This PDF file contains the front matter associated with SPIE Proceedings Volume 10826, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

In this work, we considered mixtures of ethanol and water in the form of jets as samples for THz generation based on laser-induced filamentation. The dependence of the output energy of terahertz radiation on the concentration of ethanol in water was experimentally studied. It is shown that the energy grows linearly, which can be explained by an increase in the ionization energy due to the linear replacement of low-efficient charge carriers (water) with highly-efficient (ethanol). The dependence of the THz generation on the optical angle of incidence on the mixture jets was also demonstrated. The results of this study can be further used to create universal source of terahertz radiation.

Gelatin is an irreversible hydrolyzed form of collagen having similar amino acid composites with its parent collagen. The hydrogen atoms on the side chains of the long peptide strings can weakly bond with their surrounding gelatin as well as water molecules in aqueous environment, forming a cross-linked 3D matrix. Gelatin has been used as a model for soft tissue phantoms in several medical imaging studies including in terahertz imaging. In the terahertz regime, the dielectric property of the gelatin relies largely on its hydration status and the relationship has been shown to be non-linear. It is therefore essential to study the change of the dielectric properties with respect to the hydration status so as to accurately mimic the properties of fresh biological tissues in the terahertz spectrum. In this work, we studied the hydration status of gelatin with a terahertz time-domain spectroscopy system in the reflection setup. Gelatin gels with different molar weights were prepared with the weight concentration varying from 0 to 33.3%. The complex dielectric constants of the samples were calculated and fitted with an effective medium approximation model. The results provide a quantitative knowledge on the total number of the water molecules affected by each gelatin molecule and how the hydration status is influenced by the concentration and the molar weight.

Analogous to microsphere optical nanoscope, an easy and low-price method of THz imaging is proposed and developed for improving the spatial resolution beyond the diffraction limit. By attaching a 2-4 mm diameter Teflon sphere, a narrow, high-intensity, subdiffraction-waist THz beam with a strong jet-like distribution propagates into the background medium from the sphere’s shadow-side surface and a subwavelength spatial resolution better than λ/3 is demonstrated. Furthermore, the dielectric sphere-coupled THz microscope not only enables far-field, large-area measurement but also characterizes high-throughput and broad-band imaging properties. In addition, the size effect on terajet of dielectric sphere was simulated and shows that its magnification capability and focal length are size-dependent and frequencysensitive.

As a new two-dimensional material, MoS₂ has been extensively studied in terahertz modulation due to its excellent photoelectric properties. Herein, we studied a novel method for label-free detection of the carcinoembryonic antibody based on MoS₂ nanosheets using terahertz spectroscopy. Protein molecules were fabricated on the COC substrates with MoS₂ nanosheets. A self-made simple sample cell was used to get signal of protein molecules in a liquid phase. Computational analysis was performed using a classic sandwich structure model. The refractive index and absorption coefficient were calculated. As a result, the THz signal increased with the protein concentration increasing in 0.2-1.1THz. The cole-cole plot was also investigated. The effectiveness of MoS₂ nanosheets for adsorbing protein molecules has been demonstrated, which can be used to detect carcinoembryonic antigen sensitively in the future.

Terahertz wave generation in a filament pumped by a two-color femtosecond laser has been widely used as a source for nonlinear terahertz spectroscopy. Various optical lasers and gases were explored to further optimize the source with broader radiation bandwidth and higher output pulse energy, whereas few attempts were made for this purpose with longer pump wavelengths. We examine the optical excitation-wavelength scaling behavior of terahertz waves generated from two-color laser filament in air, with particular focus on the terahertz wave frequency bandwidth and polarization. Terahertz wave generation from the photocurrent surge driven by lasers with relatively longer wavelengths yields the shift to higher central frequency with rapid increase in the output energy with increasing pump power, which is theoretically validated by considering the optical wavelength-dependent ionization rate. Furthermore, we find the terahertz polarization undergoes a continuous rotation as a function of the laser wavelength. Our results contribute to the understanding of terahertz wave generation in a femtosecond laser filament and suggest a practical way to control the polarization of terahertz pulses for potential applications.

