Applications Of Terahertz Waves Research Articles

Graphene‑vanadium dioxide ultra-wideband dual regulated absorber

In this paper, we propose a novel tunable THZ absorber consisting of a patterned graphene layer and a metal plate separated by a vanadium dioxide dielectric layer. The absorbent has a simple structure, is easy to make graphene patterns, and has a dual regulatory function. The ability to double regulate comes from the fact that the state of vanadium dioxide changes with the temperature between the insulating phase and the metal phase, resulting in a change in its conductivity. When vanadium dioxide is in the insulating phase, the total reflection characteristics of the absorber are >7 THz, and when vanadium dioxide is in the metallic phase, the absorber achieves near-perfect absorption in the ultra-wideband range of 3 THz to 12.4 THz (9.4 THz). Local surface plasmon resonance explains the perfect absorption. Compared with previous absorbers, our proposed absorbers are not only simple in structure, the graphene pattern is also easy to process, but also have double adjustment and perfect absorption in the ultra-wideband range, are not sensitive to polarized light, and have large tolerances for the Angle of incident. The further development and enrichment of terahertz metamaterials and metasurface devices will also have beneficial implications for terahertz wave applications in sensing, communications, and radar. Due to their significant application value in stealth, detection, and communication, metamaterial absorbers have become a research hotspot in the field of metamaterials.

Research on Non-Destructive Quality Detection of Sunflower Seeds Based on Terahertz Imaging Technology.

The variety and content of high-quality proteins in sunflower seeds are higher than those in other cereals. However, sunflower seeds can suffer from abnormalities, such as breakage and deformity, during planting and harvesting, which hinder the development of the sunflower seed industry. Traditional methods such as manual sensory and machine sorting are highly subjective and cannot detect the internal characteristics of sunflower seeds. The development of spectral imaging technology has facilitated the application of terahertz waves in the quality inspection of sunflower seeds, owing to its advantages of non-destructive penetration and fast imaging. This paper proposes a novel terahertz image classification model, MobileViT-E, which is trained and validated on a self-constructed dataset of sunflower seeds. The results show that the overall recognition accuracy of the proposed model can reach 96.30%, which is 4.85%, 3%, 7.84% and 1.86% higher than those of the ResNet-50, EfficientNeT, MobileOne and MobileViT models, respectively. At the same time, the performance indices such as the recognition accuracy, the recall and the F1-score values are also effectively improved. Therefore, the MobileViT-E model proposed in this study can improve the classification and identification of normal, damaged and deformed sunflower seeds, and provide technical support for the non-destructive detection of sunflower seed quality.

Topologically influenced terahertz emission in Co2MnGa with a large anomalous Hall effect

The terahertz (THz) spectral zone is one of the most exciting but least explored domains of the electromagnetic spectrum. To extend the applicability of THz waves, the present objective is to develop an efficient, compact, durable, and low-cost THz emitter source. A spintronic THz emitter consisting of a ferromagnetic/nonmagnetic bilayer heterostructure is a promising innovation that can provide an alternative solution/replacement for conventional THz emitters. To further develop these spin-based THz emitters, we demonstrate an efficient and strong THz emission from a single layer of Co2MnGa with a large anomalous Hall effect (AHE) influenced by its Weyl semimetallic nature. Strong correlations among the THz emission, AHE, and chemical ordering of the full Heusler crystal structures for Co2MnGa are shown. Based on proper structural and chemical design, the topological nature of this material facilitates systematic optimization. Our initial findings provide a new design concept for the topological influences on spin-based THz emitters, and these emitters are expected to facilitate the further development of the intriguing Weyl physics.

