Terahertz (THz) near-field imaging is an important technique that surpasses the diffraction limit of optics to achieve super-resolution THz imaging; therefore, it is crucial to investigate ultrafast dynamics processes on material surfaces. Although scanning tunneling microscope (STM) achieves atomic-level resolution, it imposes various challenges related to time scales. Early time-resolved methods derived from the inherent electrostatic approach of STM were limited by the bandwidth of electrical signal transmission. Furthermore, pump-probe methods based on optical signal coupling were restricted by the microstrip line bandwidth and severe thermal effects. Hence, due to having unique low thermal effect, high tunneling efficiency, and high stability, THz-STM has emerged as an imaging solution with both ultra-high temporal and spatial resolution of 100 fs and 0.1 nm, respectively. Besides, this technology has become a research hotspot in the field of terahertz near-field super-resolution imaging. This study discusses the developmental history from time-resolved STM to THz-STM, focusing on the fundamental principles and current status of THz-STM. It aims to offer guidance on the application and development of THz-STM technology in terahertz near-field super-resolution imaging.

Terahertz (THz) in-line digital holography is a promising full-field, lens-free, and quantitative phase-contrast imaging method with an extremely compact and stable optical configuration. Hence, it is suitable for the application of THz waves. However, the inherent twin-image problem can impair the quality of its reconstructions. In this study, a novel learning-based iterative phase retrieval algorithm, termed as physics-enhanced deep neural network (PhysenNet), is introduced. This method combines a physical model with a convolutional neural network to mitigate the twin-image issue in THz waves. Notably, PhysenNet can reconstruct the complex fields of a sample with high fidelity from just a single in-line digital hologram, without the need for constraints or a pre-training labeled dataset. Based on simulations and experimental results, it is evident that PhysenNet surpasses existing phase retrieval algorithms in imaging quality, further enhancing the application range of THz in-line digital holography.

As an important device for the application of terahertz technology, the terahertz modulator has a wide application prospect in the fields of terahertz communication, imaging, and sensing. However, current terahertz modulators exhibit problems, such as low modulation depth, narrow operating bandwidth, and poor stability. This restricts further promotion and development of terahertz technology. In this study, a new optical control GaAs/side-polished terahertz fiber (SPTF) modulator is demonstrated. GaAs film is transferred to the polished region of the terahertz fiber to enhance the interaction with the terahertz evanescent wave. The highest modulation depth of GaAs/ SPTF modulator is 97.4% with an external laser irradiation of 808 nm. The experimental results show that this new fiber modulator exhibits a good optical control modulation effect. Additionally, the compact and highly integrated design of the device suggests its broad application potential.

Terahertz (THz) wave is electromagnetic wave in the frequency range of 0.1?10 THz, which has the characteristics of low energy, broadband, fingerprint spectrum, and sensitivity for water. With the development of THz technology, THz imaging technology has shown many unique advantages in the fields of biomedical diagnosis, nondestructive testing, and security inspection, and has gained more and more widespread attention. This paper mainly overviews the common THz imaging techniques. The imaging principles and research progresses of pulsed THz imaging technology, continuous THz imaging technology, THz near-field imaging technology, and THz real-time imaging technology are introduced in detail, respectively. The typical applications of THz imaging technology in the fields of security inspection, nondestructive testing, and biomedicine are also introduced. Furthermore, the future development of THz imaging technology is prospected.

Terahertz, one of the cutting-edge research hotspots in electromagnetic waves, shows enormous application prospects in communication, radar, and biochemical detection and so on. The emergence and development of artificial electromagnetic materials, especially metasurface, provide a new avenue for efficient wavefront control at terahertz frequency. We elaborate on the relevant works of terahertz metasurface for wavefront control from the perspective of the spatial distribution of the terahertz electromagnetic field. We discuss various design details and application scenarios of terahertz far-field and near-field wavefront control and provide the development prospects of terahertz metasurface wavefront control in the future, which provide new ideas for the study of terahertz wavefront control.

Terahertz imaging technology has shown great application potential in the fields of safety inspection and medical imaging. Compared with traditional mechanical scanning imaging and focal plane array imaging, single-pixel imaging has the advantages of simple system, fast imaging speed, and low cost. By combining spatial light modulation technology, the two-dimensional terahertz spatial information of the target can be reconstructed by using the association algorithm. Starting from the basic concept and development process of terahertz single-pixel imaging, this paper focuses on the principle, materials, development applications, and possible challenges of coding masks in single-pixel imaging.

