvements in gain and return loss, as well as, tuning characteristics by changing the chemical potential. The antenna was simulated in Ansys HFSS, and the results obtained with NMP radius of 12.7 μm are S11=−37.98 and 7.9dB gain. Besides, the bandwidth increased 6.6% to 20%. These results corroborate the effectiveness and enhancement of the antenna parameters providing a great alternative for Terahertz band applications.