Applications of terahertz waves in medical diagnostics: a literature review

Applications of terahertz waves in medical diagnostics: a literature review

Illegal food chemicals sensing with photonic crystal fiber sensor in the terahertz spectrum

The method by which a photonic crystal is manufactured fiber is detailed throughout the subsequent undertaking, containing a hexahedron core and hazardous dietary additives using a hexagonal cladding. Saccharine, sorbitol, and butyl acetate are used as analytes for sensing purposes. The sensor contains five tiers of circular ventilation holes within the hexagonal framework, as well as two tiers of hexahedron circles in the central area. FEM (Finite Element Method) is implemented in the sensor design of COMSOL software version 4.2. We noticed the response of the PCF sensor to the specific substances. We examined key optical metrics encompassing specifications including V-parameter, Relational sensitivity, effective refractive index, and power fraction to assess the suitability of the sensor for precisely and effectively detecting various food additives. The revised model attains sensitivity values of 90.10%, 91.30%, and 86.60%, respectively, at a frequency of 1 THz, the identification of saccharine (1.550 in the index of refraction), sorbitol (1.375 in the index of refraction), and butyl acetate (1.394 in the index of refraction) is performed. Furthermore, the compositions of saccharine (1.550 in the index of refraction), sorbitol (1.375 in the index of refraction), and butyl acetate (1.394 in the index of refraction) demonstrate losses from confinement of 6.15 × 10−8 dB/m, 7.25 × 10−8 dB/m, and 6.35 × 10−8 dB/m, which are comparatively minimal, and 0.0235 cm−1 is an insignificant effective material loss. These structures are studied at the terahertz frequency spectrum. Owing to its superior wave-guiding properties, this proposed sensor can be used for polarization-preserving terahertz wave applications and detecting dangerous food additives. Besides, because of simple fabrication, high sensitivity, and low confinement loss, we strongly believe this optimized geometrical structure will contribute to real-life applications that lead to safer food and support a circular economy in developing countries.

Metasurface-based sensor with terahertz molecular fingerprint enhancement in trace additives identification

Terahertz (THz) wave has gained considerable interest in biosensing with its unique spectral fingerprint characteristics. However, limited sensitivity confines its applications in trace-amount identification. In this article, a magnetic dipole metamaterial for strengthening molecular fingerprint in virtue of angle-scanning strategy is proposed, which is carefully designed by elaborately arranging symmetric condition of the dipole structures. The physical mechanism is elucidated by the finite-difference time-domain method. In virtue of manipulating the incident angle, the signal of molecular fingerprint is outstandingly elevated. Consequently, three kinds of food additives including azodicarbonamide, sorbic acid and melamine can be simultaneously detected, and the corresponding identification limits are as low as 1.65 μgcm−2 , 1.20 μgcm−2 and 1.66 μgcm−2 , respectively. Our work offers a new train of thought in identifying trace-amount food additives and paves the way for more practical applications of THz wave in food industry.

Application of Terahertz Radiation in Non-Destructive Testing of Military-Designated Composite Materials

The non-destructive testing methods (NDT) are gaining significant attention due to their ability to monitor the objects structure without causing their damage. In recent years, studies focused on NDT have been directed towards imaging with the use of the terahertz (THz) waves. The presented study describes terahertz imaging-based NDT method and testing results on selected military-designated materials with intentionally introduced defects. The main aim of the work was to clearly detect various discontinuities in materials interior and thus, to show the possibilities of the newly developed terahertz-based testing method in transmission mode. The results confirmed high applicability of THz waves for monitoring various materials where each implemented flaw was easily distinguished. Therefore, the presented method looks promising for real applications in everyday practice.

Focused THz wave from a spintronic terahertz Fresnel Zone Plate emitter

Spintronic terahertz emitters (STEs) with the feature of low cost and high performance are considered as one of the most potential terahertz (THz) sources for the future applications of THz waves. At present, the demand for a STE with some additional functions apart from the generation of a THz wave only has become more and more urgent. THz lenses, as a wavefront control device, which is one of the indispensable elements in a THz system, are widely used in THz focus and image systems. However, the common THz lenses are bulky and costly and not applicable for some miniaturization and integration THz systems. Based on two nonferromagnetic film with comparable magnitude of spin Hall angle, but with opposite sign, a binary-phase spintronic terahertz Fresnel Zone Plate emitter (ST-FZPE) is proposed in this paper. Compared with reported THz emitters with a FZP structure, the ST-FZPE is low cost, easy to be fabricated and has a higher efficiency of directing emission focused THz wave. As a planar optical element, the ST-FZPE has tremendous significance to realize integration and miniaturization of THz devices and systems, and it has potential applications in the field of THz bio-sensing, THz imaging and THz communication in the future.

