Terahertz (THz) waves have shown a broad prospect in the analysis of some dielectric materials because of their special properties. For the ultrafast irreversible processes, the THz single-shot measurement is a good choice. In this paper, a single-shot system is investigated, where a pump beam is used to generate THz pulses with high electrical field by optical rectification in LiNbO₃, the probe beam with wavefront tilted by a blazed grating is used for single-shot measurement. The time window is up to 90 ps, the signal to noise ratio is 2000:1, the spectrum covers from 0.1 THz to about 2.0 THz, and the spectral resolution is 0.011 THz. The single-shot measurement result agrees well with that of a traditional electrical-optic sampling method.

We report on the investigation of optimal bias region of a wide-band superconducting hot electron bolometer (HEB) mixer in terms of noise temperature performance for multi-pixel heterodyne receiver application in the 5-meter Dome A Terahertz Explorer (DATE5) telescope. By evaluating the double sideband (DSB) receiver noise temperature (T_rec) across a wide frequency range from 0.2 THz to 1.34 THz and with a large number of bias points, a broad optimal bias region has been observed, illustrating a good bias applicability for multipixel application since the performance of the HEB mixer is uniquely determined by each bias point. The noise temperature of the HEB mixer has been analyzed by calibrating the noise contribution of all RF components, whose transmissions have been measured by a time-domain spectroscopy. The corrected noise temperature distribution shows a frequency independence relation. The dependence of the optimal bias region on the bath temperature of the HEB mixer has also been investigated, the bath temperature has limited effect on the lowest receiver noise temperature until 7 K, however the optimal bias region deteriorates obviously with increasing bath temperature.

We present a high-performance terahertz (THz) radiation source based on the photon-activated charge domain (PACD) quenched mode of GaAs photoconductive antennas (GaAs PCA). The THz radiation characteristics of the GaAs PCA under different operating modes are studied. Compared with the linear mode, the intensity of THz wave radiated by the GaAs PCA can be greatly enhanced due to the avalanche multiplication effect of carriers in the PACD quenched mode. The results show that when the carrier multiplication ratio is 16.92, the peak-to-peak value of THz field radiated in the PACD quenched mode increases by as much as about 4.19 times compared to the maximum values in the linear mode.

Prototypical three-dimensional (3D) topological Dirac semimetals (DSMs), such as Cd₃As₂ and Na₃Bi, contain electrons that obey a linear momentum-energy dispersion with different Fermi velocities along the three orthogonal momentum dimensions. Despite being extensively studied in recent years, the inherent Fermi velocity anisotropy has often been neglected in the theoretical and numerical studies of 3D DSMs. Although this omission does not qualitatively alter the physics of light-driven massless quasiparticles in 3D DSMs, it does quantitatively change the optical coefficients which can lead to nontrivial implications in terms of nanophotonics and plasmonics applications. Here we study the linear optical response of 3D DSMs for general Fermi velocity values along each direction. Although the signature conductivity-frequency scaling, σ(ω)∝ω, of 3D Dirac fermion is well-protected from the Fermi velocity anisotropy, the linear optical response exhibits strong linear dichroism as captured by the universal extinction ratio scaling law, Λ_ij = (v_i/v_j)² (where i≠j denotes the three spatial coordinates x,y,z, and v_i is the i-direction Fermi velocity), which is independent of frequency, temperature, doping, and carrier scattering lifetime. For Cd₃As₂ and Na₃Bi₃, an exceptionally strong extinction ratio larger than 15 and covering a broad terahertz window is revealed. Our findings shed new light on the role of Fermi velocity anisotropy in the optical response of Dirac semimetals and open up novel polarization-sensitive functionalities, such as photodetection and light modulation.

We report a broadband terahertz time-domain spectroscopy (THz-TDS) which enables twenty vibrational modes of adenosine nucleoside to be resolved in a wide frequency range of 1-20 THz. The observed spectroscopic features of adenosine are in good agreement with the published spectra obtained using Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. This much extended bandwidth leads to enhanced material characterization capability as it provides spectroscopic information on both intra- and inter-molecular vibrations. In addition, we also report a low-cost frequency modulation continuous wave (FMCW) imaging system which has a fast measurement speed of 40000 waveforms per second. Cross-sectional imaging capability through cardboard has also been demonstrated using its excellent penetration capability at a frequency range of 76-81 GHz. We anticipate that the integration of these two complementary imaging technologies would be highly desirable for many real-world applications because it provides both spectroscopic discrimination and penetration capabilities in a single instrument.

