The technology of terahertz wave-driven dynamic nuclear-polarization-enhanced nuclear magnetic resonance (DNP-NMR) spectroscopy can improve signal sensitivity by several orders of magnitude. The terahertz gyrotron can realize a high power output and has a certain frequency-tunable range, which meets the terahertz radiation source requirements of a NMR spectroscopic system. Herein, the development of a frequency-tunable terahertz gyrotron used for the DNP-NMR spectroscopic system was introduced. The structure of a multi-section gyrotron cavity and the variation of the electron beam quality in the frequency-tunable terahertz gyrotron with the operating voltage and the operating magnetic field were also investigated. The results show that the multi-section cavity is better than the conventional three-section cavity when the terahertz frequency-tunable gyrotron used for the DNP-NMR is operating. In the design of a frequency-tunable terahertz gyrotron, not only the variation of the velocity pitch factor but also the variation of the velocity spread and the guiding center radius spread of the electron beam related to the operating voltage or the operating magnetic field should be taken into account.

This study proposes two effective algorithms for the qualitative and quantitative analysis of components in a mixture. For cases wherein the spectrum of each component in the mixture can be obtained in advance, we develop a simple process, namely “known component analysis (KCA),” which requires a small number of samples. In contrast, for cases wherein a mixture contains many components or the spectra of some components cannot be obtained, we develop “unknown components analysis (UCA),” whose calculation time is considerably less than those of the traditional algorithms. Compared to KCA, UCA requires more samples to be analyzed to ensure the high accuracy rate. These proposed algorithms provide new ways to detect and measure key biomedical substances.

The development of large-scale terahertz (THz) array detection chips is of great significance to the promotion of related imaging and security applications. Because of the compatibility of THz microbolometer array technology with readout circuits used for infrared camera, it is the first chip to realize terahertz camera and is becoming the mainstream of THz camera. We have been devoted to the research of microbolometer array chips at room temperature. The Nb5N6 microbolometer array device has attracted the attention of international academia and industry due to its simple operation, high sensitivity, fast response and easy integration of ultra-large arrays. This paper reviews and summarizes some key technical problems encountered in the design and fabrication of THz array detection chips with Nb5N6 microbolometer, such as substrate interference effect, design of efficient coupling structure, design of low-noise readout circuits, and packaging and integration of readout circuits. It provides a reference for the design and fabrication of large-scale THz array detection chips.

Photoelectric characterization technique is an important foundation of the terahertz technology. It covers photoelectric device characterization, spectral measurement, beam improvement, and communication and imaging applications in terahertz region, and plays an important role in terahertz application field. Firstly, the working principle and the latest progress of two kinds of terahertz semiconductor quantum devices are presented. Then, their applications in terahertz photoelectric characterization such as pulse light power measurement and detector responsivity calibration, and their applications in terahertz fast modulation and detection as well as terahertz scanning imaging systems are summarized. Finally, the improvements of the above characterization techniques are also introduced and discussed, including the methods to improve the terahertz light beam quality and the effective detection area of detectors. The potential applications of devices and characterization techniques in the future are also presented.

The study of terahertz artificial electromagnetic metamaterials is constantly challenged by the problems of large loss, poor performance, and inflexible control. Superconducting materials exhibiting extremely low losses are among the best materials for high-performance functional devices in terahertz band. As such, this paper introduces terahertz superconducting artificial electromagnetic metamaterials, along with a detailed summary of their functionalities of low loss and flexible control. With respect to the application requirement of high performance, an analysis of the development trend, existing problems, and key scientific issues on terahertz functional devices is provided herein.

This study aims to introduce the optical anisotropy and modulation characteristics of different liquid crystal materials in the terahertz (THz) regime. Herein, several THz functional devices were reviewed based on the combination of liquid crystals and artificial electromagnetic microstructures, realizing the functions, such as tunable filtering, electromagnetic induced transparency, phase modulation, and polarization control of THz waves. Moreover, the interaction mechanism between liquid crystals and artificial electromagnetic microstructures was analyzed and discussed, along with the regulation law of external field and surface interaction at the THz wavelength scale. In addition, the development trend in the applications of THz liquid crystal photonic devices was prospected.

