Broadband terahertz time-domain spectroscopy and fast FMCW imaging: Principle and applications

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.

Determination of 2 Amides Pesticide Residues in Agricultural Products by Terahertz Time-domain Spectroscopy

[Objective] The aim was to explore the application of terahertz （THz） technique in pesticide recognition and residue determination. [Method] THz far-infrared spectral characteristics of 2 herbicides （butachlor and metolachlor） were measured by THz time-domain spectroscopy （THz-TDS） under conditions of room temperature and nitrogen,meanwhile their refractive index and absorption coefficient between 0.2 and 2.2 THz were calculated by using models based on Fresenl equations. [Result] The 2 pesticides both had a series of different characteristic absorption peaks between 0.2 and 2.2 THz,respectively. Their molecular structures were similar,but there were great difference in absorption and refractive index spectrum between them. [Conclusion] THz-TDS technique is feasible to detect pesticide residues,especially there is a greater advantage in identifying the structure of similar substances.

Two-Dimensional Terahertz Time-Domain Spectroscopy and Its Applications

In this work, we review the developing progress of two-dimensional terahertz time-domain spectroscopy（THz-TDS） and its diverse applications, including analyzing the polarization of THz radiation from a laser-induced plasma source and studying the corresponding physical mechanism, and characterizing the optical properties of crystals, etc.

Far-infrared conductivity of CuS nanoparticles measured by terahertz time-domain spectroscopy

This paper reports that terahertz time-domain spectroscopy is used to measure the optical properties of CuS nanoparticles in composite samples. The complex conductivity of pure CuS nanoparticles is extracted by applying the Bruggeman effective medium theory. The experimental data are consistent with the Drude-Smith model of conductivity in the range of 0.2 1.5 THz. The results demonstrate that carriers become localized with a backscattering behaviour in small-size nanostructures. In addition, the time constant for the carrier scattering is obtained and is only 64.3 fs due to increased electron interaction with interfaces and grain boundaries.

The transmission enhancement of a THz pulse through an Ag/Ag_2 O layer detected by terahertz time-domain spectroscopy

This paper reports a new way to detect the enhanced transmission of a THz electromagnetic wave through an Ag/Ag2O layer by THz-TDS （time-domain spectroscopy）. As the THz beam illuminates the sub-wavelength Ag particles gained by Ag2O thermal decomposition, the evanescent wave is generated. The evanescent wave is coupled by a 500μm-GaAs substrate, which attaches behind the Ag/Ag2O layer, and then it transmits to the far field to be detected. The experimental results indicate that the transmitting amplitude is enhanced, as well as the frequent shifting and spectra broadening.

Terahertz Time-Domain Spectroscopy and Its Applications

The terahertz time-domain spectroscopy （THz-TDS） system and the related technology and the applications in Capital Normal University are presented. The most often used THz-TDS system as a spectroscopic measurement setup in our lab is introduced in detail, including the THz radiation source, the THz detection method and its measurement, and the control system. THz spectra of various materials is summarized and discussed. These materials include but not limited to two kinds of typical matter-the illegal drugs and explosives. The biological macro-molecules, cosmetics and fine chemical materials, edible pigments and food additives, homocysteic acid and related compounds, heavy ions in soil, Chinese medicines, tobacco and crops, oil and chemical products, carbon nanotubes, superconductors, and various semiconductors and their heterojunctions, are presented. THz emissions from the InAs and InN semiconductors surface are compared. THz spectral investigation of metallic mesh structures is summarized. Finally, an outlook of THz spectroscopic applications is given.

Experimental measurements of water content in crude oil emulsions by terahertz time-domain spectroscopy

We measured the water content （0.01% 0.25% w/w） in crude oil emulsions using terahertz time-domain spectroscopy （THz-TDS）. To improve the precision and range of the measurements, we used 1 and 10 mm thick quartz cells. The experiments were performed at 20 ℃ and the THz wave was transmitted vertically to the samples and detected on the other side. The experimental results suggest linear relation for the THz absorption coefficient and the water content of the crude oil emulsions in the observed range. The linear dependence facilitates high-precision measurements of the water content of crude oil. This suggests the potential of THz-TDS in determining the water concentration in crude oil and borehole fluid identification.

Investigation of copper sulfate pentahydrate dehydration by terahertz time-domain spectroscopy

Copper sulfate pentahydrate is investigated by terahertz time-domain spectroscopy. It is shown that the terahertz absorption coefficients are correlated with the particle size of the samples, as well as the heating rates of the ambient temperature. Furthermore, the water molecules of copper sulfate pentahydrate can be quantitatively characterized due to the high sensitivity of the terahertz wave to water molecules. Based on such results, the status of water incorporated in mineral opal is also characterized using terahertz time-domain spectroscopy. It indicates that terahertz technology can be considered as an efficient method to detect the dehydration of minerals.