THz time-domain spectroscopy (THz-TDS) is used to study the THz-optical properties of a single crystal bismuth ferrite BiFeO₃ (BFO). It can be found that the anisotropy of BiFeO₃ is strongly dependent on the temperature. A giant birefringence up to around 3.6 is observed at 1 THz. The presence of a spatially modulated cycloidal antiferromagnetic structure leads to spin cycloid resonances (SCR) ψ and Φ, corresponding to the out-of-plane and in-plane modes of the spin cycloid, respectively. We distinguish the SCR with respect to their response to orthogonal polarizations of the electric fields of the incident THz beam. In addition, we observe a resonance appearing below 140 K, which might be interpreted as an electromagnon mode and related to a spin reorientation transition. Our present observations present that the temperature and polarization, as the external control parameters, can be used to modulate the THz optical properties of BFO single crystal.

Graphene has been recognized as a promising candidate in developing tunable terahertz (THz) functional devices due to its excellent optical and electronic properties, such as high carrier mobility and tunable conductivity. Here, we review graphene-based THz modulators we have recently developed. First, the optical properties of graphene are discussed. Then, graphene THz modulators realized by different methods, such as gate voltage, optical pump, and nonlinear response of graphene are presented. Finally, challenges and prospective of graphene THz modulators are also discussed.

To promote the application of far-infrared technology, functional far-infrared devices with high performance are needed. Here, we propose a design scheme to develop a wide-incident-angle far-infrared absorber, which consists of a periodically semicircle-patterned graphene sheet, a lossless inter-dielectric spacer and a gold reflecting film. Under normal incidence for both TE- and TM-polarization modes, the bandwidth of 90% absorption of the proposed far-infrared absorber is ranging from 6.76 THz to 11.05 THz. The absorption remains more than 90% over a 4.29-THz broadband range when the incident angle is up to 50° for both TE- and TM-polarization modes. The peak absorbance of the absorber can be flexibly tuned from 10% to 100% by changing the chemical potential from 0 eV to 0.6 eV. The tunable broadband far-infrared absorber has promising applications in sensing, detection, and stealth objects.

Polarization conversion is a very important electromagnetic wave manipulation method. In this paper, we investigate a high-efficiency linear-to-circular polarization and cross-polarization converter by utilizing coding metasurface. The coding particle consists of top layer metal pattern and bottom metal plate sandwiched with square F4B dielectric, which can manipulate the linear-to-circular polarization and cross-polarization converter of the reflected wave simultaneously. In the terahertz frequency range of 1.0 THz-2.0 THz, the reflection magnitudes reach approximately 90% and the axial ratio is less than 3 dB. The proposed polarization converter may lead to advances in a variety of applications such as security, microscopy, information processing, stealth technology, and data storage.

The terahertz (THz) resonance, chirality, and polarization conversion properties of a double-layer chiral metasurface have been experimentally investigated by THz time domain spectroscopy system and polarization detection method. The special symmetric geometry of each unit cell with its adjacent cells makes a strong chiral electromagnetic response in this metasurface, which leads to a strong polarization conversion effect. Moreover, compared with the traditional THz transmission resonance sensing for film thickness, the polarization sensing characterized by polarization elliptical angle (PEA) and polarization rotation angle (PRA) shows a better Q factor and figure of merit (FoM). The results show that the Q factors of the PEA and PRA reach 43.8 and 49.1 when the interval film is 20 μm, while the Q factor of THz resonance sensing is only 10.6. And these PEA and PRA can play a complementary role to obtain a double-parameter sensing method with a higher FoM, over 4 times than that of resonance sensing. This chiral metasurface and its polarization sensing method provide new ideas for the development of high-efficiency THz polarization manipulation, and open a window to the high sensitive sensing by using THz polarization spectroscopy.

We used discrete dipole approximation (DDA) to examine the scattering and absorption characteristics of spherical ice crystal particles. On this basis, we studied the scattering characteristics of spherical ice crystal particles at different frequencies and non-spherical ice crystal particles with different shapes, aspect ratios, and spatial orientations. The results indicate that the DDA and Mie methods yield almost the same results for spherical ice crystal particles, illustrating the superior calculation accuracy of the DDA method. Compared with the millimeter wave band, the terahertz band particles have richer scattering characteristics and can detect ice crystal particles more easily. Different frequencies, shapes, aspect ratios, and spatial orientations have specific effects on the scattering and absorption characteristics of ice crystal particles. The results provide an important theoretical basis for the design of terahertz cloud radars and related cirrus detection methods.

The advancement of terahertz technology in recent years and its applications in various fields lead to an urgent need for functional terahertz components, among which a terahertz switch is one example of the most importance because it provides an effective interface between terahertz signals and information in another physical quantity. To date many types of terahertz switches have been investigated mainly in the form of metamaterials made from metallic structures and optically-active medium. However, these reported terahertz switches usually suffer from an inferior performance, e.g., requiring a high pump laser power density due to a low quality factor of the metallic metamaterial resonances. In this paper, we report and numerically investigate a symmetry-broken silicon disk based terahertz resonator array which exhibits one resonance with ultrahigh quality factor for normal incidence of the terahertz radiations. This resonance, which can never be excited for regular circular Si disks, can help to realize a superior terahertz switch with which only an ultra-low optical pump power density is required to modify the free carrier concentration in Si and its refractive index in the terahertz band. Our findings demonstrate that to realize a high terahertz transmittance change from 0 to above 50%, the required optical pump power density is more than 3 orders of magnitude smaller than that required for a split-ring resonator (SRR) based terahertz switch reported in the literature.

