This PDF file contains the front matter associated with SPIE Proceedings Volume 11559, including the Title Page, Copyright information, and Table of Contents.

THz liquid photonics is a new research frontier in laser-matter interaction community. We have successfully demonstrated THz wave generation from ionized liquids, including from liquid water, liquid nitrogen, and liquid gallium. Preferable to general targets, a flowing liquid line provides a fresh area for each excitation pulse, so the chaos and debris caused by the previous pulse will not influence the next one. This makes it possible of using a kHz repetition rate laser for excitation. THz wave generation from ionized liquids presents photoionization processes that are different from those in gases.

This Conference Presentation, “Terahertz wave emission from liquid gallium,” was recorded for the Photonics Asia 2020 Digital Forum.

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.

Nonlinear THz spectroscopy extends the full-phase analysis of traditional THz-TDS onto the high-intensity regime. With THz sources reaching peak electric fields in excess of hundreds of kV/cm, it is now possible to induce large index changes in some materials. In the case of liquid water, a nonlinear index of 7.8x10-10 cm2/W is confirmed. This value is large enough to break the perturbative regime when sources with peak electric fields in the MV/cm range are used. In addition, the spectrally resolved nonlinear index dispersion can also be extracted.

A novel noncollinear phase-matching (PM) scheme by introducing a small tunable angle between two pump beams, was proposed to notably enhance the effective nonlinear coefficient (d_eff) in difference frequency generating (DFG) tunable terahertz waves in the ZnGeP₂ crystal. Compared with the collinear PM condition, the noncollinear geometry transfers the PM angles to be around θ = 90° or θ = 30° for type-II (o→e→o) or type-I (o→e→e) PM to maintain large values of d_eff in the entire output frequency band, in which tunable bands of 1.90–4.5 THz or 0.47–4.30THz can be achieved, respectively, leading to a high conversion efficiency improved by tens of times. Based on the theory of noncollinear PM, the angletuning characteristics were studied and the crystal design was provided for efficient outcoupling. Rigorous theoretical models were built for both types under small-signal approximation to show the affecting factors of noncollinear PM and reveal its superiority compared with collinear PM. The idea presented in this paper not only provides a good solution for efficient terahertz generation in ZnGeP₂, but it is also applicable in various optical frequency converters in different nonlinear materials.

Terahertz time-domain spectroscopic polarimetry (THz-TDSP) method was used to experimental study polarization properties of unaligned single-wall carbon nanotube thin films with different geometric parameters on transparent float glass substrates in a frequency range 0.2-0.8 THz at a controlled room temperature of 291–293K, and a relative humidity of 40–45%. Frequency dependences of azimuth and ellipticity angles of a polarization ellipse of electromagnetic waves transmitted through the samples were obtained for values of 0.2–1:.0Wcm^-2 of an external 980nm near infrared optical pumping, with an external static magnetic field of ~0:3T. A change of 15° in the azimuth angle, and of 10° in the ellipticity angle was achieved. The results show that by using carbon nanomaterials-based structures it is possible to devise efficient and affordable magneto-optically tunable polarization modulators that can be used in the advanced areas of terahertz nanoscience and nanotechnologies.

Through a quantitative discussion of THz modes in the racemic and enantiomeric crystals of alanine, we have examined the restriction on the THz selection rule imposed by the space-group symmetries. We have also revealed an essential role played by intramolecular vibrations in determining the IR intensities of THz modes in the low-frequency region. Although the intramolecular vibrations make marginal contributions to the vibrational energies of the low-frequency modes, they are crucial to the generation of transition moments.

InGaAs schottky diode with the property of low barrier height, simple material structure, and mature InP based integrated process, has become one of the major techniques for zero biased terahertz detector. As a thorough study from device to circuit, a “T” shape anode designed InGaAs SBD with ft over 3THz was presented, a novel device equivalent circuit model for InGaAs SBD was established, and a 75-110GHz W band zero biased detector circuit was design and fabricated, with the peak voltage responsivity of 25880V/W, indicating great potential for zero biased terahertz detection.

