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Terahertz detectors based on two-dimensional Dirac materials offer a new approach for room-temperature terahertz detection with high response and low noise. However, these devices can hardly show high response over a broad frequency range, mainly due to the poor absorption caused by their ultrathin nature. Here we apply metallic gratings to enhance the excitation efficiency of graphene plasmons. When nonzero source-drain bias is applied, graphene plasmons can generate terahertz response orthogonal to the polarization of the incidence. The response is attributed to the orthogonal overdamped plasmon rectification effect, and graphene plasmons-enhanced photo-thermoelectric effect. By comparing the normalized on/off ratio, the metallic gratings are found to effectively enhance the coupling efficiency between graphene plasmons and THz incidence, and thus the absorption and responsivity. The results are beneficial for improving the response of room temperature THz detectors.

Generation of terahertz (THz) wave from air plasma induced by femtosecond laser pulses with a single central frequency (the so-called "single-color") is one of the fundamental interactions between light and matter, and is also the basis of subsequent pumping schemes using two- or multi-color laser fields. Recently, more states of media beyond gas (e.g., atomic cluster and liquid) via photo-ionization have brought new experimental observations of THz radiation, which can no longer be simply attributed to the mainstream model of the transition-Cherenkov radiation (TCR), thus making the whole picture unclear. Here, we revisited the mechanism of this dynamic process in a new view of the traveling-wave antenna (TWA) model. By successfully reproducing the reported far-field THz radiation profiles from various plasma filament arrangements, the wide applicability of the TWA theory has been revealed. On the other hand in the microscopic view, we investigated the plasma oscillation during filamentation aiming at further bridging the plasma filament and the antenna. Accordingly, THz-plasma resonance has been theoretically and experimentally demonstrated as the elementary THz emitter, paving the way towards fully understanding this important single-color plasma based THz source.

Within the 2D theory of dissipative tunneling in the semiclassical approximation (a rarefied gas of instanton–anti-instanton pairs) at a finite temperature in the presence of an external electric field, the features of tunneling transport in the planar structures with the quantum dots made of colloidal gold—which, presumably, possess the properties of a metamaterial—are investigated. It is shown experimentally that either a single effect or a double effect of 2D tunnel bifurcations (in the form of a kink or kinks on the tunneling current–voltage curve) is observed depending on the position of the cantilever tip of a combined atomic force and scanning tunneling microscope (AFM/STM), which can be either above a single quantum dot or between two adjacent quantum dots, respectively. As our theoretical model shows, such a regime of double bifurcation (a double smoothed kink on the tunneling current–voltage curve) that is associated with the manifestation of the properties of a metamaterial by the structure under study. A convincing qualitative agreement is obtained between the experimental current–voltage characteristics and the field dependence of the probability of 2D dissipative tunneling in the two investigated regimes with due regard for the observed quantum beats (oscillations) in the vicinity of 2D bifurcation points.

A material in the form of a multilayer structure based on low-temperature LT-GaAs grown on (111)A-oriented substrates is proposed for fabrication of THz photoconductive antennas. These structures contain active LT-GaAs layers and doping acceptor GaAs:Si-based layers. At the optical pump power of 19 mW and the bias voltage of 30 V, a photoconductive antenna based on the optimized {LT-GaAs/GaAs:Si} (111)A structure emits THz pulses with the average power of 2.3 μW at the pulse repetition frequency of 80 MHz; the conversion efficiency is 1.2 × 10–4. It is shown that the dependence of the integral power of THz pulses of the antenna based on the {LT-GaAs/GaAs:Si} (111)A structure on the applied voltage is superlinear; the dependence of this parameter on the optical pump power is plotted as a curve with saturation. It is shown that the designed antennas have a practical application in THz spectroscopy of biological solutions.

Serum samples of healthy rats and rats with inoculated tumors on days 14 and 28 after transplantation of cholangiocarcinoma cells were analyzed using Raman spectroscopy (RS). It was shown that the intensity of the amide 1 band differs in the serum spectra depending on the stage of tumor development and correlates with the protein concentration in the samples. In some animals, there is a significant increase in the blood glucose concentration, leading to glycation of proteins and a change in the shape of the amide 1 band. The appearance of a band at 1670 cm–1 is most pronounced in the spectra of samples on the 28th day of the experiment and may be associated with an increase in the content of β-structural elements in the protein conformation. Using the principal component analysis, we assessed the general differences in the Raman spectra of the blood serum in rats of control groups and rats on the 28th day after tumor transplantation.