Graphene and black phosphorus have attracted tremendous attention in optics due to their support of localized plasmon resonance. In this paper, a structure consisted of graphene-black phosphorus heterostructure is proposed to realize terahertz anisotropic near-perfect absorption. We demonstrate that strong plasmonic resonances in graphene-black phosphorus heterostructure nanoribbons can both be provided along armchair and zigzag directions, and dominated by the distance between the graphene and black phosphorus ribbons. In particular, the maximum absorption of 99.6% at 10.2 THz along armchair direction can be reached. The proposed high performance anisotropic structure may have promising potential applications in photodetectors, biosensors, and terahertz imaging.

As semiconductor devices, the terahertz quantum-cascade laser is a coherent source based on intersubband transitions of unipolar carriers while the terahertz quantum-well photodetector is a kind of detector which matches the laser frequency. They are solid-state, electrically operated, and can be easily integrated with other components. This paper reviews the state of the art for the design, working performance, and future directions of the two devices. Their applications in photoelectric characterization and imaging are also discussed.

Metasurface is a kind of two-dimensional metamaterial with specially designed sub-wavelength unit cells. It consists of single-layer or few-layer stacks of planar structures and possesses certain superior abilities to manipulate the propagating electromagnetic waves, including the terahertz (THz) ones. Compared with the usual passive THz metasurfaces whose optical properties are difficult to be controlled after fabrication, the active materials are highly desirable to enable dynamic and tunable control of THz waves. In this review, we briefly summarize the progress of active THz metasurfaces, from their physical mechanisms on carrier concentration modulations, phase transitions, magneto-optical effects, etc., for various possible THz applications mainly with low-dimensional materials, vanadium dioxide films, and superconductors.

Last decade has witnessed a rapid development of the generation of terahertz (THz) vortex beams as well as their wide applications, mainly due to their unique combination characteristics of regular THz radiation and orbital angular momentum (OAM). Here we have reviewed the ways to generate THz vortex beams by two representative scenarios, i.e., THz wavefront modulation via specific devices, and direct excitation of the helicity of THz vortex beams. The former is similar to those wavefront engineering devices in the optical and infrared (IR) domain, but just with suitable THz materials, while the latter is newly-developed in THz regime and some of the physical mechanisms still have not been explained explicitly enough though, which would provide both challenges and opportunities for THz vortex beam generation. As for their applications, thanks to the recent development of THz optics and singular optics, THz vortex beams have potentials to open doors towards a myriad of practice applications in many fields. Besides, some representative potential applications are evaluated such as THz wireless communication, THz super-resolution imaging, manipulating chiral matters, accelerating electron bunches, and detecting astrophysical sources.

The aim of this study was to investigate the feasibility of detecting potassium sorbate (PS) and sorbic acid (SA) in agricultural products using THz time-domain spectroscopy (THz-TDS). The absorption spectra of PS and SA were measured from 0.2 to 1.6 THz at room temperature. The main characteristic absorption peaks of PS and SA in polyethylene and powdered agricultural products with different weight ratios were detected and analyzed. Interval partial least squares (iPLS) combined with a particle swarm optimization and support vector classification (PSO-SVC) algorithm was proposed in this paper. iPLS was used for frequency optimization, and the PSO-SVC algorithm was used for spectrum analysis of the preservative content based on the optimal spectrum ranges. Optimized PSO-SVC models were obtained when the THz spectrum from the PS/SA mixture was divided into 11 or 12 subintervals. The optimal penalty parameter c and kernel parameter g were found to be 1.284 and 0.863 for PS (0.551-1.487 THz), 1.374 and 0.906 for SA (0.454-1.216 THz), respectively. The preliminary results indicate that THz-TDS can be an effective nondestructive analytical tool used for the quantitative detection of additives in agricultural products.

Hydrodynamic calculations of the chaotic behaviors in n⁺nn⁺ In_0.53Ga_0.47As devices biased in terahertz (THz) electric field have been carried out. Their different transport characteristics have been carefully investigated by tuning the n-region parameters and the applied ac radiation. The oscillatory mode is found to transit between synchronization and chaos, as verified by the first return map. The transitions result from the mixture of the dc induced oscillation and the one driven by the ac radiation. Our findings will give further and thorough understanding of electron transport in In_0.53Ga_0.47As terahertz oscillator, which is a promising solid-state THz source.