In recent years, Terahertz time-domain spectroscopy(THz-TDS), as a new spectral analysis method, has become a hot research. Terahertz time-domain spectroscopy can acquire spectral data with high SNR over a board range of THz frequencies. Terahertz technology has been widely used in the quality control of traditional Chinese medicine. In this paper, Terahertz time-domain spectroscopy combined with chemometrics was used to analyze the differences between two kinds of medicines-Bailing capsules and Jinshuibao capsules. The time-domain signal of the samples can be obtained by THz-TDS. And the refractive index and absorption coefficient of samples can be calculated. The PCA scores of 2Dplots (PC1×PC2) of Bailing and Jinshuibao capsules were obtained through principal component analysis(PCA), and the cumulative contribution rate of the first two principal components exceeds 98%. Their absorption coefficients were used as the characteristic data of cluster analysis. Euclidean distance was used to cluster and identify the samples. The results shows that twenty different batches of TCM were divided into two categories, with a slight crossover in the middle. The clustering precision of Jinshuibao capsules is 100%, while the clustering precision of Bailing capsules is 81. 8%. The results show that the clustering method can effectively analyze the terahertz time-domain spectrum of traditional Chinese medicine.

Infrared photodetectors (IRPDs) are of importance devices with wide applications from face identification to space communication. With the investigation of thermoelectric materials, perovskite and three-dimensional (3D) graphene have been demonstrated and fabricated to thermoelectric PDs with response to terahertz bands separately. Herein, we develop thermoelectric PD based on 3D graphene and perovskite hybrid material with good IR performance. The responsivity at 1 V bias reaches to 0.1 A/W, under the illumination of 1064 nm laser with the power density of 3.1 mW, corresponding noise equivalent power (NEP) 1 nW∙Hz-^1/2 , the rise time 10.8 ms and fall time 12.8 ms, respectively. These results demonstrate these hybrid IRPDs show good IR performance, and can provide a reference for other spectral range such as terahertz bands.

A segmented RCS data measurement and processing method was proposed. Attenuation elements were introduced to improve the measurable signal range of the measurement system, and the accuracy was improved by segmented calibration of data. Based on this method, the warhead model was measured in a large dynamic range up to 63 dB. A compact-field radar cross section (RCS) measurement system applicable in the high-frequency terahertz range was built based on a seed-injected terahertz parametric generator (ips-TPG). The reliability of the system was verified by taking smooth stainless-steel spheres as the standard calibration objects and the RCS measurement of common target was performed at a high frequency point at 5 THz. The error between the measured and theoretical results was less than 4 dB.

The manufacture and use of bronze ritual vessels, as far back as the second and the first millennium BC, is an essential tool in maintaining social order and political structure in China’s Shang and Zhou periods, yet the casting techniques that made them have not been well discussed. The core issue is the manufacturing and use of clay moulds. Not only their design and positioning, archaeological studies show that their material property is also a key to the success of casting such products. The present work, among a series research, is mainly focused on the thermal property analysis of the ancient bronze casting moulds and two other contemporary moulds for comparison. Thermal diffusivity is measured with pulsed thermography as a non-destructive method. The results prove that the thermal diffusivity of ancient bronze casting moulds are relatively low which allows liquid bronze to flow for an extended duration, filling all fine patterns and corners. The difference between the three types of moulds are discussed.

During the pandemic Covid-19, infrared thermography is an efficient way to detect susceptible persons with abnormal temperatures. However, two significant factors seriously limit the temperature precision: measurement uncertainty of the infrared camera and model inaccuracy of thermal radiation. In this paper, we propose the joint maximum a posterior (JMAP) approach with a new hierarchical prior model. The advantages of JMAP are that the Bayesian inference can combine prior model and likelihood model to regulate the uncertainty from both physics and measurements. At first, we obtain the estimated parameters of the thermal radiation model from training-data. We propose that the distribution of actual temperature and the distribution of measurement error satisfy the Gaussian distribution. We take the variance of the Gaussian distribution as the latent variable and assume that the variance satisfies the inverse gamma distribution, which we control by setting hyperparameters, which determine the uncertainty of the temperature variance so that variances are updated continuously rather than constants. We apply the JMAP inference to test human-face temperature for infrared camera calibration. Although the pre-set parameters of the cost-effective infrared camera badly affect the accuracy, our proposed approach can refine the error of less than 0.1 ℃ compared with 1.0℃ without calibration at certain distances. Therefore, our proposed approach can offer an efficient and accurate way to screen people with abnormal body temperature against Covid-19.

