Jiang Li

We present a polarization-resolved dual-mode near-field terahertz single-pixel imaging (THz-SPI) scheme that enables polarization-insensitive isotropic sub-wavelength imaging. A [110]-cut ZnTe crystal enables synchronous acquisition of orthogonally polarized THz near-field images. To resolve the longstanding polarization-dependent resolution anisotropy in conventional THz near-field imaging, we develop a fusion model based on signal-to-noise ratio (SNR) and total variation (TV) criteria, yielding an optimized weight coefficient of r = 0.52. The fused image achieves an SNR of 22.97 (∼1.5-fold enhancement over single-polarization images) and eliminates resolution anisotropy, enabling high-fidelity polarization-resolved imaging of microscale structures. This work provides a robust framework for polarization-resolved THz detection and isotropic sub-wavelength imaging, broadening the applicability of near-field THz-SPI for versatile terahertz metrology.

Broadband and low-loss quarter-wave plates (QWPs) operating in the sub-THz band are crucial for polarization control in the field of high-speed wireless communications and terahertz (THz) detection systems. However, their performances are typically constrained by narrow operation bandwidth and high losses. In this paper, we propose a high-performance QWP scheme based on the effective graded refractive index subwavelength grating structure to reduce the insertion loss. This QWP is fabricated from cyclic olefin polymer (COP), incorporating tapered grating structures with graded refractive index at both ends on the basis of rectangular subwavelength gratings to reduce the air/QWP interface as well as the absorption losses. Experimental measurements demonstrate that the designed QWP achieves a transmittance of 98.17% and a polarization conversion ratio (PCR) of 99.995% at the operating frequency of 220 GHz, with a relative bandwidth of 25% (covering 205-260 GHz) and PCR maintaining higher than 98.93% within this bandwidth. Meanwhile, this QWP also exhibits outstanding manufacturing robustness, with excellent performance under a ± 20% dimensional deviation and a surface roughness of about 20 µm. Combining the high transmittance feature, near-unit polarization conversion efficiency, and the broadband operation characteristics, the proposed QWP can effectively meet the high-performance manipulation device requirements of H-band (170-260 GHz) wireless THz communications.

2D Ti ₃ C ₂ T _x MXene film exhibits exceptional electrical and thermal properties, yet the interplay between lattice thermal dissipation and carrier dynamics under photo‐excitation remains unresolved. Using time‐resolved terahertz spectroscopy, we systematically investigated photothermal‐mediated carrier relaxation mechanisms in Ti ₃ C ₂ T _x MXene film by precisely varying layers. The slowest relaxation time exhibits a linear dependence on film thickness, mirroring lattice cooling dynamics governed by thermal boundary conductance (TBC). And by engineering interfacial TBC through substrate selection, effective modulation of transient carrier relaxation dynamics was furthermore demonstrated. These findings confirm the universal photothermal mediated carrier relaxation in Ti ₃ C ₂ T _x MXene film due to strong carrier‐phonon coupling. Our insights establish interface engineering as a viable strategy to control the carrier relaxation process in Ti ₃ C ₂ T _x MXene, advancing their design for optoelectronic and energy conversion applications. This work bridges the critical gap between lattice thermal dissipation and carrier kinetics in MXene, offering new insights into heat‐managed quantum material platforms.

Terahertz (THz) imaging offers unique advantages for nondestructive evaluation, security screening, and biomedical diagnostics due to its spectral fingerprinting of low-energy excitations, high penetration. However, THz focal plane arrays are expensive and lack spectral and volumetric resolution, while scanning-based THz-time-domain spectroscopy (TDS) systems remain bulky and slow. Here, we present a compact, high-speed single-pixel THz 3D imager (THZ-TOFSPI), which integrates a DMD-based optically controlled THz spatial encoder with a reflective fiber-coupled layout. To overcome mechanical delay limitations, we introduce an ultra-stable voice-coil-motor-driven oscillating delay line (VCM-ODL), enabling THz-TDS sampling at up to 120 Hz with excellent amplitude and phase stability (0.39% and 0.0013 ps). Leveraging compressed sensing, the system achieves 3D imaging of a 16 × 16-pixel target in 5.3 s—over 1000× faster than traditional point-scanning THz-TDS. The lightweight and scalable THz-TOFSPI system offers the possibility of large-area, high-resolution 3D imaging in industrial inspection, security, and biomedical applications, with potential pathways toward integration into robotic, handheld, and wearable platforms.