Terahertz polarization detection based on the mode analysis of longitudinally polarized vortices

Polarization detection of terahertz waves is essential for applications of terahertz waves in communication, imaging, and other fields. Here, we propose a novel terahertz polarization detection approach via the mode analysis of transmitted longitudinally polarized vortex field. By finely designing the phase profiles of the proposed all-silicon metasurface, we establish a connection between the generated longitudinally polarized vortices and the polarization parameters of the incident field. Complete polarization parameters are obtained by analyzing the intensity, phase, and mode purity of the longitudinally polarized vortex field. The comparisons between the detected polarization states and the incident states prove the accuracy of the proposed metasurface for polarization detection. Such metasurface polarization detection scheme may further promote the application of terahertz technology.

Pulse Burst Multiwavelength Terahertz-Wave Spectroscopic Measurement Using Parametric Wavelength Conversion

In this study, we developed a multi-wavelength spectroscopic measurement system using a pulse burst-pumped injection-seeded terahertz (THz)-wave parametric generator (is-TPG). Real-time spectroscopy using multi-wavelength simultaneous generation has been demonstrated; however, the ability to make quantitative measurements with this method has been limited by the gain competition between wavelengths. In this study, we divided the pump beam into multiple paths and created pulse bursts with nanosecond intervals via spatial delays. Quasi-multi-wavelength simultaneous generation was achieved by generating different THz wavelengths with each pulse; in principle, this is equal to repetitive single-wavelength generation. The gain competition between multi-wavelengths was suppressed compared to the multi-wavelength is-TPG pumped by a single pump pulse. Pulse-burst pumping improved the “quantitativeness” of real-time spectroscopic measurements using an is-TPG and is expected to expand the scope of THz wave applications.

Millimetre wave antenna for biomedical applications-a review

Terahertz represents the portion of electromagnetic wave between the microwave and IR region. Recent research has been focused on the application of Terahertz waves in biomedical applications such as cancer detection. The current diagnostic methods in cancer treatment involve imaging to MRI and biopsy. Due to the low photon energy that terahertz possess, there are no health threats caused in contrast to traditional methods. Cancer cells can be detected by monitoring the blood supply to the affected tissues and water level in the local tissues. This can be diagnosed using Terahertz imaging. THz radiation can be generated and detected using various commercialized systems. On analysis of various literatures, it is observed that Meta material antennas will be the suitable candidate to enhance the existing terahertz spectroscopy techniques since it has better focusing capabilities. This paper reviews the various implementation techniques of an antenna in THz range. The design and properties of the antenna are also investigated.

Terahertz Schottky Barrier Diodes Based on Aligned Carbon Nanotube Arrays

Carbon nanotubes (CNTs) with superior electrical characteristics are among the most promising candidate materials for constructing millimeter wave and terahertz wave electronic devices. However, the reported Schottky barrier diodes (SBDs) based on CNTs usually exhibited high series resistance and severe mismatch to lower impedance external circuits, which limits the devices for high frequency applications. In this work, CNT SBDs of lateral structure with a rectify current density of -0.78 mA/μm and a short circuit current responsivity of 6.8 A/W are demonstrated based on aligned carbon nanotube arrays (A-CNTs) with high-density and high-semiconducting purity on a quartz substrate. The A-CNT SBD with a width of 10 μm and an L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ch</sub> of 50 nm under zero bias exhibits an ultra-low intrinsic capacitance of 2.5 fF and an intrinsic cutoff frequency <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of up to 840 GHz, which suggests that CNT-based SBDs are promising for millimeter wave and terahertz wave applications.

