THz Band Research Articles

Multi-function digital THz-metasurfaces based on phase change materials

An effective and digitally tunable reflective multi-function metasurface in the THz band is proposed. The introduced planar structure consists of an array of a well-designed double-split SRR (DSSRR) utilizing phase-change materials (PCMs) in the gap regions, leading to the controllable function of the metasurface due to the tuning of the crystallization level of PCMs. According to Pancharatnam-Berry's principle, this effect creates different electromagnetic responses only by employing two types of unit cells (bit-0 and bit-1) based on the proper addressing of the PCMs in the structure. It is reported that by digitally changing the coding sequence of the metasurface, it is individually capable of performing different functions such as polarization rotation, beam steering, and RCS reduction. The computational results have been verified by appropriate analytical equivalent electric-circuit modeling and the theory of antenna-arrays. The studied planar-structure with its tunable and coding properties might be suggested for advanced applications in THz sensing, communications, and imaging systems.

Compact, low-loss, and high-polarized-extinction ratio terahertz TM-pass polarizer based on a hybrid plasmonic waveguide with a graphene ridge

A compact, low-loss, and high-polarized-extinction ratio TM-pass polarizer based on a graphene hybrid plasmonic waveguide (GHPW) has been demonstrated for the terahertz band. A ridge coated by a graphene layer and the hollow HPW with a semiround arch (SRA) Si core is introduced to improve structural compactness and suppress the loss. Based on this, a TM-pass polarizer has been designed that can effectively cut off the unwanted TE mode, and the TM mode passes with negligible loss. By optimizing the angle of the ridge, the height of the ridge, air gap height, and the length of the tapered mode converter, an optimum performance with a high polarization extinction ratio of 30.28 dB and a low insert loss of 0.4 dB is achieved in the 3 THz band. This work provides a scheme for the design and optimization of polarizers in the THz band, which has potential application value in integrated terahertz systems.

Detection of optical properties of chiral substances by a photoconductive THz polarization detector

Chiral enantiomers have significant differences in biochemical functions. The use of THz wave polarization detection to characterize the optical properties of chiral substances is of great significance to the development of life science and the identification and application of chiral substances. However, the traditional polarization detection procedures of THz waves are complex, which limits the study of chiral substances. Herein, we proposed a high-sensitivity THz polarization detector, which can simultaneously obtain the change information of amplitude, phase, and polarization state through a single measurement. The optical rotation and elliptical angle of solid and liquid D/L-Glutamic acid 5-methyl ester in the THz band are studied. Then it is verified that anisotropic interference may occur in the preparation of solid samples. Finally, the effects of sample content and thickness on polarization are obtained. The experimental results show that different chirality has the opposite effect on the state of polarization, and the difference between chiral enantiomers can be detected by this method. This work is of great significance for understanding the optical properties of chiral substances and promoting the development of chiral recognition.

High FoM quad‐band THz perfect metamaterial absorber‐based RI sensor for gas sensing applications

AbstractHere, a novel design of a microstructured quad‐band terahertz metamaterial absorber is presented consisting of a copper‐based merged circular and square split ring resonator‐integrated over a lead glass substrate intended to detect changes in the surrounding medium's refractive index for gas sensing applications through THz spectroscopy. Outstanding performance is shown by the suggested absorber, which achieves a maximum absorption rate of 99.4%, 98.7%, 99.2%, and 99.4%, at resonance frequencies of 8.055 (), 8.423 (), 9.257 (), and 9.507 () THz, respectively. The quality factors (Q‐factors) attained are 201, 129, 298, and 271 for these four absorption bands. The obtained sensitivity values at these four distinct bands are 3898, 2927, 4719, and 4102 GHz per refractive index unit with high figure of merits (FoM) of 97, 95, 152, and 117, respectively, throughout a range of refractive indices from n = 1.00 to n = 1.05 (with a step size of 0.005). The study's novelty lies in its novel design and remarkable ability to excel in gas sensing with high sensitivity and high FoM across not just one but four distinct THz bands. Furthermore, each band demonstrates significantly superior performance in sensitivity, Q‐factor, and FoM, among other parameters compared to prior research in both gas detection and RI monitoring scenarios, hence exhibiting its potential across a wide range of applications.

