Terahertz Imaging Applications Research Articles

Optimized galvanometric illumination for terahertz full-field imaging and computed tomography

The pursuit of high-resolution, high-fidelity, real-time imaging is receiving significant attention in terahertz community. In this study, a versatile illumination approach based on a double-mirror galvanometer is proposed and optimized for multiple terahertz imaging approaches. We analyzed the mechanism of galvanometric illumination and elucidated two main factors affecting its homogeneity and parallelism properties. In our module, the terahertz beam is deflected rapidly by the galvanometer which is driven by triangular voltage signals, and then focused by a self-designed aspherical f-θ lens to illuminate the object at an equal lateral scanning velocity. The object beam of different transients is periodically superimposed along the Lissajous trajectory, and is recorded by an array microbolometer in a single integration of 3 s. A homogeneous illumination field is realized with a speckle contrast of 0.11, and the resultant image achieves isotropic resolution. By adopting the proposed galvanometric illumination strategy, the average intensity of the illumination field is increased by 135% compared to expanded coherent illumination, and the speckle contrast is reduced by 83.5% compared to other galvanometric illumination. By virtue of leading low speckle noise, illumination homogeneity and parallelism, a compact imaging system is built for terahertz full-field imaging and computed tomography achieving high imaging speed and fidelity. As a successful attestation of terahertz beam steering, this study is very promising to reduce the total cost, increase the performance and expand the application of terahertz imaging.

Multifunctional Reconfigurable Vanadium Dioxide Integrated Metasurface for Reflection, Asymmetric Transmission and Cross‐Polarization Conversion in Terahertz Region

AbstractIntegrating reconfigurable and diverse functionalities into a single metasurface at terahertz (THz) frequencies is an emerging research topic that faces significant challenges. Here, a reconfigurable THz metasurface is proposed, offering diversified functionalities based on the phase transition of vanadium dioxide (). The proposed metasurface can switch from wideband reflection to wideband cross‐polarization conversion (CPC) and asymmetric transmission (AT) for linearly polarized waves. When is in its fully metallic state, the metasurface efficiently reflects normally incident waves ranging from 0.2 to 2.8 THz, with total reflection exceeding 0.8. When is in its insulating state, the metasurface achieves nearly perfect wideband cross‐polarization conversion with CPC efficiency above 0.99 from 0.46 to 2.67 THz, and excellent AT effect with efficiency over 0.9 from 0.65 to 2.29 THz for normal incidence of linearly polarized waves. Moreover, the high efficiency of CPC and AT effects is maintained over a wide range of incident angles. The proposed switchable metasurface with diverse functionalities is expected to enable cutting‐edge research and innovative applications in THz communication, sensing, and imaging.

Theoretical and numerical investigation of an ultra-broadband near-perfect absorption in auxetic metamaterials

In this paper, we present and numerically investigate an auxetic metamaterial absorber based on vanadium dioxide (VO2), achieving more than 90% absorption of the incident terahertz (THz) waves between 4.15 THz to 8.43 THz with an average absorption of 98.4%. To our knowledge, the absorption bandwidth is higher than previously reported VO2-based absorbers. The proposed absorber contains two VO2 based resonator rings placed diagonally at the top of the dielectric substrate in such a way as to create an auxetic shape and provide more design space than the existing absorbers. Considering the vector nature of electromagnetic fields in three-dimensional space, numerical analysis is performed while keeping the phase-changing material VO2 in the metallic state. According to the full wave simulation, it is shown that under normal incidence, the proposed absorber provides almost flat and near-unity absorption, which covers from 4.82 THz to 7.53 THz. We describe the physical mechanism of the absorber through impedance matching theory, and the absorber performance is evaluated by observing the electric field distributions at various frequencies. The proposed structure also exhibits a satisfactory tunable range from 2% to 100%, which may satisfy the requirements of re-configurable metamaterial absorbers. We also show that the absorber provides better wide-angle absorption performance than the existing VO2-based models for both transverse polarization i.e., transverse electric (TE) and transverse magnetic (TM) modes. Owing to the higher absorption bandwidth and better tunable range, the proposed auxetic metamaterial has great potential applications in THz imaging, detectors, and sensing.