The terahertz emissions from cobalt ferromagnetic heterostructures are demonstrated. The performances of heterostructures Fe/Au and Co/Pt have been compared. The amplitude of the terahertz emission sensitively depends on the non-magnetic materials. The terahertz emission is experimentally proved to be related to the spin currents by checking the terahertz emission using reversed magnetization and sample orientation. We further studied the Pt-layerthickness dependence of the amplitude of terahertz emission. A theoretical model is developed based on a comprehensive analysis of the spin transport. The efficiency of the spin injection from the ferromagnetic to nonmagnetic layer is evaluated by taking into account the interfacial spin loss. This work reveals that interfacial spin loss needs to be carefully addressed for the design of efficient terahertz emitter based on the ferromagnetic/nonmagnetic heterostructure.

Our recent research work on artificial birefringence and broadband polarization converter in terahertz (THz) functional devices was reviewed in this paper, we proposed the subwavelength dielectric gradient grating structure with artificial high birefringence, broadband and low dispersion, and the dielectric metasurface with line-square compound lattice which can realize polarization dependent EIT effect with a large artificial birefringence effect. On the basis, we presented a compound metasurface and a coupled dielectric-metal grating for broadband THz wave polarization conversion and asymmetric transmission. Moreover, we introduced two-dimensional materials into THz polarization devices, and proposed a switchable quarter-wave plate based on graphene grating and a carbon nanotube attached subwavelength grating for broadband THz polarization conversion and dispersion control. This work has greatly promoted the development and practical application of THz polarization devices.

Multipath channel impulse response is depicted by a very few significant non-zero taps that are widely separated in time. In this paper, different estimation and equalization techniques are discussed which exploit sparse nature of radio communication channels. Various channel estimation techniques are implemented, and a comprehensive comparative analysis is presented for sparse multipath channels. The metrics used for performances evaluation are Bit error rate (BER) and Normalized channel mean square error (NCMSE). Finally, a comparative analysis of adaptive estimation algorithms is demonstrated such as LMS, NLMS, and VSSLMS for both sparse and non-sparse multipath channels.

Here, we proposed the thin-film total internal reflection geometry (TF-TIR) for sensitive material characterization. Equations to extract the material dielectric constant in the TF-TIR geometry was derived. The TF-TIR technique consumes less sample material and provides higher sensitivity compared with the traditional attenuated total reflection (ATR) geometry. The sensitivity of TF-TIR geometry was first investigated by simulation using a 10 μm thick α-lactose thin film as the sample. A THz microfluidic device was fabricated according to the TF-TIR design in the simulation with TOPAS and high-resistivity Si as the top and bottom plate, respectively. The reaction chamber was sandwiched between the TOPAS and Si plates. The device was placed on a right-angle Si prism to realize the total internal reflection. Water and alcohol mixtures were used to verify the sensitivity of the device. Our results demonstrate that the TF-TIR technique has the potential to improve the sensitivity in measuring the dielectric constant of biological samples with THz waves. Our design can be used for THz lab-on-chip devices.

Understanding of terahertz spectroscopic properties of materials is crucially important for applications of terahertz waves in sensing. Spectroscopic properties of water-rich media, such as biological tissues, gels, and aqueous solutions, are strongly affected by the amount and the dynamics of water in them. Terahertz spectroscopical measurements can clarify the dynamical and/or structural characteristics of water molecules in the hydrogen-bond network in these media. We studied the dynamical properties of water around protein molecules and polymers in aqueous solutions using terahertz spectroscopic measurements. Sensing of liquid using a terahertz waveguide composed of a metal rod array will also be described.