Discrimination of five commercial Guibourtia wood species using terahertz time domain spectroscopy combined with machine learning approaches

Abstract Terahertz waves hold significant potential for applications in wood identification, owing to their good penetration and distinctive fingerprints in wood. This study focuses on wood samples from five different Guibourtia species as the research objects. The terahertz time-domain spectroscopy (THz-TDS) is employed to acquire the spectroscopic signals of the wood samples and to extract their optical parameter data. The THz refractive indices are dimensionally reduced through principal component analysis (PCA), and three machine learning models, namely partial least squares-discriminant analysis (PLS-DA), random forest (RF), and support vector machine (SVM), are employed to classify the wood of five different Guibourtia species. Time delays of the wood samples from five different Guibourtia species are concentrated in the range of 60–62 ps and exhibit different amplitudes in the frequency domain. Refractive indices showed significant variations within the THz band. PCA for dimensionality reduction of terahertz time-domain spectral data significantly improves the recognition rate of machine learning models. Applying PCA to the refractive index data, the RF model achieves a highest recognition rate of 96.9 % and an overall classification accuracy of 98 %. Current results demonstrate that THz-TDS enables rapid, accurate, and non-destructive classification and identification of wood from the Guibourtia species.

A layered metastructure based on brewster angle for Janus ultra-wideband tunable directional thermal radiation

In this proposal, we introduce a Janus layered metastructure (JLM) composed of three distinct structures. The first and third structures (S1 and S3) are designed based on Brewster and Bragg reflection laws, which enable the selection of a specific incident angle of electromagnetic waves (EMWs) by adjusting the refractive indices of the materials. By matching the thickness of the dielectric layer to the optical wavelength, S1, and S3 operate across the 30–300 μm band, achieving angle selection (AS) of EMWs in the THz band. The second structure (S2) consists of a single layer of graphene and SiO2 arranged in quasi-periodic sequences, which absorbs EMWs in the THz band and achieves broadband absorption without angle sensitivity. By combining the characteristics of these three structures, we achieve a broadband directional thermal radiation covering the 30–300 μm band. Additionally, the adjustability of the AS of S1 and S3 and the absorption effect of S2 enable Janus ultra-wideband tunable directional thermal radiation at large angles depending on the specific needs.

Influence of cladding tube structure on transmission characteristics of negative curvature terahertz fiber

A novel type of polarization negative curvature terahertz fiber has been designed, which has a cladding region composed of two long semi-circular cladding tubes in the horizontal direction and four long semi-elliptical cladding tubes in the oblique direction. The optimal fiber dimensions are determined to investigate the internal relationship between polarization characteristics and fiber cladding structure. The transmission performances of the negative curvature fiber are investigated by varying the thickness of fiber cladding tubes and adjusting the structural characteristics of cladding tubes in different directions. Relatively good transmission performances can be obtained with a cladding tube thickness of 80 μm. The birefringence can be stably maintained at the order of 10−4, and the confinement loss of the x-polarized mode at 2.55 THz is 0.036 dB cm−1. Moreover, the dispersion coefficient of the two polarized modes is stable between ±0.6 ps (THz·cm)−1 in the 2.3–2.6 THz band. The bent performance of the designed fiber at 2.52 THz is also discussed. The results show that the fiber has low bending sensitivity, and can still maintain excellent birefringence and low confinement loss in the bending state. For the y polarization, the birefringence can be basically stabilized at the order of 10−4 within a bending radius of 10–60 cm.

Design of an ultra wideband THz reflectarray antenna based on orbital angular momentum

A Reflectarray antenna (RA) that can generate orbital angular momentum vortex waves in the THz band was proposed. First of all, a reflective phase-shifting element composed of a notched double-shaped ring and a single Matal structure was designed to form a radiation patch. The reflective phase-shifting element has the advantages of good linearity, high smoothness, and a large phase shift range, which is suitable for the subsequent production of reflective array antennas. Then, the phase distribution of the reflected wave of the OAM vortex wave carrying mode l=+1、+2、+3、+4 is given, and the size of each element is determined based on the reflected phase of each element. Using the proposed element, an OAM reflectarray antenna operating within the waveguide standard WR-5 (120GHz–220GHz) is designed. The optimal frequency band for antenna operation in mode l=+1 is 150GHz–190 GHz, and the maximum gain of the reflector array antenna is 19.4 dBi, and the minimum divergence angle of the antenna array is 5.2°, which has good directivity and beam radiation ability. The spiral structure of the OAM beam space generated by the RA in this paper is clearly visible, proving that the implementation of OAM vortex waves in the THz frequency band is feasible. This design also provides a new method for generating OAM vortex waves in wireless and microwave wireless communications. The proposed RA is suitable for 6G wireless communication systems.