Terahertz (THz) technology has broad prospect in the fields of next-generation mobile communications, radar imaging, material identification, atmospheric remote sensing, radio astronomy, etc.Among them, regulating devices that can actively manipulate the amplitude, frequency, and phase of THz waves have become one of the core components for the actual applications. Spatial terahertz modulator (STM), as a special representative of spatial wavefront control devices, has shown important applications in beam steering, beam scanning, beam forming, and even phase arrayed technology. This paper summarizes and analyzes the state-of-the-art of electronically controlled STM and optically controlled STM in recent years, with an emphasis on the semiconductor based all-optical STM because of its simple manufacture and low cost. This paper detailedly summarizes the modulation mechanism and simulation model of this kind of all-optical STM, systematically overviews the functional devices based on all-optical STM as well as the corresponding recent advances in THz imaging, discusses the technique limits of current STMs, and then introduces several new device architectures that have been proposed to enhance the modulation efficiency, reduce the device insertion loss, improve the laser utilization, increase the modulation speed, and so on. Finally, the development trend of optically controlled STMs is discussed.

Terahertz (THz) waves have broad application prospects in fields such as communication, imaging, safety monitoring and biomedicine. With the rapid development of THz technology, the demands for its transmission waveguides, especially high-performance THz waveguides with low loss, single mode delivery and flexible transmission are increasing. This will provide important support for improving the flexibility of THz systems and promoting their practical development. We review the design and fabrication process of metallic waveguides and dielectric metallic waveguides, their characteristics of loss and single-polarization single-mode transmission in the frequency bands of 0.1?0.3 THz and 2.0?5.0 THz, as well as their applications in communication and imaging. We mainly focus on the research results of our research group in the design, characteristic simulation, fabrication, evaluation, and related applications of waveguides in recent years. And preliminary exploration is conducted on the difficulties and prospects in the development of THz waveguides.

Functional devices operating in the terahertz band often use high refractive index materials such as silicon and germanium as interfaces, which can cause impedance mismatch between the interface and air, thus leading to unnecessary loss of light energy. Plating an antireflective coating at the interface can effectively reduce reflectivity and increase transmittance, thereby greatly improving the performance of the device. Therefore, the development of antireflective coating is of great significance. Considering the above requirements, the research significance of terahertz antireflective coating is first introduced, and then several common methods for preparing terahertz antireflective coating at home and abroad are presented, and the advantages and disadvantages of each are analyzed. Finally, the research on terahertz antireflective coating is summarized along with future prospects.

Bound states in the continuum (BIC) is a completely bound resonance with its frequency within the continuum spectrum. It has an infinite Q factor,and strong light-matter interaction. The BIC phenomenon is critical for developing functional devices. The introduction of BIC mechanism in terahertz metasurface provides a new way to customize high Q resonance. In this review, the classification, formation mechanism, and main properties of BIC are briefly introduced, and the emerging applications of BIC in terahertz metasurfaces, such as high sensitivity sensing, chiral enhancement, spectral coding, and near-field imaging, are emphasized. In addition, BIC carries topological charges that are defined by the winding number of the polarization vector, such charges can only be created or annihilated by drastically changing the system parameters. The topological properties of BIC also provide new possibilities for the discovery of new phenomena in topological photonics. In the future, BIC can bring more developments in the field of optics and photonics.

Terahertz mixers can mix the original signal with the intrinsic signal and transform the frequency to achieve spectrum detection, super-resolution imaging, and highly sensitive detection of astronomical signals, which have important potential applications in terahertz spectrum identification, terahertz security inspection, and radio astronomical detection. First, this paper introduces the basic principle, classification, and main technical scheme of terahertz mixer. Second, according to the application requirements of ultra sensitivity, high frequency, intermediate frequency broadband, and miniaturization integration of terahertz mixers, the global development trends in the design and manufacturing technology of terahertz mixer are analyzed. Finally, the applicability, technical challenges, and possible solutions of terahertz mixers are summarized to elucidate the technical characteristics of three types of terahertz mixers and identify the technical development trends.

Passive terahertz imaging has been utilized in strategic locations for personnel security screening owing to its safety concealment and clothing penetration. Multipolarization imaging can increase the information dimension and enhance the system's performance. In this study, we comprehensively evaluate multipolarization image quality for passive terahertz imaging detection considering the challenges in selecting the polarization mode and the unclear characteristics of multipolarization images. First, a terahertz radiation brightness temperature model of the human body and hidden objects with arbitrary polarization was examined. Then, four linear polarization images were obtained using the polarization adjustable imaging system, and six polarization parameter images were computed and analyzed. Furthermore, multipolarization images were compared using the traditional global no-reference evaluation indexes. Finally, the receiver operating characteristic (ROC) curve and differential signal-to-noise ratio (DSNR) were employed to quantitatively estimate the local image quality used for detection. Observer subjective evaluation statistical tests were also conducted to confirm the reasonability of some global and local indexes. The results show the importance of reasonable polarization selection and multpolarization fusion in improving imaging detection performance.