Horn antennas are pivotal couplers for terahertz (THz) waveguide transmission systems, yet conventional designs usually prioritize the far-field performance mainly considering the long-distance wireless communication and radar applications, often overlooking the near-field beam pattern optimization. Here, we present a monolithic polymer-based THz horn-lens system fabricated via liquid-crystal display stereolithography (LCD-SLA) 3D printing. Through the structurally designed stepped gradient and the geometric focusing advantage of the lens, a near-Gaussian emission beam without sidelobes is achieved. We found that by enhancing the dielectric constant (D_k) of photosensitive resin through Al₂O₃ doping, near-field performance can be further improved. At 30 vol% Al₂O₃, D_k increases from 2.68 to 3.68, leading to a 50% stronger near-field electric field and a 35% smaller focal spot (30 mm working distance). Experimental validation using a THz Doppler velocimetry system confirms the integrated lens antenna retains 68.16% relative signal strength at the distance of 60 mm (220 GHz), outperforming its unmodified counterpart (47.35%). The simulation results of the antenna near-field performances agree well with our measurements. This work advances beam-shaping strategies for waveguide-coupled systems and unlocks high-precision applications in velocity and distance sensing.

Aiming to address the challenge of insufficient resolution in detecting thin-layer thickness within thick materials using terahertz time-domain spectroscopy (TDS) technology, we propose two innovative time-domain fitting methods: the experimental reference signal-based method (TDF-ER), which provides a more accurate measurement, and the simulated reference signal-based method (TDF-SR), which is used when it is difficult to obtain an experimental reference signal. Through simulation analysis, it is demonstrated that the TDF-ER method enables precise measurement of surface coatings on metal substrates with a minimum detectable thickness of 4 μm and a thickness measurement deviation below 10%. Compared to the existing thickness identification methods, our method has better resolution ability for thin layer thickness measurements in thick materials. Experimental validation further demonstrates that the TDF-SR method successfully identifies layer thicknesses of 260 μm and 610 μm in 20 and 40 mm-thick polymer bonded explosive (PBX) samples, respectively. Furthermore, the TDF-ER method showcases excellent capability in accurately identifying these layer thicknesses in 50 mm-thick PBX samples. Both simulation and experimental results indicate that the proposed TDF-ER and TDF-SR methods significantly enhance the resolution of thin-layer thickness measurements within thick materials, offering practical solutions for applications involving layered material characterization.

A method of quantitatively measuring fragment velocity based on a terahertz wave Doppler interference velocimetry (TDV) system is proposed in this paper. The theoretical model of fragment group velocity measurement is established, and the quantitative extraction method of fragment group initial velocity, velocity attenuation coefficient, and fragment velocity or distance change with time are given. Using this model, the extracted maximum and minimum values of the fragment group's initial velocity and attenuation coefficient are in good agreement with the preset simulated values. An experimental demonstration test shot of a cylindrical explosive device was carried out by using a TDV system, and the extracted initial velocity, velocity attenuation coefficient, and other important parameters of this explosive device fragment group are given in this paper.

In this work, we propose what we believe to be a novel reordered Hadamard basis method, namely frequency-domain modulus-correlation Hadamard (FMH), which ensures uniform increment of spatial frequencies in both two orthogonal directions. Both simulated and experimental results reveal that the FMH can significantly decrease required sampling ratio for reconstructing high-quality images, compared with the Sylvester Hadamard and the Walsh Hadamard. By integrating a pseudo-background post-processing technique, we achieve a remarkably low sampling ratio of 4.27% for terahertz image reconstruction, with the Pearson correlation coefficient exceeding 0.9. This advancement resulted in nearly a 5-fold increase in the acquisition rate of near-field terahertz single-pixel imaging, compared with the Walsh Hadamard.

A transmissive metasurface lens thinner than a wavelength promises a potential way to replace conventional bulky components for wavefront and polarization control of incident light. Here we propose a novel approach for an ultrathin metasurface lens at the working wavelength of 1550 nm composed of silicon cuboids on the silica substrate. Taking advantage of the ‘slow-light’ effect of the electromagnetically induced transparency (EIT) phenomena, the thickness of the proposed transmissive metalens has been reduced to 130 nm (∼1/12λ₀), and it can focus incident light to a near diffraction-limited spot. The focusing efficiency of the ultrathin metalens is about 2 times larger than its peer without EIT effect. One key advantage of this metalens design is the reduction of the aspect ratio down to about 1, making this approach significantly degrade the difficulty of metalens manufacturing.

Exploring flexible electronics is on the verge of innovative breakthroughs in terahertz (THz) communication technology. Vanadium dioxide (VO₂) with insulator-metal transition (IMT) has excellent application potential in various THz smart devices, but the associated THz modulation properties in the flexible state have rarely been reported. Herein, we deposited an epitaxial VO₂ film on a flexible mica substrate via pulsed-laser deposition and investigated its THz modulation properties under different uniaxial strains across the phase transition. It was observed that the THz modulation depth increases under compressive strain and decreases under tensile strain. Moreover, the phase-transition threshold depends on the uniaxial strain. Particularly, the rate of the phase transition temperature depends on the uniaxial strain and reaches approximately 6 °C/% in the temperature-induced phase transition. The optical trigger threshold in laser-induced phase transition decreased by 38.9% under compressive strain but increased by 36.7% under tensile strain, compared to the initial state without uniaxial strain. These findings demonstrate the uniaxial strain-induced low-power triggered THz modulation and provide new insights for applying phase transition oxide films in THz flexible electronics.