In this paper, we demonstrated a passive Q-switched 2μm dual-wavelength intra-cavity PPLN optical parameter oscillator (OPO) with the advantages of compactness, large tuning range, and narrow linewidth. By using c-cut Nd:YLF crystal as the laser gain medium, the output power of the fundamental laser at 1053nm can reach up to 0.916W, corresponding to the incident pump power of 9.79W. When the temperature of PPLN crystal was varied between 40°C and 160°C, the orthogonal polarized dual-wavelength lasers can be tuned from 2026.4nm to 2193.9nm. As to the a-cut situation, the output power at the fundamental wavelength of 1047nm is about 1.192W. In the same temperature range, two orthogonal polarized wavelengths can be tuned from 1991.8nm to 2206.8nm. Such type-Ⅱ OPOs have potential abilities to work for sensing, spectroscopy, imaging, nonlinear frequency conversion and so on.

Chiral metasurfaces can realize strong chiral optical responses and various spin-dependent electromagnetic manipulations. In this paper, we propose a metal-graphene hybrid metasurface that achieves active control of the amplitude and wavefront of reflected circularly polarized terahertz wave. By introducing a single layer graphene at the bottom of the patterned metasurface, combined with the silicon layer for voltage control, continuous change of the circular dichroism (CD) from 0 to 0.6 is obtained. Then we realized dynamic switching of the terahertz beam from anomalous reflection to vertical reflection using Pancharatnam-Berry (P-B) phase. This tunable chiral metasurface provides new ideas for the design of terahertz devices.

An adaptive equivalent focal length optimization algorithm is proposed. With a complete 4π stereo angle wide field of view, this kind of imaging distortion correction simulation system is established, and the multi-level effectiveness strategy is developed. And the simulation results based on this algorithm are given in many environments.

In the process of using infrared radiation measuring equipment, it is generally required to calibrate the linear response of the linear response range of the infrared focal plane detector. The calibration process generally calculates the grayscale output value of the device by measuring the standard blackbody radiation source with different known temperatures, and calculates the linear response of the device by the fitting method. In the actual calibration process of the external field, since the calibration temperature of the blackbody radiation source cannot be set lower than the ambient temperature, the equipment cannot perform measurement calibration on the black body below the ambient temperature, resulting in the lower limit of the linear response interval of the device cannot be confirmed, lower than the environment. The extrapolation calibration of the temperature may have a large error and the error cannot be evaluated. In this paper, a calibration method for low temperature section of external field equipment is proposed. The experiment proves that the method has high calibration accuracy.

The high-frequency waves-Terahertz (THz) emission from laser-induced air-plasma is presented. The development of high power and high efficiency THz wave radiation source has significant value. The development of laser technology has led spectroscopic sensing and imaging in the THz region to be widely employed in diverse fields such as material characterization, ultrafast dynamic process studies, and biological science. The emission of THz waves has become one of the most important areas in THz research. In this research, we develop an THz time-domain spectroscopy system(THz- TDS), which is that the input laser pulses are generated from a p-polarized regenerative Ti:sapphire amplifier with a 50- fs pulse duration centered at 800 nm. A 100-μm-thick type-I BBO crystal is employed for frequency doubling the pump pulses and subsequently generating the THz wave. By this setup we investigate that when the terahertz emission from only fundamental wave and with second-harmonic wave mixing, the intensity of the terahertz wave obtained is different. The energy of THz wave generated by only fundamental wave is weak, and that generated by four wave mixing is stronger. The energy intensity of THz wave generated by four wave mixing is much higher than that of THz wave generated by fundamental wave (almost an order of magnitude higher). We believe that the different ionization modes causes the different of terahertz wave energy intensity. In addition, we find that the second harmonic wave grows in a quadratic curve with the increase of pump energy, so we believe that when the pump energy increases, the terahertz wave also grows in a quadratic curve.

We analyzed the reasons why ZnTe generates terahertz echo. Based on the Gaussian beam, the terahertz signal with echo is produced by simulation. In this paper, the deconvolution algorithm is further improved. The linear assumption that sample absorption is proportional to frequency is abandoned. We studied the method of echo based on Echo State Network. Set 200 neural units in each layer of the network. We specify 250 rounds of training. To prevent the gradient from exploding, set the gradient threshold to 1. Specify an initial learning rate of 0.005. After 125 rounds of training, multiply it by a factor of 0.2 to reduce the learning rate. This method largely removes echo. But it has not been completely removed, and there is still less than 5% of the echo signal remaining. We consider the reason may be that the number of neurons, the number of iterations, weight setting and other parameters have yet to be optimized. Although this method does not completely remove the echo, but provides a new method for removing the echo. By optimizing parameters such as the number of neurons, the number of iterations, weight setting and increasing the amount of training data, this method is expected to become a new method for echo removal.