Colossal room-temperature non-reciprocal Hall effect.

Non-reciprocal charge transport has gained significant attention due to its potential in exploring quantum symmetry and its promising applications. Traditionally, non-reciprocal transport has been observed in the longitudinal direction, with non-reciprocal resistance being a small fraction of the ohmic resistance. Here we report a transverse non-reciprocal transport phenomenon featuring a quadratic current-voltage characteristic and divergent non-reciprocity, termed the non-reciprocal Hall effect. This effect is observed in microscale Hall devices fabricated from platinum (Pt) deposited by a focused ion beam on silicon substrates. The transverse non-reciprocal Hall effect arises from the geometrically asymmetric scattering of textured Pt nanoparticles within the focused-ion-beam-deposited Pt structures. Notably, the non-reciprocal Hall effect generated in focused-ion-beam-deposited Pt electrodes can propagate to adjacent conductors such as Au and NbP through Hall current injection. Additionally, this pronounced non-reciprocal Hall effect facilitates broadband frequency mixing. These findings not only validate the non-reciprocal Hall effect concept but also open avenues for its application in terahertz communication, imaging and energy harvesting.

Terahertz spatial light modulation by phase change metasurface with pixel-wise switchable control.

Recently, the versatility of metasurfaces for pixel-wise wave regulation prompts emerging metadevices with spatially variant controllability, especially the spatial light modulator (SLM), with reliable prospects for applications in many scenarios. Somehow, the modulation depth and operating bandwidth remain elusive technical challenges, restricting the application scenarios and performance. In this paper, we propose a metasurface-based scheme for broadband terahertz (THz) SLM with large modulation depth as well as spatially varying programmable control. The phase change chalcogenide of Ge2Sb2Te5 (GST) is integrated into meta-annulus units for electrically switchable and independent control of each pixel. For proof-of-concept experiments, a THz SLM composed of an array of 5 × 5 addressable pixels with electrically switchable transmittance is demonstrated. Finally, the feasibility of our scheme is confirmed by simulations and experiments with an average modulation depth (MD) of 87.8% in the range of 0.36 THz to 0.57 THz, potentially providing a remarkable feature for broadband applications in THz imaging, sensing and communication, etc.

Graphene metamaterial (GMM) based dual mode, tunable, and broadband THz absorber with triple circular split ring structure

In this study we investigated a novel approach to designing a graphene metamaterial (GMM) based terahertz (THz) absorber with dual-mode functionality, tunability, and broadband absorption capabilities. The study leverages the unique properties of graphene, a material known for its exceptional electronic and optical characteristics, combined with metamaterials to achieve efficient THz absorption. Here we performed extensive simulation on four different types of configurations and found the optimized structure has the highest bandwidth of 3.8 THz and absorption over 90 %. The absorber is designed to operate in two distinct modes, enhancing its versatility for different applications in the THz spectrum. Moreover, the tunability of the absorber is a significant feature, allowing for dynamic adjustment of the absorption frequency, which is crucial for applications in THz imaging, sensing, and communication systems. The broadband nature of the absorber ensures effective performance over a wide range of frequencies, addressing the need for flexible and high-performance devices in emerging THz technologies. This work represents a significant advancement in the field of THz metamaterials, with potential implications for the development of next-generation THz devices.

