Tray Research Articles

Ultra-fast terahertz optical switch based on Metal vanadium dioxide

Abstract In current research, the reflection and absorption characteristics of optical switches in optical systems are often limited by the material's response speed and tunable range. We propose a novel metal grating structure with phase-change material to overcome these limitations. This paper proposes a tunable ultrafast terahertz optical switch based on vanadium dioxide (VO₂), composed of a VO₂ layer, a dielectric layer, and metal, with the VO₂ layer located on top of the plasmonic (Au) thin film. The optical performance variations in different states are revealed by analyzing the absorption rate, electric field distribution, and light absorption time of VO₂ before and after phase transition. VO2 exhibits a promising phase transition for actively controlling terahertz waves, with the insulator-to-metal transition used to switch the coupling between surface plasmon modes and normally incident electromagnetic waves on and off. These optical switches enable ultrafast switching, as the VO2 phase transition occurs on the femtosecond timescale. Experimental results indicate that the absorber demonstrates near-perfect absorption peaks, achieving absorption rates as high as 99.99%. The absorption efficiency modulation depth before and after the phase transition can reach 99.92%, allowing the absorber to achieve tunability. The reflectivity of VO2 is about -0.38 dB before the phase transition, and it significantly increases to -23.3 dB after the transition, with a switching ratio of 22.92 dB. This significant reflectivity change is attributed to the VO₂ phase transition, leading it to reflect light signals at high temperatures, preventing their transmission. The electric field diagrams and light absorption time analysis further support this conclusion, demonstrating that VO₂ material can efficiently switch optical signals by modulating temperature. This research offers new approaches for developing high-performance, fast-response terahertz optical switches, with potential applications in future optical communication and signal processing.

Perspective on the applications of terahertz imaging in skin cancer diagnosis

Applications of terahertz (THz) imaging technologies have advanced significantly in the disciplines of biology, medical diagnostics, and non- destructive testing in the past several decades. Significant progress has been made in THz biomedical imaging, allowing for the label-free diagnosis of malignant tumors. Terahertz frequencies, which lie between those of the microwave and infrared, are highly sensitive to water concentration and are significantly muted by water. Terahertz radiation does not cause ionization of biological tissues because of its low photon energy. Recently, terahertz spectra, including spectroscopic investigations of cancer, have been reported at an increasing rate due to the growing interest in their biological applications sparked by these unique features. To improve cancer diagnosis with terahertz imaging, an appropriate differentiation technique is required to increased blood supply and localized rise in tissue water content that commonly accompany the presence of malignancy. Terahertz imaging has been found to benefit from structural alterations in afflicted tissues. This study provides an overview of terahertz technology and briefly discusses the use of terahertz imaging techniques in the detection of skin cancer. Research into the promise and perils of terahertz imaging will also be discussed.

Bioinspired Disordered Aerogel for Omnidirectional Terahertz Response.

The structural disorder of the black butterfly assists in capturing sunlight across a wider spectral and angular range, injecting infinite vitality for omnidirectional and stimuli-responsive wave-absorbing materials. Here, the disordered micro-pores responding to terahertz (THz) waves through electromagnetic simulations, and then prepared via ice templating technology are analyzed and optimized. The customized disordered aerogel makes possible perfect terahertz response property with incidence-angle-insensitive and ultra-broadband. Ti3C2Tx MXene/carboxymethyl cellulose aerogels realize excellent shielding effectiveness exceeding 70.32dB and reflection loss of more than 43.02dB over the frequency range of 0.3-1.5 THz. Tailoring the structural orientation of anisotropic aerogels functions as a versatile dynamic modulation approach along terahertz propagation direction. The porous structure with moderate conductivity gradually triggers the resonance effect of the cavity, approximating a resonance sphere (pore) and waveguide system (tube). Ultimately, gradient impedance aerogel is proposed integrating THz-infrared stealth, hydrophobicity, and mechanical strength. This inspired biomimetic structural strategy will also enable various terahertz applications such as terahertz imaging, line-of-sight telecommunication, information encryption, and space exploration.