Terahertz Time-Domain Spectroscopy method for optical parameter extraction of plastic materials

A method for extracting optical parameters of plastics materials based on terahertz time domain spectroscopy is presented. The transmission-type Terahertz Time-Domain Spectroscopy(THz TDS) system is adopted to detect the refractive index and extinction coefficient on different plastic materials. Then the corresponding spectral information is obtained by Fourier transform of the terahertz time domain waveform of the sampling points, including the corresponding amplitude and phase information of the waveform. The optical parameter extraction model is built. By using the simplex optimization method, the curves of the refractive index and extinction coefficient for the plastic material are obtained. The experimental samples are made of different plastic parallel plate materials. The experimental results show that the optimization of optical parameters can improve their extraction accuracy, and the error of refractive index is ±0.005. Extraction technology with the simplex optimization method of optical parameter based on THz TDS can help to extract the optical parameters of engineering plastics. It is of great significance for the research of terahertz nondestructive testing.

Terahertz time-domain spectroscopy of a simulated pore structure to probe particle size and porosity of porous rock

The particle sizes and porosities of simulated pore structures are probed by terahertz time-domain spectroscopy.A double-peak time-domain spectrum phenomenon is observed when the terahertz（THz） pulses illuminated a pore and a particle. The amplitudes of the two peaks depend strongly and monotonically on the particle size and porosity. A model is used to study the phenomenon, and the computational results agreed with the experimental measurements. These measurements indicate the terahertz spectroscopic behaviors of pores and particles, suggesting that terahertz spectroscopy can be used as a noncontact probe of porosity.

Circular dichroism spectra of α-lactose molecular measured by terahertz time-domain spectroscopy

Researchers have long studied circular dichroism(CD) for its enormous prospects in life sciences. Many biomolecules have vibration modes in the terahertz region, and terahertz CD spectra are robust to detect biomolecular structures. However,few studies explore the terahertz CD spectra on even natural materials due to technical challenges in both fields. Here, we report a setup of home-built terahertz time-domain spectroscopy to measure the polarization states of terahertz waves.By carefully measuring the transmission Jones matrix, we obtain terahertz CD spectra of α-lactose tablets and D-glucose tablets. Our results show that the terahertz CD spectra are sensitive to vibrational motions in biochemical compounds,which will find wide applications in biosensing and biomedical diagnostics.

Investigation of solid-state reaction by terahertz time-domain spectroscopy

Terahertz time-domain spectroscopy(THz-TDS)was utilized to investigate the solid-state reaction between L(+)-Tartaric acid and sodium hydrogen carbonate.Solid sodium hydrogen L(+)-tartrate monohydrate was synthesized efficiently by mechanical grinding,which is particularly sustainable and environmentally benign.Distinct THz absorptions were observed for pure reactants and the proposed product.The reaction process could be clearly visualized by THz spectral patterns of the reaction mixtures at different grinding time.The observed results were further confirmed by synchrotron radiation X-ray powder diffraction(SRXRPD)and Fourier transform infrared (FT-IR)spectroscopy.The study demonstrates that THz-TDS is an effective novel tool to monitor solid-state reactions in pharmaceutical industry.

Temperature and Pressure Effects on Terahertz Time-Domain Spectroscopy of Crystalline Methedrine by Molecular Dynamics Simulation

In this study, the terahertz time-domain spectroscopy (THz-TDS) of crystalline methedrine, which is one of the illegal drugs, is performed using molecular dynamics simulation by the Fourier transform of time derivative auto-correlation functions of the dipole moment. In order to accurately detect the drugs from samples, it is necessary to build a complete database for terahertz spectra under different external conditions from theoretical calculation, which are hardly obtained from the experiments directly. Our results show remarkable consistency with the available experimental data in the frequency range of 10 - 100 cm-1 indicating that the presented method has significant capability to simulate terahertz spectra at various conditions. We investigated the effects of temperature and pressure on THz-TDS by simulating the system at temperature range between 78.4 K and 400 K at pressures up to 100 atm. Results show the spectral features of THz-TDS both in intensity and profile are highly sensitive to the variation of temperature and with a lower magnitude to the variation of pressure. The vanishing, rebuilding and shifting of spectral peaks are due to the complex mechanisms such as the anharmonicity, shifting in the vibration energy levels, formation and destruction of hydrogen-binding and the deformation of the potential energy surface during the environment changing. This improved our understanding for complicated THz-TDS of crystalline methedrine and would be useful for assignment of the practical measurements.