In this paper, we measure broadband terahertz spectra from a single 744-nm filament produced in tight, medium, and loose geometrical focusing conditions. Terahertz spectra are measured using interferometer coupled to a helium-cooled bolometer avoiding any cutoff frequencies. Based on THz spectrum maximum position and spectral width, we estimate electron-heavy particle (neutrals and ions) collision rate. Numerical simulations are performed using the state-of-the-art model of unidirectional femtosecond pulse propagation and convergence/divergence of optical radiation at large angles.

The main reason for the alternation of terahertz (THz) response in biological samples is the transition of a part of water from its free to bound state and back. To analyze such results, a precise spectrum of bound water within THz range should be known. The suggested wet powder spectroscopy method is optimal for this task. We measured the THz transmission of dry glucose pressed into pellets. Then, we added a small amount of water to these pellets and again measured the THz spectra. The contribution from the free water state starts to appear in resulting THz spectra after water concentration in glucose samples reaches 14–16%. Just below is the optimal point to extract the spectral contribution from bound water. We extracted the absorption and refraction spectra of bound water and confirmed that they are an order of magnitude weaker than those of free water within 0.07–1-THz range.

This article studies the generation of terahertz (THz) radiation in carbon dioxide ( ${\bf CO_2}$ ) at different levels of gas pressure. It was demonstrated that under low gas pressure, optical-terahertz conversion can be adequately described by a simplified photocurrent model. However, with the increase of gas pressure, the contribution of intra-atomic nonlinearities as a source of terahertz response irradiated by a two-color laser field is growing. Moreover, at high levels of gas pressure, the interference and dispersive effects should be taken into consideration as well. The proposed model allows for the optimization of experimental conditions aimed at flexible control of intensity and spectral properties of THz radiation.

Using the effective medium model, the complex permittivity of the ocular surface system in the THz frequency range is considered. The penetration depth of THz radiation and the sensitivity of the reflected component to the dynamics of evaporation of the tear film are discussed. The results of in vivo experiments aimed to determine the reflection coefficient of the ocular surface using continuous THz reflectometers are well described using the developed model.

Heavy metal pollution in water seriously affects human health. The disadvantages of traditional metal ion detection methods involve long and cumbersome chemical pretreatment in the early stage, and large volume of samples. In this study, microalgae were used as the medium, and terahertz spectroscopy technology was employed to collect the changes of material components in it, so as to deduce the types and concentrations of heavy metal pollution in water. Through the partial least square(PLS), we establish the prediction model of heavy metal concentration, and the results show that the best detection time for Pb

Infectious diseases are among the most severe threats to modern society. Current methods of virus infection detection based on genome tests need reagents and specialized laboratories. The desired characteristics of new virus detection methods are noninvasiveness, simplicity of implementation, real-time, low cost and label-free detection. There are two groups of methods for molecular biomarkers' detection and analysis: (i) a sample physical separation into individual molecular components and their identification, and (ii) sample content analysis by laser spectroscopy. Variations in the spectral data are typically minor. It requires the use of sophisticated analytical methods like machine learning. This review examines the current technological level of laser spectroscopy and machine learning methods in applications for virus infection detection.

Metasurfaces, the two-dimensional counterpart of metamaterials, enable an unprecedented capability in manipulating the phase, polarization and amplitude of electromagnetic waves. Here, we have proposed and experimentally demonstrated an approach to realize terahertz multi-foci metalens with polarization-rotated focal points. The THz polarization-dependent imaging is demonstrated based on the multi-foci metalens. In addition, the THz metalens with asymmetric focusing that can unidirectionally focus the incident THz waves into focal point was also designed and experimentally demonstrated.