As the development of space-based infrared remote-sensing technology, the application value of space-based infrared remote sensing instruments is gradually recognized in the field of military, meteorology, environment, resources and astronomical observation, which increasing the demand of space infrared remote sensing instruments. As the core component of space-based infrared remote sensing instruments, spaceborne infrared detectors have been used more and more, the spaceborne infrared detector technology has also made great progress. In this paper, the development history, current situation and development trend of space-based infrared remote sensing instruments were introduced; the key characteristics of spaceborne infrared detectors applied to pushbroom scanning infrared Imager, imaging spectrometer and staring imager were summarized, such as the photoelectric performance, pixel size, device scale, frame rate, power consumption and package; The applications of spaceborne infrared detectors made of different design of material, ROIC and package were presented, several existed technical problems were pointed out, and some future development requirements of the spaceborne infrared detector were also proposed.

Signal process system composed of visible information process circuit and infrared information process circuit is applied in a space-born multi-spectral Sensor. Based on pushbroom image, the system with external synchronization completes driving and signal processing of detectors including visible, short-wave and middle-wave infrared as well as long-wave infrared. Sampling and processing of analog signal along with integration and format of digital signal are also completed in the system. Multi FPA(focal plane array) synchronization, large number of analog outputs, high signal to noise ratio are characteristics of the systems. Eventually, the system samples 32 analog video signals and outputs 14bit data by 5 serial LVDS channels with a 30Mbps transmission rate in single channel. Under given test condition in laboratory, the results indicate that the SNRs of visible and short-wave infrared channel are 307 and 422, meanwhile the NETDs of middle-wave and long-wave infrared channel are 0.14K and 0.16K. An imaging experiment in VIS channel acquired good image data. The regular test and imaging experiment results validate that the system can meet the demands of multi-spectral sensor application.

The use of terahertz radiation in various fields such as biology and medicine is increasing every year. Meanwhile, people are increasingly concerned about the mechanism on the interaction between terahertz radiation and biological system. In this study, we evaluated the effect of the cellular response of bEnd.3 which exposing to the terahertz radiation in the range of 0.1-3.5 THz. We collected the spectral data of cells during the irradiation with a temperature of 20.6°C and a relative humidity of 5%. Meanwhile, the apoptosis of cell was assessed by Annexin V-FITC/PI kit. As a result, the apoptosis of cells were inhibited from 0 and 6 hours after terahertz irradiation and promoted at 12 hours after irradiation. Moreover, we also calculated the complex dielectric constant of bEnd.3 cells at different exposures. The results demonstrate that the dielectric loss of cells showed a slight decrease with the increase of exposure in the range of 0.2-1.4 THz.

In recent years, an approach of photonic true-time delay (TTD) unit which employs a linear chirped fiber grating (LCFG) and a tunable multi-wavelength laser (TMWL) for phased array antennas (PAAs) has been reported. In this paper, several key parameters of the LCFG and TMWL are analyzed. The beam-pointing resolution is effected by the parameters including the group delay slope of LCFG, the tuning accuracy of the wavelength spacing of TMWL. The relationship between the beam-pointing angle and the wavelength spacing of TMWL is an inverse trigonometric function. So linear output of the beam-pointing angle could be obtained by tuning the wavelength spacing as sine function. Furthermore, the relationship of the beam-pointing resolution and the beam-pointing angle with different tuning accuracies of the wavelength spacing of TMWL and different group delay slopes of LCFG are also analyzed.