High stability with low polarization and confinement loss based new structure of photonic crystal fiber waveguide profile

Abstract Nowadays ethanol is broadly used chemical item in various industrial purpose. So, detection of this organic chemical has become very important task in industrial basis. In this paper, a hexagonal core PCF has constructed in which silica is used as base material. A finite element method is applied and mesh analysis has been done for investigating boundary conditions of proposed PCF sensor. After simulation, 59.48 % sensitivity and an ignorable loss of confinement which is 8·956020 × 10−13 cm−1 at 2.5 THz frequency has been gained. Focusing on easy construction of fiber, conventional fabrication procedures are attainable. Having satisfactory waveguiding attributes, the proposed sensor can be applicable effectively for sensing ethanol and also polarization-based applications of terahertz waves.

Terahertz shaping technology based on coherent beam combining

The generation of terahertz (THz) waves by focusing a femtosecond pulsed laser beam at a distance is able to overcome the strong absorption properties of air and has rapidly attracted the attention of industry. However, the poor directionality of the THz wave radiation generated by this method is not conducive to THz wave applications. By controlling the morphology of the ultrafast laser-excited plasma filament and its electron density distribution through coherent beam combining technology, we achieve direct THz beam shaping and are able to obtain THz wave radiation of Gaussian or arbitrary transverse distribution. The novel experimental approach proposed in this paper opens up the research field of direct THz wave shaping using plasma. Moreover, it innovates multi-parameter convergence algorithms and, by doing so, has the potential to find beam patterns with higher energy conversion efficiency and break the energy limit of THz waves emitted by lasers at high power.

THz Systems Exploiting Photonics and Communications Technologies

Terahertz (THz) systems open up the possibility of new applications of electromagnetic waves. Enormous bandwidths up to several terahertz enable the development of powerful spectroscopy systems that can provide insights into the structure and material of objects with micrometer resolution. New high power and compact THz time-domain spectroscopy (TDS) systems will be presented. The wide bandwidth also enables high-resolution imaging under far-field conditions to analyze arbitrary objects from a distance. In addition, a near-field system-on-a-chip imaging system is presented that achieves a resolution of 10 μm. Additionally, the application of terahertz waves presents challenges due to the low transmit power of the sources, high attenuation in free space, and the high noise figures of the receivers. These challenges can be overcome by antennas with high gain. Since - depending on the application - spatial scanning or focusing in a time-varying direction is required, beam steering is an essential component of many THz systems. In terms of communications, terahertz carrier frequencies offer wide bandwidths, enabling correspondingly high data rates of 100 Gbit/s and more. As a result, the frequency bands between 250 GHz and 450 GHz have already been identified and/or allocated for communication services and are being discussed as a component of 6G mobile communications. Concepts and demonstrations for 6G terahertz mobile communications will be presented. The high bandwidth of terahertz waves can also be used for high-accuracy indoor localization.

Ultrafast Optomechanical Terahertz Modulators Based on Stretchable Carbon Nanotube Thin Films

For terahertz wave applications, tunable and rapid modulation is highly required. When studied by means of optical pump–terahertz probe spectroscopy, single-walled carbon nanotube (SWCNT) thin films demonstrated ultrafast carrier recombination lifetimes with a high relative change in the signal under optical excitation, making them promising candidates for high-speed modulators. Here, combination of SWCNT thin films and stretchable substrates facilitated studies of the SWCNT mechanical properties under strain and enabled the development of a new type of an optomechanical modulator. By applying a certain strain to the SWCNT films, the effective sheet conductance and therefore modulation depth can be fine-tuned to optimize the designed modulator. Modulators exhibited a photoconductivity change of approximately 2 times of magnitude under the strain because of the structural modification in the SWCNT network. Stretching was used to control the terahertz signal with a modulation depth of around 100% without strain and 65% at a high strain operation of 40%. The sensitivity of modulators to beam polarization is also shown, which might also come in handy for the design of a stretchable polarizer. Our results give a fundamental grounding for the design of high-sensitivity stretchable devices based on SWCNT films.