Dielectric metasurface-assisted terahertz sensing: mechanism, fabrication, and multiscenario applications

Abstract Terahertz (THz) technology has attracted significant global interest, particularly in sensing applications, due to its nonionizing feature and sensitivity to weak interactions. Recently, owing to the advantages of low optical loss and the capability to support both electric and magnetic high-quality factor (high-Q) resonances, dielectric metasurfaces have emerged as a powerful platform for multiscenario terahertz sensing applications. This review summarizes recent advancements in dielectric metasurface-assisted THz sensing. We begin with an overview of the mechanisms and properties of dielectric metasurfaces with high-Q factors. Next, we discuss typical fabrication techniques for these terahertz dielectric metasurfaces. We then explore the diverse terahertz sensing applications across various scenarios, including biomolecule sensing, biomedical detection, environmental monitoring, and chiral sensing. Finally, we provide perspectives on the future development of this promising research field.

Nondestructive testing of GFRP composites with compressive sensing-based terahertz single-pixel imaging

ABSTRACT Terahertz (THz) imaging technique has a wide range of applications, from industrial non-destructive testing (NDT) to various biomedical applications. Although the capabilities of terahertz radiation in defect detection are well-proven, the practical limitations have been associated with the long image acquisition time. Hence, to improve the image acquisition speed, the compressive sensing technique has been employed. However, the challenge is identifying the optimal modulation mask for compressive sensing as the image reconstruction metrics hugely depend on the mask. Hence, a systematic investigation of the different modulation masks has been done to reconstruct THz images of glass fibre-reinforced polymer (GFRP) composites using TVAL3 (Total Variation minimization by Augmented Lagrangian and ALternating direction Algorithm). The image reconstruction quality has been studied using mean square error, peak signal-to-noise ratio, and structural similarity index. From the results, it can be noticed with a 0.3 sampling ratio, reliable reconstruction of the THz image is possible, saving 70% of the image acquisition time. Further, the discrete cosine transform (DCT) mask is ideal for high frame rate NDT in low and medium noise scenarios. However, the Bernoulli basis offers high resistance to noise by resulting in the best image reconstruction results at high noise cases, outperforming the DCT.

Ultrathin Terahertz-Wave Absorber Based on Inorganic Materials for 6G Wireless Communications.

Terahertz waves are gathering attention as carrier waves for next-generation wireless communications such as sixth-generation wireless communication networks and autonomous driving systems. Electromagnetic-wave absorbers for the terahertz-wave region are necessary to ensure information security and avoid interference issues. Herein we report a high-performance terahertz-wave absorber composed of a composite of metallic λ-Ti3O5 and insulating TiO2 nanocrystals (λ-Ti3O5@TiO2). This material exhibits a strong terahertz-wave absorption with high values for the real (permittivity, ε') and imaginary parts (dielectric loss, ε″) of the complex dielectric constant. Furthermore, the tan(δ) (≡ ε″/ε') values are significantly high, ranging from 0.50 to 0.76 in the frequency range between 0.1 and 1 THz. An ultrathin film with a thickness of 48 μm recorded a reflection loss of -28 dB (99.8% of the terahertz wave is absorbed by the film). A terahertz-wave absorber with such a small thickness has yet to be developed. Not only does the present material exhibit resistance to heat, light, water, and organic solvents, but it can also be economically fabricated to support various applications, including outdoor uses.