With the development of aerospace industry, the detection of the earth itself is no longer limited to satellites outside the atmosphere. As the natural satellite of the Earth, terrestrial observation of the moon has unique advantages such as longterm consistency, integrity and stability. Imaging spectrometer based on the Moon, can improve the ability to observe the earth comprehensively and promote the in-depth study on scientific issues of global change. In this work, in viewing of the perspective of exploring the energy balance of the earth, the location between the earth and the moon as well as observation effect, the advantages and disadvantages of different schemes of the spectrometer component are compared and the system scheme is demonstrated and analyzed. Consequently, with the sub-satellite point ground resolution (5, 10, 20 km) and waveband (8.4-8.9 μm, 10.3-11.4 μm, and 11.5-12.5 μm), the spatial data acquisition method is determined as staring, the spectral data acquisition method is applied to a filter wheel with HgCdTe photoconductive area array detector (15 μm*15 μm) and the optical system aperture (936.96, 468.48, 234.24 mm) focal length (1152, 576, 288 mm) respectively. This work is an initial exploration of the moon-to-earth observation imaging spectrometer, which provides a theoretical basis for the following system implementation.

Peak detection is an important step in the chemical identification by Raman spectroscopy. At present, most peak detection methods have finite identification ability on overlapping peaks, especially for the spectrum measured by the portable spectrometer with low resolution. In this paper, an improved method is proposed that using continuous wavelet transform(CWT) peak detection method based on a new wavelet on the deconvolved Raman spectrum. The new wavelet has smaller linewidth and is more similar with the intrinsic line profile of Raman spectroscopy, Lorentz line profile. So it has advantages on overlapping peaks detection. The proposed method was evaluated by Raman spectrum of solid amino acid mixtures. The results show that it is better at detecting overlapping peaks than the other two wavelets.

Microwave kinetic inductance detectors (MKIDs) are playing an increasingly role in THz astronomy. Superconducting coplanar waveguide (CPW) lines are commonly adopted in MKIDs for the resonators as well as the readout through line. It is therefore of particular interest to characterize the transmission properities of superconducting CPW lines at low temperatures. A cryogenic through-line method based on two cryogenic RF switches is proposed to characterize Nb and TiN superconducting CPW lines. On-chip calibration has been successfully carried out. Detailed results and analysis will be presented.

Theoretical simulations were carried out to evaluate the properties of terahertz (THz) generation in β-BaTeMo₂O₉ (βBTM) crystal by stimulated polariton scattering (SPS) process. The effects of different polariton modes on THz generation were analyzed, from which we determined the optimal crystal design and polarizations of the coupled waves. The dispersion and absorption characteristics of these vibration modes were also given based on the first-principle calculation and correlation Raman spectrum. Finally, the angle phase matching property and THz-wave gain were calculated. Simulation results showed that β-BTM is a kind of potential material for high-power tunable THz generation.

Terahertz dual-comb spectroscopy (THz-DCS) has the potential to be used as universal THz spectroscopy with high spectral resolution, high spectral accuracy, and broad spectral coverage; however, the requirement for dual stabilized femtosecond lasers hampers its versatility due to the bulky size, high complexity, and high cost. We here report the first demonstration of dual THz comb spectroscopy using a single free-running fiber laser. While greatly reducing the size, complexity, and cost of the laser source, THz-DCS maintains the spectroscopic performance comparable to a system equipped with dual stabilized fiber lasers, and can be effectively applied to gas spectroscopy.

Common anti-reflective films on Sulfide Zinc(ZnS) Substrate has low hardness and poor protective effect, thus cannot meet the requirement of harsh environment. Composite anti-reflective and protective films with ZnS, YbF₃ and DLC films are developed. This Diamond-like Carbon(DLC) films with high infrared transmittance and high hardness is developed by quasi-molecule laser in vacuum. The deposition parameters and performance of DLC and YbF₃ films are optimized. The adhesion performance of the composite films is improved by introducing optimized adhesive coating, optimized thickness of DLC films and so on. The composite films has many advantages such as high transmittance in far IR region, good adhesion, anti-scratch performance, it also can pass boil water test, tape adhesion test and scrub test.