Terahertz (THz) special beams have unique field distributions and diffraction characteristics, showing significant advantages in the applications of particle manipulation, optical imaging, and optical communications. Some recent researches of several groups, including ours, on THz special beams are reviewed herein, such as THz Bessel beam, vortex beam, Airy beam, bottle beam, and radially polarized beam. The field distributions, diffraction characteristics, and application prospects of these beams are systematically described. The aim herein is to discuss the effect of THz special beams to the advancement of THz technology.

We review the research background and development history of the generation of high-energy strong-field terahertz radiation through the tilted-pulse-front technique in lithium niobite crystals driven by femtosecond laser pulses. We systematically analyze the influence of each component in the tilted-pulse-front setup on the energy conversion efficiency and emission properties of terahertz radiation and discuss the feasibility of the generation of mJ terahertz pulses through the proposed method. Further, the numerous promising applications of strong-field terahertz sources are demonstrated.

With the development of high-power lasers, terahertz (THz) radiation driven by high-power lasers has received extensive attention. However, it is difficult to measure the terahertz radiation produced by high-power laser devices because of their low repetition rate. This can be solved by using a single-shot time-domain spectrum detection technique. Using this technique, the electric field waveform information of terahertz radiation can be obtained in a single measurement. In this study, we review the research progress with respect to the single-shot time-domain spectrum detection for THz radiation. Based on frequency-time encoding and space-time encoding, we introduce spectral encoding detection using a chirped pulse, space-time encoding detection, and other techniques. In this review, the characteristics and parameters of each measurement technique are compared. Finally, we summarize and discuss the future prospects of single-shot detection for THz radiation.

Terahertz time-domain spectroscopy (THz-TDS) uses coherent detection method for spectrum analysis. The THz-TDS system integrates terahertz emitter and detector, which can simultaneously acquire the field intensity and phase information of terahertz pulses and be widely used in biology, materials, security, and other fields. The THz pulse induced by the ultrafast laser filament is an important way to generate wide-spectrum and high-intensity THz radiation. In this paper, the physical mechanisms of ultrafast laser filament radiated THz waves and the methods of enhancing and regulating THz waves are introduced in detail. The detection principle and methods of the THz-TDS system based on laser filamentation are described.

Owing to its unique terahertz properties, graphene has potential applications in terahertz sources, terahertz detection, and terahertz control. In this paper, the terahertz properties of graphene and graphene-based terahertz devices are reviewed; further, the potential applications of graphene in the aforementioned fields are prospected. As for the terahertz properties of graphene, this paper mainly introduces the conductivity model, the characteristics of equilibrium and ultrafast spectral response, and the properties of surface terahertz wave emission. As for the graphene-based terahertz devices, this paper mainly reviews the recent developments in the research on the active terahertz devices based on optical, electrical, and magnetic control, the terahertz modulators with graphene-based metamaterial, the antireflection devices based on impedance matching, and the tunable terahertz sources.

Organic optoelectronic materials exhibit various advantages, including low cost, low processing complexity, and relatively easy industrial production. Because of their excellent optoelectronic properties, such materials have been extensively used in flat panel displays, lighting, photovoltaic cells, and other industrial fields. Recently, organic optoelectronic materials have been extensively employed in terahertz wave dynamic modulation research. In this study, we introduce a series of research achievements with respect to the terahertz wave modulators of organic optoelectronic materials. Further, the principles, advantages, and disadvantages of optical modulation and electrical modulation are analyzed. Additionally, the recent scientific research achievements associated with the application of organic optoelectronic materials to terahertz wave modulation are discussed. This study indicates that organic optoelectronic materials can provide new research ideas to realize efficient terahertz wave modulators.

Starting with a few common cell lines including fibroblasts, epithelial cells, nerve cells, stem cells, lymphocytes, and germ cells, we describe the effects of terahertz radiation on the function, protein expression, and genotoxicity of different cells in detail. Around irradiation conditions and response mechanisms, some suggestions on the terahertz biology research and specific applications are put forward based on existing cytology research results. With the development and maturity of the terahertz theory in the field of biomedicine, the terahertz technology will certainly be of significance to pioneer new diagnosis technologies.