We demonstrate that the tunneling magneto-resistance (TMR) can be used to modulate terahertz (THz) wave propagation through a magnetic tunnel junctions (MTJ). Operating at the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with the thickness of 77.45 nm, by using a small magnetic field of 30 mT. The THz conductivity spectrum of the MTJ is governed by the spin-dependent electron tunneling. This findings open that the MTJ device will have potential applications in THz magnetic modulators, filtering and sensing.

We use the terahertz attenuated total reflection (ATR) technology to evaluate different types of handcream and establish the theoretical model to investigate the reflectivity after interacting with the skin. In addition, a novel reflection sensing platform for enhanced fingerprint detection of samples in an ATR configuration with metasurface is also proposed.

We report on the creation and investigation of a new class of crystals for nonlinear optical applications in the NIR-THz range, requiring non-critical phase matching. A narrowband two-frequency THz radiation source is demonstrated. The spectral properties of THz radiation emitted via optical nonlinear rectification in a semiorganic molecular crystal guanylurea hydrogen phosphite (GUHP) are described in relation to the crystal axis orientation.

The monoclinic crystal of guanylurea(1+) hydrogen phosphite (GUHP) is a novel efficient nonlinear-optical medium having high χ(3) and χ(2) values. We used THz-TDS with high polarization contrast to study the absorption coefficient and the refractive index of the crystal in the range of 0.2—1.5 THz. We found that the dielectric frame xyz at 1 THz is rotated 16 degrees from the visible range. Absorption spectra show peaks at 0.93, 1.02, and 1.45 THz representing high-Q resonances. The last two modes show very high Raman activity that can be associated with the interlayer collective movement of the crystal. We assume that GUHP has the potential to be a basis for active and passive terahertz devices manufacturing.

A supersonic jet consisting of noble gas clusters is considered as a source of terahertz (THz) radiation pumped by high-power femtosecond laser system. We present the results of experimental studies on optimizing the intensity and diagram pattern of the THz radiation generated in cluster jet and demonstrate that under certain conditions of the jet excitation, it is possible to form a cluster filament that radiates THz waves with a maximum intensity along the direction of the laser beam propagation. Provided phenomenological model is in a good agreement with experimental results and well explains observed phenomenon.

In this work, we propose a structure consisting of three metamaterial layers and a metallic grating layer to rotate the polarization of arbitrary linearly polarized incidence to the y-direction with high transmissivity by electrically tuning these metamaterials. The transfer matrix method together with a harmonic oscillator model is adopted to theoretically study the proposed structure. Numerical simulation based on the finite difference time-domain method is performed assuming that the metamaterial layers are constituted by graphene ribbon arrays. The calculation and simulation results show that the Drude absorption is responsible for the polarization rotation. Fermi level and scattering rate of graphene are important for the transmissivity. For a polarization rotation of around 90°, the thickness of either the upper or lower dielectric separations influences the transmission window. For a polarization rotation of around 45° and 135°, the lower dielectric separations decide the frequency of the transmission window, while the upper dielectric separations just slightly influence the transmissivity.

A multi-frequency terahertz radiation source based on quantum-cascade lasers is described. The prospects of its application for the study of atmospheric phenomena and the creation of lidars and remote sensing equipment are discussed. The influence of radiation attenuation in the atmosphere for three frequency ranges was studied using the proposed terahertz source. Estimates of the influence of the laser spectral line width on terahertz radiation attenuation coefficient measurements in the atmosphere were obtained.

Actively controlling the phase of terahertz wave has received much attention. Here, we present a complete theoretical treatment by using coupled-mode theory to actively control the reflective wave phase in a attenuated total reflection-based metasurface. Controlling is achieved by variations in the air gap, enabling the radiative loss (radiative Q factor) to be continuously adjusted over a narrowband frequency range. A singularity can be found in the phase slope spectrum when the radiation loss is equal to the intrinsic loss determined by material properties. The theoretical results have been verified by simulation and the experimental results. The physical mechanism of this phase transition in attenuated total reflection configuration would pave the way for potential applications such as polarization modulation, and guide further research works for terahertz physics.