High Resolution Terahertz (THz) Imaging

Terahertz (THz) imaging is essential for non-contact and non-destructive testing due to its ability to penetrate numerous materials. Typically, the sample is raster-scanned through the beam waist of a confocal optical setup to generate an image in a single-pixel detection scheme. However, the spatial resolution achieved using such imaging configurations remains no less than millimeters, restricting the application of THz imaging. Here in this work, a simple hollow-core metal waveguide (HCMWG) based terahertz imaging setup has been designed and implemented in transmission configuration to record THz hyperspectral images of a sample. The sample is kept in the near-field range of the HCMWG to exploit the THz electric field confinement of the guided mode toward attaining high-resolution imaging. The THz images are acquired by raster scanning the sample in front of the HCMWG output aperture using a single-pixel detection setup. Additionally, spectroscopic sensing using the same setup has been shown by extracting the absorption spectrum of a chemical compound. Further, a plant leaf is used as a sample to demonstrate the applicability of this technique, where the highly resolved transmitted THz image is acquired using the proposed setup. This image is compared with the image recorded using a conventional confocal lens-based THz optical setup. The results show that the technique can resolve subwavelength features (approx. 0.8λ) of the sample under study while preserving spectroscopic information.

High brightness terahertz quantum cascade laser with near-diffraction-limited Gaussian beam

High-power terahertz (THz) quantum cascade laser, as an emerging THz solid-state radiation source, is attracting attention for numerous applications including medicine, sensing, and communication. However, due to the sub-wavelength confinement of the waveguide structure, direct beam brightness upscaling with device area remains elusive due to several mode competition and external optical lens is normally used to enhance the THz beam brightness. Here, we propose a metallic THz photonic crystal resonator with a phase-engineered design for single mode surface emission over a broad area. The quantum cascade surface-emitting laser is capable of delivering an output peak power over 185 mW with a narrow beam divergence of 4.4° × 4.4° at 3.88 THz. A high beam brightness of 1.6 × 107 W sr−1m−2 with near-diffraction-limited M2 factors of 1.4 in both vertical and lateral directions is achieved from a large device area of 1.6 × 1.6 mm2 without using any optical lenses. The adjustable phase shift between the lattices enables a stable and high-intensity surface emission over a broad device area, which makes it an ideal light extractor for large-scale THz emitters. Our research paves the way to high brightness solid-state THz lasers and facilitates new applications in standoff THz imaging, detection, and diagnosis.

Terahertz super-resolution imaging with large depth of field enabled by the terajet of bilayer dielectric spheres.

Terahertz imaging has found extensive applications in non-destructive testing, security inspection, and other various fields. Intensive research on terahertz imaging systems has been executed to pursue high performance on imaging resolution and depth of field (DOF). However, the terahertz imaging systems with both high imaging resolution and large DOF have rarely been reported. In this paper, a mesoscopic-sized dielectric bilayer sphere-assisted super-resolution imaging method was proposed to simultaneously achieve enormously improved imaging resolution and extended DOF. Simulation analyses revealed that the ultrathin and long terajets were generated by the well-designed bilayer sphere. The THz super-resolution image for the samples can be captured by the point-by-point terajet scanning. The experimental results demonstrated that the best resolution reached up to 0.4λ, the DOF with super-resolution was up to 2λ, and the DOF with sub-wavelength resolution was up to 4λ. This method holds great potential for widespread application in terahertz imaging and detection, especially for curved or complex sample structures.

A high-performance sub-THz planar antenna array for THz sensing and imaging applications

Terahertz (THz) spectral region from 0.1 to 3 THz is envisaged to hold immense potential in the next generation of wireless technologies. Recently, research has focused on this terahertz gap, because of its unprecedented channel capacities. At the physical layer, the design complexities and fabrication of THz devices, especially antennas are the prime bottlenecks to realize its full potential. This article introduces a cost-effective, easy-to-fabricate, and reproducible sub-THz antenna design based on a single-layer planar printed circuit board technology. The antenna incorporates carefully designed quasi-cross slots and applied machine learning-assisted global optimization techniques to achieve the desired performance metrics. The antenna performance is elucidated through numerical simulations and verified through a rigorous in-house THz experimental framework around 100–110 GHz. The proposed antenna offers a peak gain of 13.90 dBi with less than 1 dB variation within the entire band of 100–110 GHz. The antenna holds the potential to achieve terabits per second data rates and futuristic high-resolution short-range THz imaging applications.