Graphene has an extremely weak coupling of electrons to phonons due to its nonionic character of lattice. This remarkable property makes graphene very attractive for hot electron bolometers (HEBs). In this paper, we present the development of a graphene-based terahertz hot electron bolometer (HEB) with Johnson noise readout. The HEB is essentially a graphene microbridge that is connected to a log spiral antenna by Au contact pads. We study the responsivity, noise equivalent power (NEP) and time constant of the graphene-based HEB in a perpendicular magnetic field. In order to understand the thermal transport inside the graphene microbridge, we also measure the graphene-based HEB at different bath temperatures between 3 K and 10 K. Detailed experimental results and analysis will be presented.

The small video camera of the video life detector, which can go into the narrow space that the rescuers can hardly reach, is used to get color video of the trapped people in the ruins after the disasters such as earthquake. Due to the covering of rubble and dust, it is difficult to detect the position of the trapped persons effectively and judge their physical condition from the video, which affects the search efficiency during the golden time of 72 hours for rescue. In this paper, a small dual-band (LWIR/VIS) video camera with common optical path is designed. The dual-band video camera contains a micro color video camera for details of the scene and a miniaturized thermal imaging component which has a micro uncooled infrared focal plane array (IRFPA) as the detector for capturing the location and vitality of trapped people. The field of view (FOV) of the color video camera, is partly matched with the FOV of the IRFPA component through a hot mirror as the visible and infrared optical splitter. In other words, the FOV of the dual-band imaging system is designed in common path. Then a real-time fusion algorithm of dual-band video is implemented on a DSP hardware image processing platform. As a result, the people and other hot targets in the scene are highlighted in the fused video, which can provide a basis for target detection and decision-making in the rescue process.

Recently, terahertz (THz) computed tomography (CT) has emerged as a possible effective technique for 3D structural information detection. However, THz-CT is difficult to be applied to high refractive index object, due to the severe refraction phenomenon occurred during the acquisition of raw data. We propose a novel experimental procedure to solve this problem. Including the use of a sink filled with liquid whose refractive index is close to the sample, and a correction algorithm to eliminate the noise of liquid. The proposed method is applied to the high-density polyethylene samples of different shapes.

In recent years, biotechnology has been widely used in production and living. The research on processing biomaterials and developing them into new functional types has also been gradually carried out. In this paper, the influence of water content of microbial material on the extinction properties of infrared band was studied from the aspects of composition and structure. The sample of moisture content of microbial was established. The qualitative law between the water content of microbial particle and its absorption property was given. The quantitative relationship between the water content of microbial particle and its scattering property was calculated. Under the conditions of strong attenuation, the optimal solution of microbial materials based on water content is designed. The results show that when the transmittance is less than 10%, the complex refractive index n of microbial particles can be changed within the range of 0 < Δn ≤ 0.072 by controlling the amount of intracellular bound water. The controllability and variability of complex refractive index n can improve the extinction performance of microbial materials in the infrared band by up to 50%.

The precise target identification is significant for commanding decisions and enemy identification. The micro-Doppler effect (MDE) can reflect the subtle movement characteristics of the target, which provides a new way for the target detection and recognition. However, the current research is mainly on the micro-motion feature extraction and classification of the targets, which is not capable for identifying the targets of the same type. This also reduced the application of the MDE. In fact, by accurately estimating the micro-motion parameters and combining sufficient prior knowledge, the target can be accurately identified. Further, the MDE detected by laser in infrared band has higher sensitivity and resolution than microwave detection, especially for the MDE generated by weak vibrations. Thus, in this paper, the photocurrent model of the laser detected MDE echo signal is established. The all-fiber coherent laser detection system for target micro-motion is designed. The detection sensitivity of and resolution requirements of the multicomponent micro-Doppler features are analyzed. Based on the time varying auto-regression (TVAR) model, the precise parameter estimation method for micro-motions are proposed, which provides the basis for target identification. The validity of the theoretical analysis and estimation method is verified through simulation. This research is helpful for extending the application of MDE from classification to precise identification in the future.