Fourier synthetic-aperture-based time-resolved terahertz imaging

Terahertz (THz) microscopy has attracted attention owing to distinctive characteristics of the THz frequency region, particularly non-ionizing photon energy, spectral fingerprint, and transparency to most nonpolar materials. Nevertheless, the well-known Rayleigh diffraction limit imposed on THz waves commonly constrains the resultant imaging resolution to values beyond the millimeter scale, consequently limiting the applicability in numerous emerging applications for chemical sensing and complex media imaging. In this theoretical and numerical work, we address this challenge by introducing, to our knowledge, a new imaging approach based on acquiring high-spatial frequencies by adapting the Fourier synthetic aperture approach to the THz spectral range, thus surpassing the diffraction-limited resolution. Our methodology combines multi-angle THz pulsed illumination with time-resolved field measurements, as enabled by the state-of-the-art time-domain spectroscopy technique. We demonstrate the potential of the approach for hyperspectral THz imaging of semi-transparent samples and show that the technique can reconstruct spatial and temporal features of complex inhomogeneous samples with subwavelength resolution.

A Broadband Mode Converter Antenna for Terahertz Communications

The rise of artificial intelligence (AI) necessitates ultra-fast computing, with on-chip terahertz (THz) communication emerging as a key enabler. It offers high bandwidth, low power consumption, dense interconnects, support for multi-core architectures, and 3D circuit integration. However, transitioning between different waveguides remains a major challenge in THz systems. In this paper, we propose a THz band mode converter that converts from a rectangular waveguide (RWG) (WR-0.43) in TE10 mode to a substrate-integrated waveguide (SIW) in TE20 mode. The converter comprises a tapered waveguide, a widened waveguide, a zigzag antenna, and an aperture coupling slot. The zigzag antenna effectively captures the electromagnetic (EM) energy from the RWG, which is then coupled to the aperture slot. This coupling generates a quasi-slotline mode for the electric field (E-field) along the longitudinal side of the aperture, exhibiting odd symmetry akin to the SIW’s TE20 mode. Consequently, the TE20 mode is excited in the symmetrical plane of the SIW and propagates transversely. Our work details the mode transition principle through simulations of the EM field distribution and model optimization. A back-to-back RWG TE10-to-TE10 mode converter is designed, demonstrating an insertion loss of approximately 5 dB over the wide frequency range band of 2.15–2.36 THz, showing a return loss of 10 dB. An on-chip antenna is proposed which is fed by a single higher-order mode of the SIW, achieving a maximum gain of 4.49 dB. Furthermore, a balun based on the proposed converter is designed, confirming the presence of the TE20 mode in the SIW. The proposed mode converter demonstrates its feasibility for integration into a THz-band high-speed circuit due to its efficient mode conversion and compact planar design.

Improvement of least mean square adaptive algorithm for non-monotonic systems

This paper introduces the zlLMS algorithm, an improvement over the traditional least mean square (LMS) algorithm, addressing its limitations in handling nonlinear and non-monotonic transfer functions commonly encountered in engineering systems. The proposed method replaces the LMS algorithm’s error inputs with a function monotonically correlated with the controllable signal. Through mathematical derivations and simulations, the zlLMS algorithm demonstrates superior performance in nonlinear adaptive filtering scenarios, such as the raised-cosine transfer function of Mach-Zehnder modulators and the Lorentz transfer function of diode lasers. Simulation results reveal that the amplitude difference between the ideal and zlLMS-equalized signals is reduced to 1/1000th of the original input signal’s amplitude, underscoring its effectiveness in optics and terahertz technologies. These findings highlight the algorithm’s robustness and potential for broad applications in engineering, especially where nonlinear dynamics are involved.

Temperature-controlled adjustable metamaterial terahertz demultiplexer design

Abstract This paper introduces a novel temperature-controlled terahertz metamaterial demultiplexer that capitalizes on the phase transition characteristics of vanadium dioxide (VO2). The proposed demultiplexer is structured in three distinct layers: the top layer serves as a reflective layer, comprising a double-open resonance ring and a factory rectangular metal sheet; the middle layer is a polyimide dielectric isolation layer; and the bottom layer is a cross-shaped VO2 temperature-controlled layer. This design enables the effective separation of four distinct terahertz frequency bands: 0.72 THz, 1.11 THz, 0.98 THz, and 1.26 THz. Notably, the demultiplexer achieves high isolation levels exceeding −20 dB across all target frequencies, while maintaining an insertion loss of less than 1 dB. The device also exhibits extremely low insertion loss and high isolation properties, which are crucial for terahertz communication applications. Therefore, the presented design holds significant potential for application and development in the realm of terahertz communication technology, offering a viable solution for frequency separation in upcoming high-speed and broadband communication systems.