Graphene-based terahertz reconfigurable printed ridge gap waveguide structure

Graphene-based microwave devices have enabled reconfigurability, thus paving the way to the realization of flexible wireless terahertz systems with featured performances. Despite great progress in the development of graphene-based terahertz devices in the literature, high insertion loss and wide tunable range are still significant challenges at such high frequencies. In this work, we introduce the use of graphene to implement a reconfigurable printed ridge gap waveguide (RPRGW) structure over the terahertz frequency range for the first time. This guiding structure is suitable for both millimeter and terahertz wave applications due to its supporting quasi-TEM mode, which exhibits low dispersion compared to other traditional guiding structures. The presented solution is featured with low loss as the signal propagates in a lossless air gap, which is separated from the lossy graphene elements responsible for the reconfigurable behavior. In addition, this guiding structure is deployed to implement a tunable RPPGW power divider as an application example for the proposed structure.

Broadband generation of abruptly autofocusing terahertz ring-Airy beams with all-silicon metasurface

Abruptly autofocusing terahertz (THz) ring-Airy beam has been generated by transmissive plasmonic metasurface at a single frequency in the earlier work. However, the transmission efficiency of it is low. Based on the waveguide effect, a design strategy for broadband generation of THz ring-Airy beams with transmissive all-silicon metasurface is proposed in this paper, which has an improved transmission efficiency and is more favorable in fabrication. Under the linearly polarized plane wave incidence, the amplitude and phase of the cross-polarized transmitted beam can be simultaneously modulated by controlling the rotation angles of the identical rectangular silicon pillars arranged on the silicon substrate. A polarization-independent antireflection square silicon pillar array is also proposed to be patterned at the other side of the substrate. The metasurface promises a working bandwidth from 2.12 to 2.72 THz with the overall transmission efficiency being over 51.18% (the highest transmission efficiency is up to 77.57% at 2.52 THz). The peak intensity positions of the generated beam during the propagation follow a parabolic trajectory. For the generated ring-Airy beam, the focal length is positively proportional to the frequency, while the abruptly autofocusing property and full width at half maximum (FWHM) of the focus are positively and negatively related with the frequency, respectively. This work can be expected to facilitate the applications of terahertz wave in biomedicine and manipulation of particles.

Beam steerable terahertz antenna based on VO&lt;sub&gt;2&lt;/sub&gt;

To realize the diversified applications of terahertz wave, a new method to realize 2-bit (4 states) coding metasurface with only two hybrid units is proposed, which combines the phase transition characteristics of VO&lt;sub&gt;2&lt;/sub&gt; and is different from the traditional metasurface. The metal wire threaded through the patch makes single-line control possible. The method of preparing the VO&lt;sub&gt;2&lt;/sub&gt; thin film and the voltage control mechanism make the design more practical. The highlight of this design is that the fixed structure array can encode different state sequences and then tune the reflected beam. On this basis, a visual design is carried out for the calculation of the coding metasurface array antenna by MATLAB. The state sequence and radiation results are predicted by actively setting the operating frequency, beam deflection angle, etc., so as to achieve active adjustment. The system does not limit the unit structure and is universal to all arrays that meet the coding conditions. In addition, a deep neural network is introduced into the array arrangement, and the structure sequence is predicted by algorithm training and verified by numerical calculation and full-wave simulation. The results show that the proposed method is effective in beam deflection angle and structure arrangement. This study presents a new way of actively and flexibly controlling terahertz waves, which has potential applications in terahertz imaging, phase-controlled radar, communication and other fields.

Nano-printing technology based double-spiral terahertz tunable metasurface

Electromagnetic devices made of artificially constructed metasurfaces can achieve filtering, modulation, sensing, and detection functions in the terahertz frequency band, which is essential for the applications of terahertz waves in the fields of communication and imaging. We design and prepare a flexible and transparent double spiral metasurface based on nano-printing technology, and use the metasurface to construct a rotating tunable filter, which can achieve regular tuning of the terahertz wave transmittance by rotating the metasurface. After rotating 90°, the transmittance at 0.52 THz increases from 8% to 67%, and the transmittance at 0.92 THz decreases from 68% to 3%, thus realizing active tuning with modulation depth greater than 88%. Moreover, the proposed nano-printing metasurfaces have excellent properties of ultra-thinness, flexibility, and visible light transparency, which are conducive to the miniaturization, light-weight and large-area preparation of terahertz tunable devices.