Effect of crystal-water on the optical and dielectric characteristics of calcium sulfate in the THz band

The effect of crystal-water contents on the optical properties and dielectric characteristics of calcium sulfate in the THz band is investigated. The complex dielectric constant and conductivity are analyzed using the Drude-Smith model. The refractive index and absorption coefficient are linearly increased with the content of crystal-water, and the corresponding linear fitting lines of R2 over 0.97 are obtained. The dielectric properties of calcium sulfate are significantly affected by the crystal-water content. These results indicate that a new method to quantitative measurement of the crystal-water content in hydrous minerals is provided.

Concave Version Printing of High-Performance Polymer THz Filters.

The development of high Q and tunable narrowband filters that can efficiently manipulate THz beams is critical for various THz applications, such as imaging and sensing. However, for filters made of metals and dielectrics, issues such as high losses, limited tunability, and lengthy process flows exist. Here, a scalable concave version reprinting technique to mass produce high-performance microstructured polymer filters is presented. The technique is extremely simple, eliminating the demand for the use of any large equipment including injection molding and thermal press printing machines, and is reliable; in the reprinted structures, there are no defects including gaps and air bubbles. The produced narrowband filters exhibit a high Q factor of 57 with wide tunability over the THz band from ∼80 to 160 μm in wavelength. The presented technique can be adopted to realize other devices as well using polymer materials with simplicity and high precision.

Wafer-level substrate-free YIG single crystal film for a broadband tunable terahertz isolator

Yttrium iron garnet (YIG) is a promising material for various terahertz applications due to its special optical properties. At present, a high-quality YIG wafer is the desire of terahertz communities and it is still challenging to prepare substrate-free YIG single crystal films. In this work, we prepared wafer-level substrate-free La:YIG single crystal films, for the first time, to our knowledge. Terahertz optical and magneto-optical properties of La:YIG films were characterized by terahertz time domain spectroscopy (THz-TDS). Results show that the as-prepared La:YIG film has an insertion loss of less than 3 dB and a low absorption coefficient of less than 10 cm−1 below 1.6 THz. Benefitting from the thickness of the substrate-free YIG films and low insertion loss, their terahertz properties could be further manipulated by simply using a wafer-stacking technique. When four La:YIG films were stacked, there was an insertion loss of less than 10 dB in the range of 0.1−1.2 THz. The Faraday rotation angle of the four-layer-stacked La:YIG films reached 19°, and the isolation could reach 17 dB. By further increasing the stacking number to eight pieces, a remarkable Faraday rotation angle of 45° was achieved with an isolation of 23 dB, which is important for practical application in the THz band. This material may provide a milestone opportunity to make various non-reciprocal devices, such as isolators and phase shifters.