Tunable wideband terahertz absorber based on single-layer patterned graphene

In this study, we propose and validate a broadband terahertz perfect absorber. The absorber has a simple structure, which consists of a gold (Au) substrate, a TOPAS dielectric layer and a patterned graphene layer. By the simulations, a broad absorption band can be obtained with a high absorption above 90% between 3.31 THz and 7.15 THz, and the relative bandwidth reaches 73.4%. Meanwhile, the absorption higher than 99% is between 3.81 THz and 6.6 THz with a relative bandwidth of 53.6%. The mechanism of the surface excited plasma (SPP) and electromagnetic dipole resonance are responsible for its almost perfect broadband absorption. By changing the Fermi energy level of the graphene layer, the flexible broadband absorption spectra can be acquired. By varying the geometrical parameters of the structure, a wider spectral range of absorption wavelengths can be realized. For both TE and TM polarization, the absorption can remain above 90% over a wide range of incidence angles. Due to the independent tunability and high absorption of the proposed device, it has great potential applications in terahertz imaging, smart absorption, detectors and communications.

Graphene-based polarization insensitive structure of ultra-wideband terahertz wave absorber

Graphene, a new smart material, has gained a lot of attention recently because of its tunable band structure and strong light-matter interaction. In comparison with typical absorbers, graphene absorbers have emerged as a highly attractive substitute for various types of applications. To broaden the absorption bandwidth, a two-dimensional graphene-based ultra-wideband and polarization-insensitive novel terahertz (THz) absorber with absorption over 95 % from 3.0 THz to 8.5 THz is proposed, with peak absorption of 99.9 % at 6.6 THz and average absorption of 98.4 % for the 5.5 THz bandwidth. The surface plasmon increased by the gold-graphene structure, the Mei resonance, and the interference cancellation between metasurfaces collectively contribute to the ultra-wideband absorption. To obtain nearly perfect broadband terahertz absorption, a single layer of graphene is used. The finite difference time domain (FDTD) approach is used for absorber analysis. In addition, the absorber is insensitive to the polarization angles and incidence angles of incident electromagnetic waves. The proposed graphene-based absorber dominates previously reported wideband THz absorbers in terms of over 95 % bandwidth, absorption peak value, and overall volume. This research will present a novel idea for the design of an ultra-wideband absorber, which can be considered a suitable candidate for different potential applications in terahertz imaging, sensors, photodetectors, and modulators.

Single-Shot Direct Transmission Terahertz Imaging Based on Intense Broadband Terahertz Radiation.

There are numerous applications of terahertz (THz) imaging in many fields. However, current THz imaging is generally based on scanning technique due to the limited intensity of the THz sources. Thus, it takes a long time to obtain a frame image of the target and cannot meet the requirement of fast THz imaging. Here, we demonstrate a single-shot direct THz imaging strategy based on a broadband intense THz source with a frequency range of 0.1~23 THz and a THz camera with a frequency response range of 1~7 THz. This THz source was generated from the laser-plasma interaction, with its central frequency at ~12 THz. The frame rate of this imaging system was 8.5 frames per second. The imaging resolution reached 146.2 μm. With this imaging system, a single-shot THz image for a target object with a size of more than 7 cm was routinely obtained, showing a potential application for fast THz imaging. Furthermore, we proposed and tested an image enhancement algorithm based on an improved dark channel prior (DCP) theory and multi-scale retinex (MSR) theory to optimize the image brightness, contrast, entropy and peak signal-to-noise ratio (PSNR).

Terahertz angle sensor based on the asymmetry coupling of the square and L-shaped structure

This letter presents a terahertz angle sensor using a rectangular and double L-shaped structure. The structure unit of the sensor consists of a typical metal-medium structure, with the top metal pattern comprising a rectangle and a double L-shaped structure. At the same time, the bottom layer is made of polyimide, a dielectric material. By varying the position and number of L-shaped structures, a terahertz angle sensor based on a spring-shaped structure is created. The terahertz angle sensor achieves a Q value of 120.6 with a sensitivity of 3.45 GHz per degree. The terahertz angle sensor offers high-angle resolution and may find applications in terahertz communication, imaging, sensing, and other related fields.