Low-Frequency Vibrations of Indole Derivatives by Terahertz Time-Domain Spectroscopy

Several indole derivatives with different ＇3-＇ substituents have been investigated by terahertz （THz） time-domain spectroscopy. The low-frequency absorption spectra and refractive indices were obtained in the range of 0.2 THz to 2.5 THz （7 cm-1 to 83 cm-1）. These derivatives with different substituents present distinct features, which suggests that THz spectroscopy is sensitive to different structures and components of these chemicals. The density functional theory was employed to calculate the low-frequency vibrational properties of indole-3-carboxylic acid and indole-3-propionic acid based on their crystal structures, and the intermolecular interactions were involved. Meanwhile, the temperature dependence of the spectra agreed with the calculated results. The quantitative analysis of a ternary mixture was studied by taking the THz fingerprints into account, and the results demonstrate THz spectroscopy has great potential for the practical applications in biochemistry and pharmaceutics.

Different Types of Bone Fractures Observed by Terahertz Time-Domain Spectroscopy

Microcracks are common in compact bone,but their continued propagation can lead to macroscopic fractures.These microcracks cannot be visualized radiographically,necessitating alternative noninvasive methods to identify excessive microcracking and prevent fractures.In this study,terahertz time-domain spectroscopy(THz-TDS)was used to examine bone interiors near cracks resulting from loading in bovine tibia samples.Various loading configurations,such as impact,quasi-static loading,and fatigue loading,known to induce different types of micro-scale damage,were applied.The values of refractive index and absorption coefficient of the bone samples were then determined from the THz-TDS spectra acquired before loading and after fracture.The study revealed that different loading configurations led to varying terahertz optical coefficients associated with various types of bone fractures.Specifically,the refractive index notably increased under fatigue loading but remained relatively stable during quasi-static bending.The absorption coefficient of bone decreased only under fatigue loading.Furthermore,samples were subjected to axial and radial impacts without sustaining damage.Results indicated that in the undamaged state,the change in refractive index was smaller compared to after impact failure,while the change in absorption coefficient remained consistent after failure.Under radial impact loading,changes in refractive index and absorption coefficient were significantly more pronounced than under axial loading.Prior to loading,the measured value of refractive index was 2.72±0.11,and the absorption coefficient was 6.33±0.09 mm^(−1)at 0.5 THz.

Electronic properties of 2H-stacking bilayer MoS_(2)measured by terahertz time-domain spectroscopy

Bilayer (BL) molybdenum disulfide (MoS_(2)) is one of the most important electronic structures not only in valleytronics but also in realizing twistronic systems on the basis of the topological mosaics in moiré superlattices. In this work, BL MoS_(2) on sapphire substrate with 2H-stacking structure is fabricated. We apply the terahertz (THz) time-domain spectroscopy (TDS) for examining the basic optoelectronic properties of this kind of BL MoS_(2). The optical conductivity of BL MoS_(2) is obtained in temperature regime from 80 K to 280 K. Through fitting the experimental data with the theoretical formula, the key sample parameters of BL MoS_(2) can be determined, such as the electron density, the electronic relaxation time and the electronic localization factor. The temperature dependence of these parameters is examined and analyzed. We find that, similar to monolayer (ML) MoS_(2), BL MoS_(2) with 2H-stacking can respond strongly to THz radiation field and show semiconductor-like optoelectronic features. The theoretical calculations using density functional theory (DFT) can help us to further understand why the THz optoelectronic properties of BL MoS_(2) differ from those observed for ML MoS_(2). The results obtained from this study indicate that the THz TDS can be applied suitably to study the optoelectronic properties of BL MoS_(2) based twistronic systems for novel applications as optical and optoelectronic materials and devices.

Research on Detection of Food additives Based on Terahertz Spectroscopy and Analytic Hierarchy Process

Terahertz time-domain spectroscopy is a kind of far-infrared spectroscopy technology,and its spectrum reflects the internal properties of substances with rich physical and chemical information,so the use of terahertz waves can be used to qualitatively identify food additives containing nitrogen elements.Analytic hierarchy process(AHP)was originally used to solve evaluation-type problems,and this paper introduces it into the field of terahertz spectral qualitative analysis,proposes a terahertz time-domain spectral qualitative identification method combined with analytic hierarchy process,and verifies the feasibility of the method by taking four common food additives(xylitol,L-alanine,sorbic acid,and benzoic acid)and two illegal additives(melamine,and Sudan Red No.I)as the objects of study.Firstly,the collected terahertz time-domain spectral data were pre-processed and transformed into a data set consisting of peaks,peak positions,peak numbers and overall trends;then,the data were divided into comparison and test sets,and a qualitative additive identification model incorporating analytic hierarchy process was constructed and parameter optimisation was performed.The results showed that the qualitative identification accuracies of additives based on single factors,i.e.,overall trend,peak value,peak position,and peak number,were 80.23%,70.93%,67.44%,and 40.70%,respectively,whereas the identification accuracy of the analytic hierarchy process qualitative identification method based on multi-factors could be improved to 92.44%.In addition,the fuzzy characterisation of the absorption spectrum data was binarised in the data pre-processing stage and used as the base data for the overall trend,and the recognition accuracy was improved to 94.19%by combining the fuzzy characterisation method of such data with the hierarchical analysis qualitative recognition model.The results show that it is feasible to use terahertz technology to identify different varieties of additives,and this paper constructs a hierarchical analytical qualitative model with better effect,which provides a new means for food additives detection,and the method is simple in steps,with a small demand for samples,which is suitable for the rapid detection of small samples.