Terahertz emission spectra of the surface of silicon crystals with different types of conductivity were experimentally recorded upon excitation by femtosecond laser pulses at various temperatures. The observed features in the terahertz spectra of the silicon surface correspond to the energy structure of the impurity centers determining the type of conductivity of the sample. Comparison was made with the results obtained in the case of deposition of gold nanoparticles on a semiconductor surface. The spectral features of the surface with deposited nanoparticles are discussed using the terahertz re-emission mechanism in two-phonon absorption.

Aqueous solutions of guanylurea hydrogen phosphite (GUHP) have been synthesized. The temperature dependence of equilibrium GUHP concentration in water was obtained. The change in the density of aqueous GUHP solutions with a change in their concentration was investigated. The influence of the solution pH on the GUHP solubility in water in the temperature range of +25–45°C was analyzed. The crystallization range of monoclinic GUHP crystals, belonging to the sp. gr. Cc, was determined. GUHP crystals were grown using different methods, based on cooling or maintaining a constant temperature, with mixing aqueous solutions or without mixing. The properties of the grown crystals in the optical and THz ranges were investigated.

Generation of terahertz radiation upon filamentation in air of two-color radiation in loose focusing conditions with focal lengths from 30 to 312 cm and a beam diameter of 0.8 cm was studied. Two-color radiation was generated in a nonlinear BBO crystal irradiated by a converging laser beam. Measurements show that the energy input into the medium increases dramatically with a decrease in the focal length to 50 cm or less; at F = 30 cm, the energy of a terahertz pulse linearly depends on the laser pulse energy and increases significantly at F = 50 cm. Terahertz radiation is still not observed at longer focal lengths, which is apparently due to a spatio—temporal mismatch of the radiation of the first and second harmonics.

A new approach to terahertz (THz) diagnostics of the human psychoemotional state is proposed, based on the analysis of the THz contribution to the total signal while simultaneously recording the IR and THz emissions from the human body (hereinafter referred to as the IR – THz image). The developed image-processing algorithm allows extraction of the informative contribution determined by the THz radiation from the total signal perceived by the recording system. Simultaneous registration of the IR – THz images of the human face and the psychophysiological indicators are carried out in situations of physical stress (short-term intense physical exercise – squats functional test), electrical stimulation, and information stress (cognitive load – simple arithmetic mental calculations). The obtained data are compared with those of similar measurements at rest. It is shown that using cluster analysis of IR – THz images it is possible to divide the test subjects into classes according to the type of reaction of the circulatory system under stressful conditions.

Co-crystals series of different mono- and diaminopyridines non-centrosymmetric complexes, namely 3,4-diaminopyridine-4-nitrophenol-4-nitrophenolate, 2,6-diaminopyridine-4-nitrophenol-4-nitrophenolate and 2-aminopyridine-4-nitrophenol-4-nitrophenolate were grown by slow solvent evaporation from solution with constant temperature technique. Terahertz absorption and refraction properties of the materials were measured, and the co-crystals were tested as generators of THz pulses under irradiation with 800 nm Ti:Sapphire femtosecond laser. Efficiency of the optical-to-THz conversion was compared to that of ZnTe crystal.

Significance: The creation of fundamentally new approaches to storing various biomaterial and estimation parameters, without irreversible loss of any biomaterial, is a pressing challenge in clinical practice. We present a technology for studying samples of diabetic and non-diabetic human blood plasma in the terahertz (THz) frequency range. Aim: The main idea of our study is to propose a method for diagnosis and storing the samples of diabetic and non-diabetic human blood plasma and to study these samples in the THz frequency range. Approach: Venous blood from patients with type 2 diabetes mellitus and conditionally healthy participants was collected. To limit the impact of water in the THz spectra, lyophilization of liquid samples and their pressing into a pellet were performed. These pellets were analyzed using THz time-domain spectroscopy. The differentiation between the THz spectral data was conducted using multivariate statistics to classify non-diabetic and diabetic groups’ spectra. Results: We present the density-normalized absorption and refractive index for diabetic and non-diabetic pellets in the range 0.2 to 1.4 THz. Over the entire THz frequency range, the normalized index of refraction of diabetes pellets exceeds this indicator of non-diabetic pellet on average by 9% to 12%. The non-diabetic and diabetic groups of the THz spectra are spatially separated in the principal component space. Conclusion: We illustrate the potential ability in clinical medicine to construct a predictive rule by supervised learning algorithms after collecting enough experimental data.