Tunable broadband absorption and broadband linear polarization converter based on vanadium dioxide

A composite dielectric metamaterial based on vanadium dioxide (VO2) is proposed to achieve flexible switching between two functions, broadband absorption, and polarization conversion, by adjusting the VO2 conductivity. The designed metamaterial functions as a broadband absorber when VO2 is in the metal phase. The absorber consists of a VO2 top structure, a silicon dioxide (SiO2) dielectric layer, and a VO2 thin film. Numerical simulation ns show that the absorber can absorb up to more than 90% in the frequency range of 3.22 ∼ 8.51 THz, and due to the symmetry of the structure, the absorber is characterized by polarization-insensitive properties and good absorption over a wider incidence angle. When VO2 is in the insulator phase, the designed metamaterial has a cross-polarization conversion function. The linear polarization converter primarily comprises an I-beam metal, a SiO2 dielectric layer, and a gold substrate layer. Numerical simulations demonstrate that the linear polarization converter accomplishes a line polarization conversion rate (PCR) greater than 90% within the 1.40 ∼ 4.11 THz frequency range, attains a close to 100% cross-polarization conversion rate (PCR) at 1.46, 1.95, 3.0, and 3.97 THz. To confirm the wave absorption mechanism of the absorber, we utilize the impedance matching theory to analyze it. The proposed switchable bifunctional metamaterials present significant potential for broader applications in future terahertz communication, imaging, stealth technology, and related fields.

All-Silicon Polarization-Insensitive Metamaterial Absorber in the Terahertz Range.

All-silicon terahertz absorbers have attracted considerable interest. We present a design and numerical study of an all-silicon polarization-insensitive terahertz metamaterial absorber. The meta-atoms of the metamaterial absorber are square silicon rings which can be viewed as gratings. By properly optimizing the structure of the meta-atom, we achieve a broadband absorptivity that is above 90% ranging from 0.77 THz to 2.53 THz, with a relative bandwidth of 106.7%. Impedance matching reduces the reflection of the terahertz waves and the (0, ±1)-order diffraction induce the strong absorption. The absorption of this absorber is insensitive to the polarization of the terahertz wave and has a large incident angle tolerance of up to 60 degrees. The all-silicon metamaterial absorber proposed here provides an effective way to obtain broadband absorption in the terahertz regime. Metamaterial absorbers have outstanding applications in terahertz communication and imaging.

Helicity-Sensitive Terahertz Detection in Monolayer 1T'-WTe2.

Valleytronics, identified as electronic properties of the energy band extrema in momentum space, has been intensively revived following the emergence of two-dimensional transition metal dichalcogenides (TMDCs) as their valley information can be controlled and probed through the spin angular momentum of light. Previous optical investigations of valleytronics have been limited to the visible/near-infrared spectral regime through which the carriers of most TMDCs can be excited. Monolayer 1T'-WTe2 with broken time-reversal symmetry provides a fertile platform to study the long-wavelength photonic properties in different valleys. Here, we employed a circularly polarized terahertz (THz) laser to selectively excite the valley of monolayer 1T'-WTe2 and demonstrate that the helicity-dependent photoresponse is generated via the photogalvanic effect (PGE). We also observed that the photocurrent is controlled by circular polarization and the external electric field. Because of the tunable Berry curvature dipole derived from the nontrivial wave functions near the inverted gap edge in monolayer WTe2, the bandgap can be tuned efficiently. Our results provide a versatile venue for controlling, detecting, and processing valleytronics and applications in on-chip THz imaging and quantum information processing.