In this study, we propose a design method to develop an arc multiple-input multiple-output (MIMO) antenna array based on the equivalent array concept. Further, an arc MIMO antenna array containing 50 transmitters and 60 receivers is designed. The simulation experiment and analysis conducted in the terahertz band denote that the imaging resolution of the arc MIMO array is almost equal to the theoretical limit. When compared with an equivalent array containing 3000 transmitters, the main-lobe widths of both the point spread functions are observed to be almost identical, and the positions of the side lobe peaks and valleys correspond accordingly. Subsequently, the equivalence between the arc MIMO array and the equivalent array can be verified based on the simulation results. When compared with the traditional linear MIMO array, the arc MIMO array exhibits a larger imaging range, and its side gating artifacts are almost completely reduced. These factors are important to obtain better imaging results with respect to the side view of the curved surface imaging targets.

A flexible metamaterial-based resonator was used to monitor the alcohol-water mixtures with different volume fractions using the terahertz time-domain spectroscopy (THz-TDS) in a transmission geometry. As the volume fraction of alcohol increases, the resonant frequencies of the biosensor blue-shift. The frequency shift of the second peak is higher than that of the first peak, the maximum shift value is 52 GHz, and the sensitivity is around 100 GHz/RIU. The developed resonator is flexible and has extremely low solution consumption (≈20 μL), indicating the proposed method shows possibility for the applicability of THz-TDS and the industrial application of THz resonators in the field of biology.

In this study, a terahertz amplitude modulator based on metasurface/ion-gel/graphene hybrid structure was designed and fabricated. The modulation performance of the device was simulated and experimentally demonstrated. This device uses the ion-gel medium embedded between graphene and metasurface as the electrolyte, and the graphene as the active material. The enhancement of the interaction between terahertz wave and graphene is realized on the metasurface. Further, an external bias voltage was used to tune the electrical conductivity of graphene for actively controlling the terahertz waves. The results indicate that the device can achieve a modulation depth of up to 73% at the resonant frequency with a relatively small bias voltage. Moreover, the resonant frequency remains almost constant in the modulation process. Thus, the proposed device provides one novel tool in the large terahertz amplitude modulation under low voltages.

The characteristic fingerprint spectrum of levodopa (L-DOPA) in the frequency range of 0.5-14.5 THz was obtained by using an air plasma terahertz time-domain spectroscopy system, and the variation in absorption spectrum with temperature was investigated. Density functional theory was used to calculate the L-DOPA crystal cell structure and for the spectral analysis of terahertz vibration. The results show that L-DOPA terahertz absorption peaks correspond to different collective vibrations and molecular local vibrations. The collective vibrations of L-DOPA have a wide distribution in the terahertz range, and the benzene ring and molecular side chain exhibit distinct vibrational modes. The vibration specificity is closely related to the molecular conformation and hydrogen-bonding interactions.

In this study, the undamaged and damaged areas in fused silica induced by laser with different energies were tested with the transmission terahertz time-domain spectral system. The time domain signal and frequency spectra of 0.8-1.25 THz are obtained. The peak-to-peak value, amplitude, transmission spectrum, refractive index, and absorbance are analyzed. The results show that the peak-to-peak value, amplitude, transmission spectrum, and refractive index of the damaged area are obviously reduced compared with those of the undamaged area, and with the increase of laser energy and the enlargement of damaged area, these parameters gradually decrease. In contrast, the absorbance exhibits an opposite change. Therefore, the variation in the optical properties of fused silica in the terahertz wave band could be applied to distinguish the damages and analyze the degree of damage, which provides a good basis for applying the terahertz time domain spectral technique to the identification and analysis of laser-induced damage.

A fiber-type terahertz (THz) time-domain spectrometer is designed by combining a fiber femtosecond laser with a fiber-coupled THz photoconductive antenna. The effect of laser polarization on the THz time-domain waveform, intensity, and spectral characteristics are studied experimentally. Via the precise control and the optimization of the polarization state of the femtosecond laser, the time-domain pulse splitting is eliminated, and a single-peak THz time-domain pulse with a high signal-to-noise ratio is obtained.

To characterize a wide-band and high-absorption terahertz radiometer, it is necessary to study the properties of the absorbing coating material. First, we simulated the absorption rates of common absorbing materials to find materials that have high absorption in terahertz range. Further, we mixed silicon carbide and 3M black lacquer to increase the absorption rate of the coating. In addition, we simulated the variation in the internal particle size of the coating. Finally, the mixed coating was prepared based on the simulation results, and the samples were measured using the terahertz time-domain spectrometer. The measurement results show that the spectral absorbance of the mixed coating is greater than 0.99, which is basically consistent with the simulation results.