This paper proposes a novel terahertz metamaterial modulator utilizing vanadium dioxide phase-change materials, which adopts three nested square split rings as a unit cell. The modulator can achieve dual functions of band-stop filter and bandpass filter by vanadium dioxide filled in the gap of split ring changing phase. The simulation results show that the central frequency of the modulator is around 0.83 THz, whether the modulator is at band-stop state or band-pass state. The modulator shows that the -3 dB bandwidth reaches to 30 GHz at the state of band-pass, and that the -20 dB bandwidth reaches to 160 GHz at the state of band-stop.

Montmorillonite plays an important role in mineral research, pharmaceutical research and treatment of polluted wastewater. The purpose of this study is to detect different porosity, thickness, calcined products of montmorillonite and to distinguish different types of montmorillonite by terahertz time-domain spectroscopy combining with other conventional analytical methods. In addition, optical parameters such as absorption coefficient, phase difference and refractive index can significantly characterize different porosity, thickness and calcination temperature. Therefore, it is of great significance to use terahertz time-domain spectroscopy for nondestructive testing of montmorillonite.

In this article, we propose a novel ultra-wideband, polarization-independent and wide-angle metamaterial absorber for the long-wave infrared region (8~14μm). The topology in this design is obtained by directly covering the surface of photoresist discs with a dielectric-metal-dielectric film, where the substrate supporting discs is a metal ground plane. The topology period is only 1.29 μm and the metamaterial absorber thickness is 1.305 μm. The broadband absorption of the entire long-wave infrared region mainly comes from resonance absorption at two wavelengths of 8.92 μm and 12.4 μm, and these two resonance absorptions are generated at different positions of the topology. The peak absorptions of the metamaterial absorber at 8.92 μm and 12.4 μm are 93.7% and 98.5%, respectively, and the average absorption in the infrared region of 8 to 14 microns is 91.6%. We calculate that the average absorption of the metamaterial absorber for TE mode still exceeds 85% up to an incident angle of 40°. However, for the TM mode, as the incident angle increases, the absorption increases. When the incident angle is 40°, the average absorption exceeds 95%. This metamaterial absorber with high performances in both bandwidth and absorptivity shows a promising prospect in applications such as thermal emitters, thermal coolers, and microbolometers.

This Conference Presentation, “A tunable broadband and graphene-based THz absorbers,” was recorded for the Photonics Asia 2020 Digital Forum.

This article uses terahertz (THz) spectroscopy combined with Ramp-SVM to distinguish different sources of traditional Chinese medicine. The spectra of four different herbs (Curcuma Wenyujin, Curcuma phaeocaulis,Curcuma longa,Curcuma kwangsiensis) were obtained in the range of 0.5-2THz. Apply principal component analysis (PCA) to reduce the dimensionality of the original spectral information. Three classification algorithms, Support Vector Machine (SVM), Extreme Learning Machine (ELM) and Random Forest (RF) are used to distinguish herbs. Compared with the above models, Ramp-SVM has good robustness and high accuracy. The confusion matrix is combined with the classification accuracy to evaluate the performance of the three classification algorithms. The Ramp-SVM method achieves 95% prediction accuracy. The experimental results show that the combination of terahertz spectroscopy and chemometric algorithm is an effective method to quickly identify Same-based Chinese Medicine.

The frequency range of the terahertz wave makes it have partial characteristics both of microwave and light waves. This also makes the terahertz wave has the advantages of strong penetrability and small influence on the detected object when applied in the field of velocity measurement compared with other non-contact velocity measurement technologies. We designed and built a non-invasive, high-precision, free-space terahertz wave Doppler velocimetry (TDV) , and amplified the Doppler frequency difference to solve the problems such as incomplete terahertz wave interference fringes and low velocity resolution due to the long wavelength of terahertz when TDV applied in the diagnosis of detonation process and other instant process with high acceleration. Combined with the velocity measurement model introduced by our optical system and terahertz interference velocity measurement method, we used high-resolution TDV to measure the flying process of flyerplate. Then we completed the extraction of flyerplate velocity and displacement at velocity up to 1.2 km/s, and proved the high velocity resolution of measurement results. The terahertz Doppler velocimetry combined with the frequency difference multiplication provides a non-destructive, non-contact, high-resolution measurement method for the research of detonation and shock wave physics, internal vibration detection of large equipment, and many other fields.