In this work, we demonstrate a high-selectivity terahertz (THz) band-stop filter with a wide range of center wavelengths (CWLs) from 150 to 600 μm (2.0 THz to 0.5 THz). The dip transmission is lower than 10 % at the center CWLs, even to 5 % at 1.9 THz. The band-stop terahertz filter is based on periodic metallic Cross Cell (CC) structures deposited on the top of a 50 μm thick polyimide film (Kapton) with low absorption and good mechanical properties, resulting in a large-area, freestanding and flexible membrane with a low intrinsic loss. The excellent tunable terahertz band-stop filter properties are investigated using terahertz time-domain spectroscopy. The measured and simulated results are coming to an excellent agreement. The THz band-stop filter possesses not only a light weight and polarization insensitivity but also a simple structure and high integration.

Owing to the similar absorption between the region of interest and background, an inferior contrast may be shown at the terahertz (THz) image with the reconstruction of unbefitting parameters. In this experiment, due to the small difference of THz pulse between the liver tumor tissue and mirror, the parameter with the sum of the maximum and minimum (SMM) value of the THz pulse was used to reconstruct the THz image. We compared it with the pulse imaging of amplitude (Amp) and maximum (Max) value, indicating that SMM image achieved the best result. K-means and fuzzy c-means clustering results of the two combination parameters (Amp and SMM, Max and SMM) further validate the effect of SMM. The differences of THz pulses from three regions explain the reliable imaging results.

Alcohol is a kind of flammable substance. It is strictly forbidden in public places such as airports and high-speed railway stations. So the non-destructive testing of the alcohol is very crucial. In this paper, we used the reflective terahertz (THz) time-domain spectrometer to measure THz signals of alcohol aqueous solutions in different concentrations, the relationship between the pulse peak and concentration is obtained. And then reflectance spectra were obtained by using Fourier transform. Linear regression was employed to fit the relationship between the terahertz reflectance and the alcohol concentration at different frequencies. This pilot work has preliminarily formed a testing method for the detection of liquids concentration by using THz reflectance spectra. In the future, we can establish the THz spectral library for dangerous and inflammable liquids.

Terahertz nondestructive testing is an increasingly important technology in recent years. Compared with visible and infrared bands, terahertz wave can easily penetrate common nonpolar and nonmetal materials without ironize the sample because of low energy. Digital holography can reconstruct the quantitative amplitude and phase distributions of the object wavefront. We proposed a continuous-wave terahertz reflective off-axis digital holography to measure the morphology of encapsulated object. An experimental system was built using a 2.52 THz (118.83 μm) far-infrared gas laser 295-FIR and a pyroelectric array detector. The morphology of the metallic bookmark hidden behind the optically opaque materials such as polytetrafluoroethylene and Polypropylene plates were obtained by angular spectrum integral and phase unwrapping algorithm. It proves that THz digital holography is an effective nondestructive testing method.

Visible and near-infrared spectral reflectances of surface vegetation are basic data for applications in remote sensing classification, multispectral imaging and color reproduction. Leaves are the objects of this study. Firstly, The 400-700 nm visible light spectral reflectance and 700−1000 nm near infrared spectral reflectance data of 12 kinds of trees such as camphor tree, ginkgo tree and peach tree (etc.) are measured by visible and near-infrared portable hyperspectral cameras. The spectral reflectance data is obtained by denoising the using the Minimum Noise Fraction (MNF). Secondly, the Principal Component Analysis (PCA) is used as a method of processing spectral reflectance in the visible and near infrared bands. At last, the correlation analysis is used for spectral reflectance in the visible and near-infrared bands. The obtained data and results provide a theoretical basis for the subsequent establishment of a spectral reflectance data base of surface vegetation spectroscopy and multispectral imaging.