A Compact THz Photometer for Solar Flare Burst Studies from Space

Recent solar flare observations reveal the presence of THz components having a few orders of magnitude higher amplitude than the Radio Frequency (RF) and microwave radiations in the solar flare spectrum. This feature makes it remarkably beneficial for the detection of solar flares of low magnitudes such as Geostationary Operational Environmental Satellite (GOES) Class C, and M flares. Hence, space agencies including National Aeronautics and Space Administration (NASA) and European Space Agency (ESA) are gearing up with THz instruments for solar flare observations. In line with this, Laboratory for Electro-Optics Systems (LEOS) has completed the development of a photometer that operates in continuum THz band, from 1.5 to 15 THz. The entire electro-opto-mechanical design was carried out to realize a compact, light-weight and power-efficient photometer to meet the size, mass and power constraints posed by inherent resource limitations of experimental satellite platforms such as Nanosatellite, Microsatellite and Polar Satellite Launch Vehicle’s 4th Stage Orbital Platform (PS4-OP). A Cassegrain optical configuration was designed to collect the sun radiation with an aperture effective area of 0.0027 m2 with ceramic low pass filters and resonant mesh filter for selective perception of THz radiation, rejecting the huge background of visible and Near Infrared (NIR) with a spectral selectivity of 107:1. Sensitivity of the Photometer was estimated to be 41.5 K/mV in 1.5 to 15 THz band by calibration using a blackbody radiation source. The radiometric resolution of the payload is 125 Solar Flux Unit (SFU) (1 SFU=1×10-22 Wm-2Hz-1). The payload was successfully developed and space-qualified for its deployment in Nanosatellite or PS4-OP platform.

The RCS Scaled-Measurement and Inversion Method for Metal Targets With Rough Surfaces Based on the Distribution of Surface Elements in the THz Band

The RCS Scaled-Measurement and Inversion Method for Metal Targets With Rough Surfaces Based on the Distribution of Surface Elements in the THz Band

System-Level Assessment of Massive Multiple-Input–Multiple-Output and Reconfigurable Intelligent Surfaces in Centralized Radio Access Network and IoT Scenarios in Sub-6 GHz, mm-Wave, and THz Bands

System-Level Assessment of Massive Multiple-Input–Multiple-Output and Reconfigurable Intelligent Surfaces in Centralized Radio Access Network and IoT Scenarios in Sub-6 GHz, mm-Wave, and THz Bands

Polarization independent tunable bandwidth absorber based on single-layer graphene

As the demand for cognitive understanding of the THz band continues to grow, there is significant potential for practical applications in designing optical devices operating in this frequency range. This paper proposes a polarization-independent tunable bandwidth absorber based on a monolayer graphene, capable of achieving an absorption rate higher than 0.9 in the 3–6 THz band. The absorber exhibits polarization incoherence due to the structural symmetry. Analysis of the electric field distribution indicates that the strong absorption effect of the absorber primarily arises from the coupling of localized surface plasmon resonance (LSPR). The absorber also demonstrates excellent tunability, both in terms of physical and chemical properties, while maintaining optimal absorption performance at a 45° incidence angle. This implies that the absorber holds significant potential for applications in fields such as biomedicine and communications.

Design of an ultra-compact reconfigurable IDC graphene-based SIW antenna in THz band with RHCP

Design of an ultra-compact reconfigurable IDC graphene-based SIW antenna in THz band with RHCP

Organohydrogel-based transparent terahertz absorber via ionic conduction loss

The fast-growing terahertz technologies require high-performance terahertz absorber for suppressing electromagnetic interference. Since the dissipation mechanism in terahertz band usually focuses on electronic conduction loss, almost all terahertz absorbers are constructed with electronically conducting materials being opaque, which limits their applications in scenarios requiring high visible transmittance. Here, we demonstrate a transparent terahertz absorber based on permittivity-gradient elastomer-encapsulated-organohydrogel. Our organohydrogel-based terahertz absorber exhibits a high absorbing property (average reflection loss of 49.03 dB) in 0.5–4.5 THz band with a thin thickness of 700 μm and a high average visible transmittance of 85.51%. The terahertz absorbing mechanism mainly derives from the ionic conduction loss of the polar liquid in organohydrogel. Besides, the hydrophobic and adhesive elastomer coating endows this terahertz absorber high absorbing stability and interfacial adhesivity. This work paves a viable way to designing transparent terahertz absorbers.

High-Order Rotational-Migration Correction and ISAR Imaging in the THz Band Considering Impact of Scattering Intensity

High-Order Rotational-Migration Correction and ISAR Imaging in the THz Band Considering Impact of Scattering Intensity

THz Band Channel Measurements and Statistical Modeling for Urban Microcellular Environments

THz Band Channel Measurements and Statistical Modeling for Urban Microcellular Environments

Reconfigurable fork shaped plasmonic graphene based nano-patch antenna for wireless network-on-chip application in THz band

Reconfigurable fork shaped plasmonic graphene based nano-patch antenna for wireless network-on-chip application in THz band