The liquid and lyophilized blood plasma of patients with benign or malignant thyroid nodules and healthy individuals were studied by terahertz (THz) time-domain spectroscopy and machine learning. The blood plasma samples from malignant nodule patients were shown to have higher absorption. The glucose concentration and miRNA-146b level were correlated with the sample’s absorption at 1 THz. A two-stage ensemble algorithm was proposed for the THz spectra analysis. The first stage was based on the Support Vector Machine with a linear kernel to separate healthy and thyroid nodule participants. The second stage included additional data preprocessing by Ornstein-Uhlenbeck kernel Principal Component Analysis to separate benign and malignant thyroid nodule participants. Thus, the distinction of malignant and benign thyroid nodule patients through their lyophilized blood plasma analysis by terahertz time-domain spectroscopy and machine learning was demonstrated.

This review considers glioma molecular markers in brain tissues and body fluids, shows the pathways of their formation, and describes traditional methods of analysis. The most important optical properties of glioma markers in the terahertz (THz) frequency range are also presented. New metamaterial-based technologies for molecular marker detection at THz frequencies are discussed. A variety of machine learning methods, which allow the marker detection sensitivity and differentiation of healthy and tumor tissues to be improved with the aid of THz tools, are considered. The actual results on the application of THz techniques in the intraoperative diagnosis of brain gliomas are shown. THz technologies’ potential in molecular marker detection and defining the boundaries of the glioma’s tissue is discussed.

The investigation of converged twisted beams with a helical phase structure has a remarkable impact on both fundamental physics and practical applications. Geometric metasurfaces consisting of individually orientated metal/dielectric meta-atoms provide an ultracompact platform for generating converged vortices. However, it is still challenging to simultaneously focus left-handed and right-handed circularly polarized incident beams with pure geometric phase modulation, which hinders the independent operation on topological charges between these two helical components. Here we propose and experimentally demonstrate an approach to design terahertz geometric metasurfaces that can generate helicity-independent converged vortices with homogeneous polarization states by the superposition of two orthogonal helical vortices with identical topological charges. Furthermore, the multiplexing of polarization-rotatable multiple vortices in multiple dimensions, i.e., in both longitudinal and transverse directions, and a vortex with an extended focal depth is confirmed by embedding polarization modulation into the geometric metasurfaces. The demonstrated approach provides a new way to simultaneously manipulate orthogonal helical components and expand the design dimension, enabling new applications of geometric metasurface devices in polarization optics, twisted-beam related image and edge detection, high capacity optical communication, and quantum information processing, to name a few.

This work describes the promising approach for the diagnostic of diseases, such as diabetes, by using the THz spectroscopy and THz holography method. In this paper, dry pellets consisting of human blood plasma (normal and diabetic) were designed; these objects were analyzed in various THz Russian laboratories, PCA-analysis of THz spectra was applied to separate the normal and diabetic groups; the holographic approach was proposed that provides sufficient information to reconstruct the spatial distribution of complex refractive index of the samples.

We successfully demonstrate broadband THz wave emission from cryogenic fluid - liquid nitrogen - by focusing laser pulses on a flowing liquid nitrogen line in the ambient environment. The THz signal from liquid nitrogen shows a comparable field strength with a broader bandwidth to that from liquid water. In contract to solid materials or other liquids, liquid nitrogen is a “debris-free” target, which can sustain the excitation with high repetition rate laser pulses. This work provides more possibilities in studying laser-induced ionization of cryogenic liquids.

Using terahertz time domain spectroscopy (THz TDS), the dielectric properties of liquid and lyophilized blood plasma of patients with thyroid nodules were studied for the first time. THz absorption and refraction were compared with the data of biochemical analysis of blood plasma; a correlation with the concentration of glucose, protein, and some tumor miRNAs were revealed. Thus, the sensitivity of THz TDS to thyroid cancer was confirmed in the frequency range from 0.05 to 1.6 THz.