Quantitative modeling of spintronic terahertz emission due to ultrafast spin transport

In spintronic terahertz (THz) emitters, THz radiation is generated by exciting an ultrafast spin current through femtosecond laser excitation of a ferromagnetic-nonmagnetic metallic heterostructure. Although an extensive phenomenological knowledge has been built up during the last decade, a solid theoretical modeling that connects the generated THz signal to the laser induced-spin current is still incomplete. Here, starting from general solutions to Maxwell’s equations, we model the electric field generated by a superdiffusive spin current in spintronic emitters, taking Co/Pt as a typical example. We explicitly include the detector shape, which is shown to significantly influence the detected THz radiation. Additionally, the electron energy dependence of the spin Hall effect is taken into account, as well as the duration of the exciting laser pulse and the thickness of the detector crystal. Our modeling leads to realistic emission profiles and highlights the role of the detection method for distinguishing key features of the spintronic THz emission. Published by the American Physical Society 2025

Amplitude and Phase Imaging of CW-THz Waves Using a Photomixing System with Coherent Detection

Continuous-Wave Terahertz (CW-THz) phase and amplitude imaging provides valuable insights into the interaction between THz waves and matter, particularly in low-absorption materials. This information is also essential for enhancing CW-THz beam profiling, a critical aspect in the design of free-space THz devices. Hereby, we introduce a single-pixel THz amplitude and phase imaging technique based on frequency scanning and fringe analysis, incorporated into a straightforward experimental setup. We validate the usefulness of the proposed approach by demonstrating two representative case studies. The first is a 3-D measurement of the amplitude and phase profile of a THz wave that is transmitted through a 3-D printed metalens. The second is beam profiling of a THz beam emitted from a photomixer antenna. In both cases, our results are in excellent agreement with previous predictions and validate the usefulness of this imaging technique. As such, we believe that the demonstrated approach will provide an important tool in the quest for establishing THz as an important method for a myriad of applications, including imaging, range finding and metrology.

Detection of melatonin and 5-HTP in dietary supplements based on multiple spectra

IntroductionMelatonin and 5-hydroxytryptophan (5-HTP), known for benefits in regulating sleep and combating depression, respectively, are incorporated into dietary supplements. Rapid and accurate identification of dietary supplement types and their contents remains a significant challenge in ensuring food safety.MethodsIn this study, qualitative and quantitative analysis of melatonin and 5-HTP was performed using Raman spectroscopy, powder X-ray diffraction (PXRD), and terahertz time-domain spectroscopy (THz-TDS). Purity and crystal structures of the samples were investigated using Raman spectroscopy and PXRD, establishing the foundation for terahertz (THz) simulations.Results and discussionThe Raman spectroscopy results demonstrate that the characteristic Raman peaks of melatonin and 5-HTP in the range from 170 cm−1 to 1700 cm−1 were observed at 1356 cm−1 and 1,304 cm−1, respectively. Results of THz revealed that melatonin and 5-HTP each have five THz characteristic peaks, which distinguish these substances. The peak of melatonin at 1.23 THz shows a good linear fit with the mass fraction, while 5-HTP has a similar relationship at 1.14 THz. Then, L-tryptophan, a common contaminant in the production of melatonin and 5-HTP, was successfully identified within the mixture. Finally, it is demonstrated that THz technology can effectively detect melatonin and 5-HTP in commercial dietary supplements. This study establishes a rapid, efficient, and non-destructive approach for the regulation and quantitative analysis of dietary supplements.