Liquid Crystal Metasurface for On‐Demand Terahertz Beam Forming Over 110° Field‐Of‐View

AbstractThe terahertz spectral region, which bridges between electronics and optics, is poised to play an important role in the development of transformative wireless communication and imaging systems with unprecedented functionality. Currently, a major challenge in terahertz technology is to develop high‐performance terahertz beam‐forming devices that can dynamically shape the terahertz radiation in a flexible manner. Existing terahertz beam‐forming devices have limited coding bits, field‐of‐view, and beam gain. Here, a reconfigurable liquid crystal‐integrated terahertz metasurface is experimentally demonstrated, with each metasurface unit cell being independently addressable. The metasurface has a 260° continuous phase tuning range with a liquid crystal layer thickness of only 1% of the free‐space wavelength. The terahertz wave diffracted from the metasurface can be steered toward a wide range of directions is shown, covering a record‐large 110° field‐of‐view with a peak gain of 25 dBi. The metasurface also features a low power consumption and sub‐second switching time. Furthermore, the formation of multiple terahertz beams is demonstrated, with the direction of each beam and the power ratio between beams adjustable on demand. The proposed liquid crystal metasurface possesses compelling prospects for future terahertz communication and imaging applications.

THz radiation distribution for the identification of infiltrating ductal carcinoma in human breast model: a computational study

With numerous biomedical applications of terahertz (THz) imaging, spectroscopy, and sensing, the THz regime of the spectrum is quickly emerging as an important area of research with the potential to usher in a new era in the healthcare industry. In the medical field, THz radiation has been explored to diagnose and monitor several ailments, including foot diabetes, skin dryness, wounds, and burns. Compared to X-rays, THz waves' phase sensitivity to bodily fluid levels and water results in superior contrast and stronger absorption. This feature makes THz potentially useful for the diagnosis of various malignant tumor forms because cancerous tumors have higher water molecule contents than healthy tissues. The current study presents a computational analysis of a CW-THz point-by-point scanning technique for breast cancer detection. COMSOL multiphysics platform was utilized to create a multilayer three-dimensional breast model containing an irregular-shaped infiltrating ductal carcinoma in the glandular layer. The propagation of the THz radiation in the constructed breast model was simulated based on the radio frequency module and electromagnetic waves in the frequency domain interface. Furthermore, different volumes and locations of the induced tumor were examined. The results demonstrated significant disparities in the acquired electric distribution at different tumor sizes and sites between normal and diseased breasts. The resultant electric field for healthy breast ranged from 1.1 × 10–5 to 0.61 V/m, however, the electric field ranges following tumor addition fluctuate based on its volume and location. Additionally, the position of the THz source and detector array influences the reflected THz signal.

Asymmetric metamaterial sandwich structure with NIM characteristics for THz imaging application

This study presented a unique, miniaturised asymmetric interconnected vertical stripe (IVS) design for terahertz (THz) frequency applications. Therefore, this research aimed to achieve a frequency response of 0 to 10 THz using a 5 × 5 µm2 Silicon (Si) substrate material. Meanwhile, various parametric examinations were conducted to investigate variations in the performance. For example, the unit cell selection process was carefully examined by using various design structures and modifying the structure by adding split gaps and connecting bars between vertical stripes. Furthermore, the proposed sandwich structure design was used to compute the absorbance and reflectance properties. All the analytical examinations were executed utilising the Computer Simulation Technology (CST) 2019 software. The introduced IVS metamaterial exhibits negative index behaviour and has a single resonance frequency of 5.23 THz with an acceptable magnitude of − 24.38 dB. Additionally, the quadruple-layer IVS structure exhibits optimised transmission coefficient behaviour between 3 and 6 THz and 7 to 9 THz, respectively. However, the magnitude of the transmission coefficient increased with the number of material layers. Besides that, the absorbance study shows that using a quadruple-layer structure obtains unique and promising results. Overall, the proposed asymmetric IVS metamaterial design achieves the required performance by using a compact structure rather than extending the dimensions of the design.