Terahertz waves have a wide range of applications in biology and hazardous explosive detection, but they suffer limitations in imaging owing to the sensitivity of the corresponding detectors. Herein, we demonstrate a self-mixing coherent imaging system with a terahertz quantum cascade laser. We achieve imaging with a large depth of field in the terahertz band by combining the holographic imaging technology and self-mixing interferometric measurement. We imaged an area of 2 pair line per millimeter on a resolution board in the off-focus plane in two dimensions using a feature like self-mixing of a transceiver. We obtain the same image contrast as that on the focal plane when the imaging plane is 56 times the wavelength offset.

Terahertz nondestructive testing (THz-NDT) technology is a new noninvasive and noncontact detection technology with strong penetration capability for nonmetallic and nonpolar composite materials, and has a great potential in the field of NDT. In practical detection, for complex terahertz echo signals, such as dispersive echoes and overlapping echoes, it is difficult to meet the requirements of localization accuracy for the time-of-flight only via traditional signal processing methods, such as the direct and deconvolution methods. In this context, the sparse representation method, as a new method for THz signal processing, has good localization accuracy and noise immunity. In this study, we propose a sparse representation method based on the least absolute shrinkage and selection operator (LASSO) to reconstruct an impulse response function from the complex THz echo signal and construct a double-over-complete dictionary to complete the dispersion compensation for the THz reflection signal. An amplitude decay coefficient is proposed to address the problem of inaccurate amplitude of the reconstructed impulse response function. This correction can effectively improve the peak-to-peak imaging quality in the time domain. The effectiveness of the proposed method is verified through numerical calculations and experimental analysis. The proposed method is expected to provide a novel solution for signal processing in THz-NDT.

In order to improve the transient performance and system integration level of terahertz spectral analysis, this study proposes a novel terahertz spectrum-measurement method that combines metasurface spectral encoding and computational reconstruction. High-random spectral encoding of incident terahertz waves is achieved using spectral encoder devices that utilize multiple metasurface structures. Furthermore, a sparse recovery algorithm based on dictionary learning is developed to accurately restore the spectrum. Theoretical calculations and numerical simulations demonstrate that under a 4% noise level, the proposed method achieves a reconstruction error of less than 3% for the lactose transmission spectrum. Thus, the proposed method provides a new pathway for developing on-chip integrated terahertz spectrometers.

Herein, a method for extracting the Jones matrix of a material in the terahertz (THz) wave band is proposed. A THz focal-plane imaging system was utilized to coherently measure the longitudinal component of a converging THz field and to determine the THz polarization. Furthermore, the Jones matrix of the material was extracted by measuring the polarization of a reference THz field and the polarizations of the THz fields transmitted from a sample in its original position and with a 90° rotation. The proposed method was employed to measure the Jones matrices of a THz polarizer and a wave plate with different azimuthal angles, and the experimental results were consistent with theoretical calculations. This demonstrated the feasibility of the proposed method to characterize the anisotropy of materials in the THz frequency range.

Efficient regulation of terahertz (THz) waves is crucial for their utilization in THz optical system, communications, imaging, etc. High-efficiency THz wave reflection regulation can be achieved through interface impedance matching. However, there have been no reports of THz wave regulation achieved via silicon-based interface impedance design. In this study, MXene films were fabricated on high-resistance silicon substrates using the self-assembly method, and their resistance were changed by increasing the thickness of the film. The impedance of the Si/MXene/air interface was continuously adjusted to achieve efficient THz wave attenuation. When approaching the impedance-matched state, the THz reflectivity at the interface is reduced by 83%, while the transmittance decays by approximately 30%. This study also confirmed the variation trend in THz wave reflection intensity resulting from the impedance change of the Si/MXene/air interface, employing THz wave tomography imaging technology. The silicon-based functional interface designed in this work, which offers efficient THz wave reflection regulation, presents a novel approach for achieving THz wave transmission regulation.

The terahertz (THz) field can be confined and manipulated at the sub-wavelength level using spoof surface plasmon polaritons (SSPPs) formed on structured metal/dielectric interfaces. Furthermore, it is feasible to manipulate THz waves on a two-dimensional (2D) scale by utilizing the dispersion properties of the unit structure on the geometrical parameters, which provides a solution for the development of integrated and miniaturized on-chip THz multi-functional devices. This study proposes a 2D gradient refractive index lens based on the dispersion characteristics of the metallic pillar structure. THz SSPP devices were designed according to the proposed design scheme for manipulating the propagation, such as flat telescopes, waveguide couplers, and dual-function lenses, and the working performance of each functional device was analyzed through electromagnetic simulations. This study not only enhances the different THz surface wave control devices but can also help develop THz on-chip systems further based on surface plasmon polariton chains.