When using near infrared spectroscopy to predict the moisture content of potato leaves, a large amount of spectral data needs to be processed, resulting in a time-consuming and labor-intensive calculation process. This paper proposes to use a variety of feature wavelength extraction methods to reduce the amount of calculation of near-infrared spectral data, and according to the comparison of prediction results, the feature wavelength extraction method with the best extraction effect is obtained. First, the spectral reflectance information of 110 fresh potato leaves in the 900~2100nm band is collected, and then the Regression Coefficient (RC), Principal Component Analysis (PCA), first-order derivative correlation extraction are used respectively Method, extract the characteristic wavelengths from the full-band spectral data, and finally establish a BP neural network prediction model according to the characteristic wavelengths extracted in three different ways, and compare the prediction results to obtain the optimal characteristic wavelength extraction method. The results show that the BP neural network model established by the characteristic wavelength extracted by the Regression Coefficient (RC) has the best prediction effect, the prediction set decision coefficient (R2) is 0.9698, and the root mean square error (RMSE) is 0.3177. In this experiment, on the basis of reducing the amount of near infrared spectroscopy data by more than 90%, a good prediction effect was achieved, and the purpose of quickly and concisely predicting the moisture content of potato leaves was achieved.

Terahertz (THz) time-domain spectrometer is used to analyze the response of tea and insect foreign bodies to distinguish foreign bodies in tea. Collect the absorption coefficient and dielectric constant of green tea and foreign matter, and analyze the spectral characteristics of the foreign matter. Principal component analysis (PCA) was explored to find clustering patterns. It can be found that terahertz spectroscopy can effectively distinguish insect foreign bodies in tea. It provides a new method for detecting insect foreign bodies in tea.

In order to achieve infrared imaging spectrometers for environmental gas monitoring in the medium-wave infrared and long-wave infrared spectra, meanwhile detecting the characteristic spectra of a variety of ambient gas molecules, it is necessary to do research on the design of the infrared wide spectrum (3.2μm～14μm) imaging spectrometers optical system with a large relative aperture. First of all, the lens power of reasonable distribution will be available by solving of the wide spectrum correcting chromatic aberrations equation and athermalization equation. Secondly, the list compares the common infrared optical material in the infrared wide spectrum dispersion characteristics and thermal characteristics differences, optimizes the three infrared optical materials of the Germanium single crystal, wide spectral Zinc sulfide, and Chalcogenide glass for transmission optical system design. Thirdly, using CODE V optical design software to optimize the optical parameters, the system uses four lenses, introduces four aspherical faces, the rest are spherical. Finally, the modeling gives the 3D layout of the system, and the image evaluation and tolerance analysis of the optical system are carried out. The results show that the modulation transfer function (MTF) value of the optical system at the spatial frequency 30lp/mm is greater than 0.5. The average square root (RMS) value of the diffused spot diameter is less than 17μm; The working band is 3.2μm ~14μm, F number 1, the optical system in the temperature range of -40°C ~+60 °C has a good imaging quality. The optical system has the characteristics of large relative aperture, wide working band, good process and compact structure, and can be used for 640×480 the infrared wide-spectral segment focal plane detector.

Nanoparticles are a favorable way to enhance ionization for ultrashort laser pulse focusing on liquid targets. We experimentally investigate how nanoparticles affect the THz generation from deionized water for different concentrations. However, no obvious enhancement is observed comparing with the signal from deionized water under the same excitation condition. Thus, the concentration of nanoparticles is not a crucial factor to enhance THz wave generation within the order of 10^8~10^11 particles/ml. We believe our results provide useful information on enhancing the liquid THz emission source by nanoparticles

For spatial-modulation imaging spectrometers, the conventional non-uniformity correction method cannot efficiently reduce the pattern noise caused by stationary interference fringes on the imaging plane. In this study, we apply optical the defocus method to realize scene-based non-uniformity correction. By applying severe defocusing, the diffraction effects can be neglected, and the point spread function of the system is equivalent to the geometric projection at the exit pupil. When the light of each spot is within the field of view, it illuminates the entire detector, and the detector obtains uniform irradiance. We can implement a single-point calibration or single-plus-two calibration using the uniform irradiance.