In this study, we experimentally demonstrated the enhancement of a terahertz (THz) signal generated by a femtosecond laser-filament array in air using a simple experimental setup in which a step phase plate is used to generate multiple parallel filaments. The results show that at a given incident-femtosecond-pulse energy, compared with the case of single filament, the proposed method enhances the THz radiation intensity, and the THz radiation intensity is proportional to the number of filaments. Further, at an incident energy of 5.8 mJ, the THz energy generated by eight filaments is 31.73 nJ and the radiation efficiency is 5.47×10-6. This simple method can be used to solve the energy saturation problem in the generation of THz radiation through femtosecond laser filamentation in air.

Herein, a 190-GHz frequency doubler with a high output power is designed and realized using a balanced double-frequency construction based on the anti-series gallium arsenide planar Schottky varactors. A method that combines a three-dimensional electromagnetic field and nonlinear harmonic balance is employed to conduct a simulation. The simulation results demonstrate that the proposed frequency doubler has been effectively fabricated, assembled, and tested. The frequency-doubling efficiency of the proposed doubler can reach more than 8% in the output frequency range of 182-196 GHz. Moreover, at an output frequency of 187 GHz, the frequency-doubling efficiency and the output power can reach 15.4% and 85 mW, respectively.

This study presents the design of a triple-band metamaterial absorber with a composite structure that can fully absorb the incident terahertz radiation at a certain frequency. The absorption remains high at incident angles up to 50°. The conditions for perfect absorption and the frequency shift caused by the permittivity of the dielectric spacer were analyzed via the interference theory. Further, the effect of the length of the Jerusalem cross on absorption was analyzed by applying the transmission line theory combined with the interference theory. With the increase of the short edge length, the absorption peak redshifts. The experimental results are consistent with those from the simulation, the interference theory, and the transmission line theory, thus the results here provide guidance for the design of a metamaterial absorber.

This study aims to propose a structure of axicon-lens-axicon phase lens group. The structure is simulated and analyzed theoretically. The experimental verification is conducted using the rapid 3D printing technology to prepare an axonal pyramid, and a terahertz Bessel beam with 1000 mm diffraction-free length is generated. The results indicate that the diffraction-free region of such a beam is no longer proximal to the axial pyramid, but has a 100 mm distance from the final axicon. The findings in this study are helpful for the application of a terahertz Bessel beam in the field of large depth of field imaging.

Herein, terahertz spectra of four kinds of commonly used three-dimensional (3D) printing materials are detected and analyzed. Experiments show that these printing materials have obvious differences in the range of 0.20-2.60 THz. The transmittance of 3D printing polycarbonate measured in terahertz is highest. The refractive index of the four materials ranges from 1.45 to 1.65, and the refractive index of the photosensitive resin is the highest, which is approximately 1.62. The real and imaginary parts of the dielectric constant of these materials range from 2.20 to 2.75 and from 0.05 to 0.35, respectively. From the viewpoint of the absorption spectra, there are no absorption peaks among the four materials. This study lays the foundation for the selection of materials for 3D printed terahertz devices.

This study aims to investigate the process of terahertz wave generation using a vortex beam generated by spatial light modulator rather than a Gaussian beam as the light source. Herein, we explore the differences among vortex beams with different topological charge numbers when generating terahertz waves. We find that the generation of terahertz waves is affected by the phase singularity position. In addition, we investigate the changes of terahertz wave energy, spectrum, and polarization produced by vortex beams with different pulse intensities and laser wavelengths. The results demonstrate that the intensity of the terahertz wave changes with the topological charge number, which is also closely related to the position of the vortex center. Under the same vortex beam, the variation of the generated terahertz wave with pulse intensity and laser wavelength is consistent with that for a Gaussian beam. Specifically, the terahertz waves generated by a vortex beam and a Gaussian beam are consistent in terms of spectrum and polarization.