Chirality of structure is prevalent in nature, which is usually manifest as the inability to coincide with its mirror structure by translation or rotation. Because spectrum detection techniques can reflect the abundant information of the interaction between light and matter, chiroptical spectroscopy has become a common method to investigate and identify chiral substances. Chiral metasurface can be artificially designed to achieve strong circular dichroism (CD), which is a research hot spot in the fields of matter detection and sensing. We propose a terahertz chiral metasurface that can dynamically control the CD response while achieving high sensing performance. The metasurface is based on a flexible material, and its top and bottom surfaces are J-shaped metal structures with four-fold rotational symmetry. The simulation results show that the chiral metasurface can produce a high CD value up to 0.805 at 0.760 THz. And by equally proportional stretching in two-dimensions, the CD peak redshifts from 0.760 THz to 0.650 THz maintaining a strong CD signal. Meanwhile, its sensing sensitivity can reach 327 GHz/RIU, and its chiral response and sensing performance are well maintained during the stretching process with the relative stretching deformation up to 20%. The designed chiral metasurface has potential applications in the field of dynamic multifunctional devices and wearable sensors.

With the development of terahertz technology, the global research and anticipation regarding 6G communication based on terahertz waves has gained considerable research attention. However, the transmission of terahertz waves is directional, involves end-to-end propagation, and is often blocked by obstacles. Therefore, realizing wide angle and directional transmission of terahertz waves for communication is technically difficult. Herein, a new metasurface was designed and verified to realize the wide angle and directional reflection of terahertz waves. To realize the focusing function of metasurface and control the phase of light field, the transmission phase modulation principle was adopted in designing the unit structure. This unit structure was arranged based on the lens focusing principle and phase compensation principle. Simulations and experiments were conducted at a working frequency of 220 GHz. Our findings verified that the proposed metasurface can reflect the terahertz waves over a wide angle range of 5°?45° and focus it with a focal length of 600 mm along the direction of 13°. The design scheme proposed provides an effective approach to solve the problems associated with 6G communication based on terahertz waves and has a certain application prospect.

Metasurfaces based on bound states in the continuum (BIC) are capable of manipulating light in both temporal and spatial dimensions. In the temporal dimension, BIC possesses an infinite photon lifetime and is widely used in the laser cavity, nonlinear optics, and sensing. In this study, we investigate the properties of band-folded resonances and quasi-BIC based on a hybrid BIC metasurface. The key role of a toroidal dipole in distinguishing the two-type resonances is investigated via multipolar analysis, and numerical simulations, and verified by terahertz experiments. Moreover, hybrid BIC metasurfaces with supercells of different band folding features are compared, further demonstrating the essential role of the toroidal dipole in determining quality factors of these resonances. A deep understanding of the scattering properties of multipolar in resonance plays an important role in obtaining high-quality factor resonant cavities, laying a theoretical foundation for promoting the application of metasurfaces in terahertz sensing, modulators, and nonlinear interactions.

Based on terahertz time domain spectral (THz-TDS) system, we use two mutually perpendicular photoconductive antennas construct a 1×2 GaAs photoconductive terahertz source array. Its radiation terahertz wave polarization direction is studied by adjusting the bias voltage of each array element. The results indicate that, on the basis of the efficient synthesis of photoconductive emission antenna arrays and the detection antenna that can simultaneously detect the amplitude, phase and polarization state of pulsed terahertz waves, the intensity of terahertz waves radiated by two parallel and vertical elements are changed by adjusting the bias voltage of each array element. After 1×2 GaAs photoconductive terahertz source array synchronously synthesized in the far field, pulse terahertz waves with different polarization directions are generated, realizing a photoconductive terahertz radiation source that can generate terahertz waves with arbitrary polarization direction in a fully electrically controlled manner.

Terahertz radiation has broad application prospects in many fields, including high-speed communication, biomedicine, nondestructive testing, space exploration, and security. However, the development of a highly sensitive room-temperature terahertz detector is an urgent issue that must be addressed. The special optoelectronic properties of emerging topological materials open up new paths for terahertz detection. In this study, the band structure and topological surface states of the type II Dirac semimetal NiTe2 were calculated based on first-principles calculations. NiTe2 nanosheets were obtained by mechanical exfoliation, and metal-NiTe2-metal field effect transistors were fabricated using integrated-circuit processing technology. The photoelectric response of the device to terahertz radiation was measured. The results show that the NiTe2-based terahertz detector has a high response rate of 2.44 A/W and a noise equivalent power of approximately 14.96 pW/Hz1/2. Particularly, even at a zero bias voltage, the response rate remains 2.25 A/W, and the noise equivalent power decreases to 9.55 pW/Hz1/2. These characteristics are better than those of similar terahertz detectors, and the device is stable in air and has excellent linearity within a certain range. These results are of great significance for further promoting the practical application and integration of room-temperature terahertz detectors.