The rotational mode and vibrational mode of many biomolecules are all located in terahertz band, and the photon energy of terahertz wave is low, which is only millivolt level, and will not damage the internal structure of molecules, so terahertz time-domain spectroscopy technology has a good application prospect in biological detection. In addition, most biomolecules can only exert their biological activity under liquid conditions, so it is necessary to study the interaction between biomolecules in liquid environment. In the study of the interaction between biomolecules and terahertz wave in solution, minimizing the absorption of terahertz waves by water molecules has become a research hotspot in recent years. The microfluidic chip reduces the absorption of terahertz waves by reducing the distance between the measured sample and the terahertz waves, and has important application value in the detection of biological samples. By changing the concentration of hydrogen ions in the aqueous solution, different pH values can be obtained. In this paper, Zeonor 1420rs, which has high transmission rate to terahertz wave, is selected as the material of microfluidic chip, and the microchannel depth is 50 μm. Then we combine terahertz technology and microfluidic technology, and prepare three kinds of strong acid solutions with different pH values. By analyzing the terahertz time-domain and frequency-domain spectra of strong acid solutions, we can explore the influence of different pH solutions on terahertz wave absorption, and provide necessary reference for the application of terahertz technology in the field of biochemistry.

Terahertz (THz) waves, referring to electromagnetic waves with frequency range from 0.1 THz to 10 THz, have many novel and unique properties for a wide range of applications from security to medicine. However, it is relatively difficult to study the physical and chemical processes within this band due to limitation of sensitive terahertz detectors. As one of transition group metal oxides, two-dimensional van der Waals α-MoO3 crystal has relatively high carrier mobility and strong phonon polaritons effects. The relatively weak van der Waals forces between adjacent layers allow various atoms to easily enter into the crystal and interact with the lattice to produce defect states, enabling excellent optoelectronic conversion characteristics. In this study, we fabricate terahertz detectors based on hydrogen atom-intercalated α-MoO₃ two-dimensional crystal integrated with log-periodic antenna, which operate from 0.25 THz to 0.37 THz with a maximum optical responsivity of 0.04 V/W.

Water molecules are polar molecules, and intermolecular hydrogen bonds interact strongly with terahertz waves, which makes it very difficult to study the dynamic characteristics of biological samples in aqueous solutions by using terahertz technology. Microfluidic technology is famous for its ability to accurately control micro scale fluid, and its channel depth can reach 50 μm or even smaller. Because microfluidic technology reduces the propagation distance of terahertz wave in the fluid, it greatly reduces the absorption of water to terahertz wave. In addition, pH value also has a certain impact on biological activity. In this study, different pH values are generated by changing the concentration of hydrogen ions and hydroxyl ions in aqueous solution. THz technology and microfluidic technology are combined to explore the THz spectrum characteristics in different pH value of strong acid and alkali solution, and analyze the influence of pH value on the absorption characteristics .

Enhance sensitivity of the terahertz (THz) wave sensor by the THz wave control characteristic based on artificial electromagnetic meta-materials, is a hot spot in current research. The performance of split ring resonators (SRRs) THz sensor on Mylar substrate is numerical simulated with COMSOL Multiphysics software in this paper. The two key characteristic parameters of the THz sensor are analyzed with COMSOL Multiphysics software, which are the thickness of the Mylar substrate and the gap width of the gold SRRs. By comparing the relationship between different parameters and the corresponding resonance frequency curve, the optimal parameters of the THz sensor are obtained which are 10 microns for the Mylar substrate thickness, 2 microns for the gap width of gold SRRs. Then the optimized THz sensor model is used to test the different thickness and relative dielectric constant material with the COMSOL Multiphysics software. The results show that the resonant frequency (f₀) move to lower frequency with the increase of the relative dielectric constant of tested materials, and has a good linear relationship between the f0 and the relative dielectric constant of tested materials. While the f₀ also move to lower frequency as the thickness of tested materials are increase, but when the thickness are more than 5 microns, the resonance peak stay at near 1.997 THz. They are show that the SRRs THz sensor on flexible substrate has high measurement precision, sensitivity, and reliability when the tested materials are common metal, semiconductor and medium layer which layer thickness is less than 5 microns, and the relative dielectric constant for the range of 1-4 by the linear relationship between the resonance frequency and the different parameters of tested materials, as well as it can provide theory references for actual testing of such terahertz sensors.