In this study, the high-energy terahertz-wave pulses were generated using a potassium titanyl phosphate (KTiOPO4, KTP) crystal and an experimental scheme that combines a Stokes parametric oscillator and a Stokes parametric amplifier involving the vertical surface emission of terahertz waves. The pump source was a Q-switched laser with an output wavelength of 1064.2 nm, a pulse width of 7.5 ns, and a pulse repetition rate of 1 Hz. The Stokes light wavelength is 1086.2 nm, the angle between the pump and Stokes beams is 4.4°, and the terahertz wave frequency is 5.7 THz. The time-delay device on the pump-light path can ensure that the pump light pulse has a good time coincidence with the Stokes light pulse to be amplified. When the pump light pulse energy is 770 mJ and the Stokes light pulse energy to be amplified is 16.8 mJ, the amplified Stokes light pulse energy is 185.4 mJ and the maximum terahertz wave pulse energy is 6.4 μJ.

Single-shot measurement of terahertz pulses is mainly used to rapidly measure the terahertz spectra during ultrafast irreversible processes. It has broad prospects for applications in protein denaturation, nuclear-explosion-simulation analysis, and other fields. The optical pulse from a femtosecond laser amplifier is divided into two beams and passes through two gratings to generate a tilted wavefront. One beam is used as the pump light to excite a lithium niobate crystal and generate a high-power terahertz wave based on the optical-rectification effect. The other beam is used as the probe light. We use the crossed and balanced detection method. After the terahertz pulses are generated, we implement a single-shot measurement of terahertz pulses using the proposed light paths. The time window of the system ranges up to 16.8 ps, and the spectral range is 0.1-1.6 THz.

A THz linear array scanning imaging system was built using a 0.3 THz radiation source and a linear array detector. The imaging system only needs 1 min to achieve an imaging measurement over a 100 mm×100 mm region at an imaging resolution of 1.5 mm. Using this imaging system, the defects embedded in polyethylene samples and water pipes as well as the scissors and knives concealed in envelopes are quickly and accurately identified. The research results demonstrate that the proposed imaging system can rapidly inspect various objects and is suitable for non-destructive testing and security inspection.

This paper presents the design of a dual-band terahertz metamaterial antireflection coating with a multilayered structure (two layers of metal-polyimide). The formation of the two low-reflectivity bands is analyzed by numerically calculating the electric field distributions on metal surfaces of the two layer of metal-polyimide structure. After adjusting the thickness of the polyimide layer and the size of the metal unit structure, ultra-low reflectivity is achieved with the minimum percentages of 0.0028 and 0.0025 at 0.471 and 1.560 THz, respectively (with <10% reflectivity for the bandwidths of 0.26 and 0.21 THz). The results provide a reference for the applications of multi-band terahertz metamaterial antireflection coatings.

In this study, fully-polarimetric information is introduced to a standoff holographic imaging system. A 0.14-GHz fully-polarimetric wideband radar is designed for detecting concealed targets in a standoff fully-polarimetric holographic-radar-imaging experiment. The low-entropy polarization scattering of concealed targets is analyzed through the target-polarization decomposition method, and the average polarization scattering mechanism is investigated at different irradiation angles. The experimental results show that in the 0.14 THz band, the covering by the typical clothing materials is found to alter the speckle pattern of the background clutter, but has a negligible impact on the polarization scattering of a strongly scattered echo target. The polarization scattering entropy of the artificial target is smaller than that of the background clutter. The main scattering mechanism of a model pistol is the low-entropy surface scattering from its smooth surface, in which the inclined stripes on the sleeve contribute to the low-entropy multiple scattering, which is the main source of the cross-polarization echo of the target.

In this study, we propose a broadband polarizer based on the “double-split ring” structure of a Dirac semimetal metamaterial. Further, we investigate the influences of the Dirac semimetal Fermi level and the intermediate dielectric thickness on the polarization conversion performance. The results show that the polarization conversion efficiency is 100% at two resonance frequencies of 1.44 THz and 1.95 THz for an intermediate dielectric thickness of 22 μm and a Fermi level of 70 meV. In addition, For a 22 μm thick intermediate dielectric, the two resonant peaks at high and low frequencies show a blue shift as the Dirac semimetal Fermi level increases from 64 meV to 70 meV. Moreover, for a Dirac semimetal Fermi level of 70 meV, as the substrate dielectric thickness increases from 19 μm to 22 μm, the resonant peak at low frequency does not shift, whereas that at high frequency exhibits a red shift.