High photoelectric conversion efficiency and fast relaxation time of FA_(0.4)MA_(0.6)PbI_(3) applied in ultrafast modulation of terahertz waves

Active control of terahertz(THz)waves is attracting tremendous attentions in terahertz communications and active photonic devices.Perovskite,due to its excellent photoelectric conversion performance and simple manufacturing process,has emerged as a promising candidate for optoelectronic applications.However,the exploration of perovskites in optically controlled THz modulators is still limited.In this work,the photoelectric properties and carrier dynamics of FA_(0.4)MA_(0.6)PbI_(3)perovskite films were investigated by optical pumped terahertz probe(OPTP)system.The ultrafast carrier dynamics reveal that FA_(0.4)MA_(0.6)PbI_(3)thin film exhibits rapid switching and relaxation time within picosecond level,suggesting that FA_(0.4)MA_(0.6)PbI_(3)is an ideal candidate for active THz devices with ultrafast response.Furthermore,as a proof of concept,a FA_(0.4)MA_(0.6)PbI_(3)-based metadevice with integrating plasma-induced transparency(PIT)effect was fabricated to achieve ultrafast modulation of THz wave.The experimental results demonstrated that the switching time of FA_(0.4)MA_(0.6)PbI_(3)-based THz modulator is near to 3.5 ps,and the threshold of optical pump is as low as 12.7μJ cm^(-2).The simulation results attribute the mechanism of ultrafast THz modulation to photo-induced free carriers in the FA_(0.4)MA_(0.6)PbI_(3)layer,which progressively shorten the capacitive gap of PIT resonator.This study not only illuminates the potential of FA_(0.4)MA_(0.6)PbI_(3)in THz modulation,but also contributes to the field of ultrafast photonic devices.

Stretchable,Transparent,and Ultra-Broadband Terahertz Shielding Thin Films Based on Wrinkled MXene Architectures

With the increasing demand for terahertz(THz)technology in security inspection,medical imaging,and flexible electronics,there is a significant need for stretchable and transparent THz electromagnetic interference(EMI)shielding materials.Existing EMI shielding materials,like opaque metals and carbon-based films,face challenges in achieving both high transparency and high shielding efficiency(SE).Here,a wrinkled structure strategy was proposed to construct ultra-thin,stretchable,and transparent terahertz shielding MXene films,which possesses both isotropous wrinkles(height about 50 nm)and periodic wrinkles(height about 500 nm).Compared to flat film,the wrinkled MXene film(8 nm)demonstrates a remarkable 36.5%increase in SE within the THz band.The wrinkled MXene film exhibits an EMI SE of 21.1 dB at the thickness of 100 nm,and an average EMI SE/t of 700 dBμm^(−1)over the 0.1-10 THz.Theoretical calculations suggest that the wrinkled structure enhances the film’s conductivity and surface plasmon resonances,resulting in an improved THz wave absorption.Additionally,the wrinkled structure enhances the MXene films’stretchability and stability.After bending and stretching(at 30%strain)cycles,the average THz transmittance of the wrinkled film is only 0.5%and 2.4%,respectively.The outstanding performances of the wrinkled MXene film make it a promising THz electromagnetic shielding materials for future smart windows and wearable electronics.

Time-domain dynamic constitutive model suitable for mucky soil site seismic response

Soil nonlinear behavior displays noticeable effects on the site seismic response.This study proposes a new functional expression of the skeleton curve to replace the hyperbolic skeleton curve.By integrating shear modulus and combining the dynamic skeleton curve and the damping degradation coefficient,the constitutive equation of the logarithmic dynamic skeleton can be obtained,which considers the damping effect in a soil dynamics problem.Based on the finite difference method and the multi-transmitting boundary condition,a 1D site seismic response analysis program called Soilresp1D has been developed herein and used to analyze the time-domain seismic response in three types of sites.At the same time,this study also provides numerical simulation results based on the hyperbolic constitutive model and the equivalent linear method.The results verify the rationality of the new soil dynamic constitutive model.It can analyze the mucky soil site nonlinear seismic response,reflecting the deformation characteristics and damping effect of the silty soil.The hysteresis loop area is more extensive,and the residual strain is evident.