We applied the harmonic oscillator model combined with the transfer matrix method to study the polarization conversion for transmitted waves in metallic grating/plasmon-excitation layer/metallic grating structure in the terahertz (THz) region. By comparing the calculated spectra and the simulated (by the finite-difference-time-domain method) ones, we found that they correspond well with each other. Both methods show that the Drude background absorption and the excited plasmon resonances are responsible for polarization conversion. The transmission is close to 0 when the distance between the top/bottom metallic gratings and gated graphene is an integer multiple of half the wavelength of the incident wave (in the dielectrics), at which points the plasmon resonances are greatly suppressed by the destructive interference between the backward/forward electromagnetic waves and that reflected by the top/bottom metallic gratings. Away from these points, the transmission can be higher than 80%. The electron density and the excitation efficiency of the plasmon-excitation layer were found to be important for the bandwidth of the polarization conversion window, while the scattering rate was found to influence mainly the polarization conversion rate. Multi-broadband polarization conversion is realized by exciting plasmon modes between the 0 transmission points in the THz region.

This work is devoted to the development of a suitable phantom of a biological object for measurements in the THz frequency range and for approbation with various diagnostic methods developed in different THz laboratories. The phantoms were represented as a pellet of human and a laboratory rat blood plasma in the diabetic and the control groups. These objects were analyzed in various laboratories, using THz pulsed spectroscopy and a high-resolution THz spectrometer based on a backward wave oscillator. The components of the dry blood plasma were identified by the detected spectral lines.

The monoclinic organic crystal of guanylurea hydrogen phosphite (GUHP) was considered. The absorption and refractive properties of an optically negative biaxial crystal were investigated. The terahertz time-domain spectroscopy under cryogenic conditions was carried out for observation of the temperature dependence of the phonon structure of GUHP crystal.

Terahertz radiation emission from the photoexcited metallic microdroplets was observed for the first time. We found two separated in time femtosecond pulses excitation scheme to be efficient for such targets, while signal at one-pulse excitation was negligible. Spectral and polarization properties of the emitted terahertz radiation and its dependence on delay between pulses were measured.

Liquid targets are capable of offering rapid refreshment for per shot, which makes it viable of using high repetition rate lasers. Here, we push the liquid targets to a low temperature condition. By generating a stable flowing line of liquid nitrogen in ambient environment, we successfully observe broadband terahertz wave generation under the excitation of subpicosecond laser pulses. This demonstration provides more possibilities in choosing potential materials for studying the THz wave generation process, and in understanding the laser-induced ionization from a new perspective.

We show that an individual droplet of liquid metal can be a source of coherent terahertz radiation when it is excited by two femtosecond laser pulses of the same frequency. Under certain delays between these pulses, the intensity of terahertz radiation increases by more than 3 orders of magnitude. We describe the experimental results with the model of dynamic gain control, which considers the interaction of both laser pulses with the droplet and explains the terahertz-generation process by taking into account the dynamics of electrons and ions after photoionization of the metal droplet. The spatial distribution of terahertz radiation has a forward-directed contribution, whose polarization properties are well described by a nonlinear susceptibility of the second order. Our theoretical estimations based on the experimental data show that under the dynamic gain control the observed terahertz output can be considerably increased. Joint generation of x-ray, ultraviolet, and, as shown in the present work, terahertz radiation allows one to forecast that a free-falling photoexcited droplet of liquid metal is a promising source of multifrequency electromagnetic radiation for extreme nonlinear laser science.

Magnetic tunnel junctions (MTJs) are a key technology in modern spintronics because they are the basis of read-heads of modern hard disk drives, nonvolatile magnetic random access memories, and sensor applications. In this paper, we demonstrate that tunneling magnetoresistance can influence terahertz (THz) wave propagation through a MTJ. In particular, various magnetic configurations between parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the MTJ have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. Operating in the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with a thickness of 77.45 nm, using a magnetic field as low as 30 mT. The THz conductivity spectrum of the MTJ is governed by spin-dependent electron tunneling. It is anticipated that the MTJ device and its tunability scheme will have many potential applications in THz magnetic modulators, filtering, and sensing.