Recent years, polarimetry in the terahertz frequency range has gained popularity. Polarimetry is a technique used to measure the polarization state of electromagnetic waves transmitted through samples. The ellipticity angle, the azimuth rotation angle, complex optical properties of materials can be obtained by terahertz timedomain polarimetry. This allows for obtaining more comprehensive information about the object. In this paper, we study diagonal and off-diagonal components of the permittivity tensor of thin bismuth-based films using terahertz time-domain polarimetry

In this paper, a novel strategy for diagnosis of bTBI is proposed by measuring terahertz (THz) spectroscopy of serum in a rat model. A compressed air-driven shock wave generator was used to establish bTBI model of rats. It was validated by a neurological deficit assessment method. The spectral differences of serum for different degrees of bTBI rats have been demonstrated by THz time-domain spectroscopy (THz-TDS). Moreover, based on the THz spectra of serum, the principal component analysis (PCA) and support vector machine (SVM) were performed to automatically identify the degrees of bTBI. The results indicate that THz spectroscopy could provide a sensitive, rapid and label-free diagnosis method for bTBI.

The dusty plasma sheath formed during the reentry process of hypersonic vehicle will interrupt the propagation of the communication signals, which is called the blackout problem. One of the most effective solutions to the blackout problem is to detect with terahertz wave, which refers to the electromagnetic wave with a frequency range from 0.1 to 10THz. Recently, the propagation characteristics of terahertz wave in dusty plasma have been studied based on the analyzed of the dielectric properties. Unfortunately, the influence of scattering caused by ablation particles is neglected in these studies, which can be ignored no more, especially for high terahertz wave. In this work, the propagation characteristics of dusty plasmas are analyzed considering both intrinsic absorption and scattering. Firstly, the electron densities and collision frequencies have been calculated based on the simulation of flow field around a return capsule model, and then the dielectric properties are analyzed. Secondly, the propagation characteristics are calculated and the results show that with the increase of detection frequency, the transmittance of THz wave in dusty plasma increases due to the decrease of absorption. But for higher frequency, the stronger scattering leads to the decrease of transmittance. Moreover, the influences of the flight speed of the vehicle, the diameter and density of ablation particle are also discussed. This research provides a basis for the selection of the best frequency band for the detection of return capsule.

We proposed a novel interference elimination method for CW-THz reflection imaging. Based on the study of interference mechanism in the imaging window, inverse processing was proposed to realize the interference elimination. Theoretical calculation showed that the interference phenomenon was closed related to the characteristics of imaging window. The refractive index and thickness of the window were the two main factors for the interference. Moreover, polarization was also taken into consideration for the CW-THz reflection imaging because the reflectivities of the s and p polarized waves were different from each other. This method was applied in different imaging systems for the demonstration of its feasibility. The high resistivity float-zone silicon (HRFZ-Si) plate was employed as the imaging window in the normal incidence system with a terahertz quantum cascade laser (THz-QCL) operating at 4.3THz. The oblique system employed quartz plate as the imaging window with an optical pumped THz laser operating at 2.52THz. Pork and liquid samples of water and edible oils were chosen as the imaging samples to demonstrate the interference elimination method.

The influence of water vapor absorption is inevitable in optical path from the THz source to detector without nitrogen gas, resulting signal distortion at the tail of THz pulse. In the frequency spectrum, the unwanted water-vapor absorption lines will emerge, making it difficult to identify the possible specific absorption lines of the measured sample near the water-vapor absorption lines. To eliminate water-vapor influences can greatly expend the applications of THz-TDS system in the most common open-air environment. In this paper, we used a Support-Vector-Machine algorithm (SVM) in the recent advanced machine learning technology to recover the actual THz signal by removing the influence of water vapor degradation. The learning and prediction of the water vapor absorption effects is completed by iterative training process of the SVM algorithm. After the SVM model is built, we found that it can effectively eliminate the fluctuations of the THz-TDS signal obtained in the open-air environment, thus the corresponding water-vapor absorption peaks in the frequency spectrum are greatly suppressed. We also compared the signal recovering ability of our SVM algorithm with traditional Back- Propagation(BP) neural network algorithm with the same training data as well as the same training time and found that the SVM algorithm outperforms the traditional BP neural network algorithm. To furtherly verify the generalization ability of our SVM model, the THz signals measured under different humidity are set as the inputs of the SVM model. It turns out that the SVM algorithm can still effectively eliminate the water-vapor absorption effects.