A broadband terahertz metamaterial absorber is proposed based on the double composite structure layers (CSLs). The composite structure layer I and composite structure layer II comprise four different-sized gold rings and square plates, respectively. Further, the finite difference time domain (FDTD) method is employed to theoretically investigate and discuss the effects of absorption spectrum, magnetic field distribution, surface current distribution, polarization angle, and incident angle on absorption. The results show that the high-order resonance peaks can be effectively suppressed by this absorber. Furthermore, in case of normal incidence, the absorption bandwidth corresponding to absorptivity greater than 90% is 0.722 THz, and its central frequency is approximately 2.041 THz.

A 430 GHz tripler circuit in the terahertz range, which consists of all parts of a tripler, has been designed and fabricated on a 12 μm thick GaAs film substrate. A pair of two matched anti-parallel Schottky diodes without application of direct current bias is used to realize series balanced tripler structure. One distinct advantage of the balanced configuration is that it generates only odd harmonics, which simplifies the tripler analysis and optimization. Harmonic nonlinear simulations and three-dimensional electromagnetic simulations are combined to simulate radio-frequency performance of the entire tripler circuit accurately. The monolithically integrated 430 GHz tripler circuit is mounted in a split waveguide block for test. It has produced a peak output power of 215.7 μW with 4.3% efficiency at 430 GHz.

Herein, a super-resolution terahertz wave spectrometer system is proposed based on optical interference theory. The interference system can be used for the multipoint simultaneous detection of signals based on a self-developed broadband GaN array detector. The proposed spectrometer has high precision and can achieve super-resolution. The beam quality of the source to be tested can be measured based on the two-dimensional intensity distribution obtained by the proposed array detector. Continuous terahertz waves, generated by a multiplier-chain emission source with a frequency range from 470 to 720 GHz, are used as the testing source to verify the proposed spectrometer. At a detection distance of 75 mm, the spectral resolution and the measurement accuracy are determined to be 100 MHz and 0.1%, respectively. These values are 20 times the resolution and accuracy limits under the same detection conditions.

This study proposes a model to measure the real and imaginary parts of the permittivity of a dielectric material using the S -parameters of a coplanar waveguide (CPW), and discusses the detailed derivation and application of the proposed model. Based on this model, the permittivity of the substrate material is calculated at 200 GHz using the measured S-parameters of the CPW. The calculated result agrees well with the theoretical value. The proposed model can be employed to characterize the dielectric properties of many materials within the sub-terahertz frequency regime.

In this study, we designed a metamaterial-based terahertz multi-band sensor integrated with microfluidic channels. We simulated the reflection spectra of the sensor during the detection of ethanol-water mixtures containing different concentrations of ethanol. The simulation results show that an increase in the ethanol concentration correlates with a decrease in the reflectivity at the resonant dips and a blue shift of the resonant frequency. We analyzed the quantitative relationship between resonant frequency or reflectivity of the sensor with ethanol concentration, which is in turn used for the prediction of the ethanol concentration in an ethanol-water mixture. Three resonance dips are used for predicting the ethanol concentration and the predicted errors are smaller than 1%. The above results demonstrate the utility of the terahertz time-domain spectroscopic technique in the rapid, real-time, and infinitesimal material identification and bio-sensing.

This paper proposes a method for identifying organic compounds by applying a differential principal-component-analysis (PCA)-support-vector-machine (SVM) to the terahertz time-domain spectral data. First, the terahertz absorption spectrum is calculated according to the terahertz time-domain signal of the material sample; then, the features of the data in the frequency range of 0.2-2.5 THz are extracted. During the feature extraction, an expansion-of-sample-size method based on differential data is proposed and combined with the PCA method to achieve the feature extraction. Finally, the SVM is used to establish a mathematical model for the corresponding relationship between the extracted features and the material category, and the unknown samples are identified according to this model. The terahertz-spectral data of 15 organic compounds are identified using the proposed method, and the correct recognition rate is 93.33%. The experimental results show that the correct recognition rate of organic compounds by the proposed method is the highest when compared with those by the linear-discriminant analysis method and the absorption peak frequency-amplitude method.