Insulator–metal transition in VO2 film on sapphire studied by broadband dielectric spectroscopy

Vanadium dioxide () is a favorable material platform of modern optoelectronics, since it manifests the reversible temperature-induced insulator-metal transition (IMT) with an abrupt and rapid changes in the conductivity and optical properties. It makes possible applications of such a phase-change material in the ultra-fast optoelectronics and terahertz (THz) technology. Despite the considerable interest to this material, data on its broadband electrodynamic response in different states are still missing in the literature. This hampers the design and implementation of the -based devices. In this paper, we combine the Fourier-transform infrared (FTIR) spectroscopy, THz pulsed spectroscopy (TPS), and four-contact probe method to study the films prepared by magnetron sputtering on a c-cut sapphire substrate. Considering different temperatures of a substrate and pressures of atmosphere, we reconstruct complex dielectric permittivity of film in the frequency range of 0.2–150 THz, along with its static conductivity. The dielectric response is modeled using Lorentz and Drude kernels, which make possible splitting contributions from vibrational modes and free charge carriers to the total dynamic conductivity. By studying at different substrate temperatures and atmosphere pressures, we show that IMT appears to be pressure-dependent, which we attribute to the different thermostatic conditions of a sample. Finally, we estimate somewhat optimal thickness and temperature of the film in metallic phase for the THz optoelectronic applications. Our finding should be useful for further developments of the -based devices and technologies.

Optically Pumped Ultrafast and Broadband Terahertz Modulation with Scalable Molecular Beam Epitaxy Grown MoTe2/Si Heterostructures

AbstractDespite advancements in terahertz (THz) modulators, achieving a balance between large modulation depth (MD) and fast modulation speed in scalable devices remains a significant challenge. Optically pumped THz modulators with high MD, broad bandwidth, and fast response times are essential for progress in THz technology. Here, scalable MoTe2/Si van der Waals (vdW) heterostructures are grown via molecular beam epitaxy (MBE) as an optically pumped THz modulators, leveraging its favorable band alignment and seamless integration with silicon complementary metal‐oxide semiconductor (CMOS) technology. High‐quality bilayer, 5‐layer, and 7‐layer MoTe2 films are grown on high‐resistivity silicon, with the bilayer modulator achieving a maximum 74.6% modulation depth under 355 nm illumination of 4.8 W mm−2 at 0.25 THz. The bilayer device exhibited fast rise and fall times of 1.8 and 0.6 µs, respectively, and provided broadband modulation across the 0.1 to 1.0 THz range. Furthermore, this work demonstrates that higher modulation depth significantly enhances THz imaging quality with promising applications in detecting pharmaceutical forgeries. Overall, these ultrafast, broadband THz modulators provide a promising route for advancing next‐generation THz technology, enhancing applications in wireless communication, precision detection, and high‐resolution THz imaging, fostering future innovation.

Slow Interband Recombination Promotes an Anomalous Thermoelectric Response of the p–n Junctions

Thermoelectric effects in p–n junctions are widely used for energy generation with thermal gradients, creation of compact Peltier refrigerators and, most recently, for sensitive detection of infrared and terahertz radiation. It is conventionally assumed that electrons and holes creating thermoelectric current are in equilibrium and share the common quasi-Fermi level. We show that lack of interband equilibrium results in an anomalous sign and magnitude of thermoelectric voltage developed across the p–n junction. The anomalies appear provided the diffusion length of minority carriers exceeds the size of hot spot at the junction. Normal magnitude of thermoelectric voltage is partly restored if interband tunneling at the junction is allowed. The predicted effects can be relevant to the cryogenically cooled photodetectors based on bilayer graphene and mercury cadmium telluride quantum wells.

Optimized PCF architectures for THz detection of aquatic pathogens: Enhancing water quality monitoring.