In this work, the stepped heating thermography was used to detect the ancient tomb murals in the early Yuan Dynasty. During the experiment, the heating and cooling processes were collected at the same time. The collected data will be post-processed using methods such as: Thermal Signal Reconstruction, Pulsed Phase Thermography, Principal Component Thermography, Thermal Tomography Method. By comparing the thermal images and results of different processing methods, it is found that different methods have different effects when facing different types of defects in the murals. The Thermal Tomography Method can reflect the depth information of the defects to a certain extent. Each method has its own advantages and disadvantages. In view of the complicated murals, various methods can be combined to more accurately and clearly identify the type of disease and the location and other information, while providing effective scientific means for the protection of cultural relics.

This research uses a two-dimensional photocurrent model to propose and theoretically demonstrate an effective method for generating polarization-controllable terahertz wave from gaseous plasma according to experimental conditions. The simulation results show that in a combined field composed of a linear two-color laser field of 1600 nm+800 nm and a third circularly polarized laser pulse, the phase of the third pump laser is the key factor to control the polarization of the THz wave generated by the emitted air plasma. Theoretical simulation also shows that based on the two-color field, by selecting the appropriate available laser wavelength for the third pulse, the terahertz output can be effectively increased, and the terahertz conversion efficiency is significantly improved, which improves the production efficiency of the lasers. For such an incommensurate three-color femtosecond laser field, the intensity ratios of the fundamental beam and the third laser can also be used to control the polarization state and the direction of the terahertz radiation. Using current laser technology, our scheme can be implemented in experiments.

The study of terahertz band has been widely concerned, and the combination of metamaterials and various reconstruction mechanisms has made great progress in recent years. In this paper, we simulate the effect of splitting the gallium arsenide(GaAs) layer with different conductivity in the twisted split-ring resonator(SRR) pairs structure on the inductive coupling strength. We find that with the increase of the conductivity of the doped GaAs layer, the transmittance of the nonresonant region decreases, the resonance intensity decreases, and the frequency shows a blue-shift. When the conductivity increases to more than 64 S/m, the two resonant dips merge into one, and the inductive coupling gradually weakens until disappears. At the same time, we simulate the current and electric field diagram to confirm our results. Our findings are helpful to adjust the resonant intensity of metamaterials by optical pumping or DC bias in the following experiments, which improves the basic understanding of metamaterials and reconfiguration mechanisms.

Nowadays, the combination of metamaterials and molybdenum disulfide (MoS₂) plays more and more important roles, due to its broad applications in many areas. Here, we propose a novel hybrid structure for terahertz (THz) wave manipulation with the integration of MoS₂ layer and metamaterials consisting of split ring resonators (SRRs) arrays on Si substrate. Compared with the simulation results of single ring resonator and double ring resonators, it is found that the original dual-mode resonance transforms four mode resonance after inserting the inner ring. When the inner ring is rotated, the multi-mode resonance does not change. The resonance intensity decreases and the frequency moves to the low frequency with the red-shift effect, when MoS₂ is added. The hybrid MoS₂-SRRs structure THz modulator has multi-mode resonance, and the interaction between SRRs and MoS₂ is revealed through the analysis of multi-mode resonance.

We demonstrate the active control of resonant frequency in terahertz (THz) metamaterial comprised of split ring resonator arrays (SRRs). It is found that the introduction of different substrates can greatly modify the sensing capabilities of the SRRs structure, i.e., the SRRs designed on PET (polyethylene terephthalate) is more sensitive to THz wave accompanied with higher resonance frequency as well as wider non-resonant region. Furthermore, our simulated findings indicate that THz response sensitivity can be distinctly tuned by changing the gap of SRRs on PET. The mechanism of inductance-capacitance (LC) resonance and dipole resonance is exploited to explain the varied THz transmission responses. Our work infers that the SRRs structure based on PET with low dielectric constant is a significantly better option for biological and chemical sensing applications.