Waterborne bacteria pose a serious hazard to human health, hence a precise detection method is required to identify them. A photonic crystal fiber sensor that takes into account the dangers of aquatic bacteria has been suggested, and its optical characteristics in the THz range have been quantitatively assessed. The PCF sensor was designed and examined as computed in Comsol Multiphysics, a program in which uses the method of "Finite Element Method" (FEM). At 3.2 THz operating frequency, the proposed sensor performs better than the others in all tested cases, with a high sensitivity of 96.78% for Vibrio cholera, 97.54% for E. coli, and 97.40% for Bacillus anthracis. It also has a very low CL of 2.095 × 10-13 dB/cm for Vibrio cholera, 4.411 × 10-11 dB/cm for E. coli, and 1.355 × 10-11 dB/cm for Bacillus anthracis. The existing architecture has the potential to produce the sensor efficiently and scalable, opening the door for commercial applications. The innovation is in the optimization of structural parameters to increase the fiber's sensitivity to bacterial presence, thereby improving the interaction between terahertz waves and bacterial cells. It targets bacterial macromolecule absorption peaks to increase sensitivity. Localized field augmentation, which concentrates THz vibrations where bacteria interact more, may arise from optimization. By improving scattering, structural alterations can help identify bacteria by their characteristic scattering patterns. These improvements improve the sensor's trace bacteria detection. These factors increase the sensor's aquatic germ detection when combined. In aqueous environments, this results in a more precise and efficient detection, which could facilitate the real-time monitoring of bacterial contamination. Public health and water quality control may be significantly affected by these developments.

Terahertz Multicolor Imaging of Opaque Objects Using Self-Mixing Interferometry with Quantum-Cascade Lasers

Self-mixing interference in a terahertz quantum-cascade laser has been demonstrated to be suitable for the detection of weak signals scattered or reflected by the target. This technology has achieved the high-sensitivity detection of complex refractive indices, surface/interface morphologies and molecular feature spectra. Here, a set of terahertz quantum-cascade lasers with different lasing frequencies is used to inspect a tiny amount of powder concealed inside a polytetrafluoroethylene tablet by using self-mixing interferometry combined with the penetration properties of terahertz waves. Multicolor spectral images were acquired, which were synthesized by absorption contrast images obtained at different lasing frequencies. They enable the detection of the spatial distribution of hidden objects which are totally opaque in visual light and allow for them to be identified with spectral absorption characteristics. Self-mixing interference technology can also obtain phase information when a terahertz wave interacts with a tablet, showing the difference between the hidden object and surroundings from another dimension. Our research may provide a strategy for the development of terahertz multispectral imaging technology for the inspection of hidden trace residues.

A Slotted Elliptical Patch Antenna for Six Generation Communication System in Terahertz Band

A Slotted Elliptical Patch Antenna for Six Generation Communication System in Terahertz Band

Predicting and synthesizing terahertz spoof surface plasmon polariton devices with a convolutional neural network model

With the increasing global attention to deep learning and the advancements made in applying convolutional neural networks in electromagnetics, we have recently witnessed the utilization of deep learning-based networks for predicting the spectrum and electromagnetic properties of structures instead of traditional tools like fully numerical-based methods. In this study, a Convolutional Neural Network (CNN is proposed for predicting spoof surface plasmon polaritons, enabling the examination of the absorption spectrum of metallic multilevel-grating structures (MMGS) and designing various sensor devices and absorbers in the shortest time possible. To expedite the training process of this network, a semi-analytical method of rigorous coupled-wave analysis (RCWA) enhanced with the fast Fourier factorization (FFF) technique has been employed, significantly reducing the data generation time for training. This CNN enables the prediction of existing spoof surface plasmon polaritons (SSPPs) and their intensity within a 110% frequency range. In addition to the speed of dataset generation, the simple framework of this network has also facilitated the training of the network in a short time. By comparing the network in this study with previous works, it is apparent that in addition to structural and geometrical changes in the unit cell, the designer is afforded greater freedom in determining the material of the incident medium and, for the first time, specifying the angle of incidence of the source. Finally, for the validation of the suggested network, the predictive power of the absorption spectrum of various structures is compared with traditional methods. Three examples are provided for inversely designing several sensor devices and absorbers in the terahertz band using the proposed CNN and the genetic optimization algorithm.