In the spectral analysis for biological fluids compositions, the sensitivity of trace compositions reflected in the spectral is very small, which results in the low measurement accuracy. This paper proposes a “Multi-dimension and Multi-mode spectroscopy method” to improve the measurement accuracy of trace compositions by two aspects of spectral acquisition and data processing: measuring the biological fluids sample at multiple modes so that multiple spectral for each sample can be obtained which will carry more information about trace compositions, then connecting these spectral to increase the spectral data dimension that is equivalent to increase the number of constraint equations in the modeling process, the error will be reduced through more constraint equations. An experiment was designed: taking the cholesterol concentration (2.57-8.1mmol/L) in blood plasma as the tested object, the blood plasma was irradiated with tungsten lamp and ultraviolet light source respectively, ultraviolet light stimulates blood plasma can produces fluorescence, the obtained transmission spectrum and fluorescence spectrum were rearranged to build the model. Experimental results showed that the analysis accuracy of cholesterol had been significantly improved. This research provided a new thinking of the analysis for biological fluids trace compositions.

With the continuous development of spectral analysis technology, on-line spectral analysis technology has been widely applied. It can real-time monitor the key links in the process of liquid transportation, and provide real-time guidance for reliable and efficient liquid delivery. In the process of non-sampling real-time measurement of the solution in a flexible conveying tube, the accuracy of the spectral analysis is reduced due to the differences in optical parameters of the flexible conveying tubes. Therefore, this paper studies the effects of differences in flexible conveying tubes on on-line spectral analysis. A standard solution calibration method was proposed to suppress the interference caused by the differences of flexible conveying tubes based on the modified Lambert Beer's law. The calibrated spectral data is modeled by partial least squares regression to reduce the analysis error introduced by the optical differences of the flexible conveying tubes. An experiment was designed to verify the feasibility of the method by using a mixed solution of Intra-lipid and India-Ink as an analytical sample and using a polyvinyl chloride (PVC) material tube as a flexible conveying tube. The experimental results show that the method of calibrating the differences of the flexible conveying tubes by the standard solution is feasible, and effectively inhibits the impact of the differences of the conveying tubes on the online spectral analysis.

In this paper, a novel terahertz modulator using the phase transition characteristics of vanadium dioxide to achieve both band-pass and band-stop filter states is proposed. The regular hexagonal split-ring is used as a unit structure, and the center frequency can be changed by changing the size of the regular hexagon. The simulation results show that any frequency between 0.1~1THz can be used as its center frequency. In the case of 140 um regular hexagon cell, the band-pass filter state shows that the -3dB bandwidth reaches to 100 GHz, and the band-stop filter state shows that the -20dB bandwidth reaches to 75 GHz. Furthermore, a mirror-type regular hexagonal structure is designed as unit cell, which can improve the modulator performance in both states.

Terahertz imaging technology has been widely used in various fields. In continuous-wave terahertz imaging system, when the large size object is located at the unfocused position, Bessel beam with non-diffractive properties show its large depth of focus advantage over Gaussian beam. Bessel beam can be generated by the axicon, which has high conversion efficiency. The non-diffraction distance and the main lobe size of the Bessel beam depend on the parameters of the axicon and incident light wavelength. We analyzed that the influence on the axial two-dimensional intensity distribution of a zero-order Bessel beam by changing the axicon parameters and the incident Gaussian beam size. Experimentally, the axicon with different parameters were fabricated using different materials. Then the two-dimensional intensity distribution of the Bessel beam in the axial and transverse direction were recorded and analyzed. The experimental results is basically consistent with the theoretical ones.

In this work, we propose and theoretically investigate the first dynamically tunable metasurface based on new twodimensional (2D) material - multi-layer graphene (MLG). As a basis for metasurface development, the results of experimental studies in THz frequency band of 80-layered graphene on dielectric substrate under external optical pumping were used. The metasurface consist of the Polymethylpentene (TPX) substrate and cross-shaped MLG pattern. In addition, the structure of the metasurface is very simple and can be fabricated by chemical vapor deposition and laser engraving. Proposed non-metallic metasurface is high-potential candidate for designing an active THz devises.

The methods of terahertz (THz) wave generation has attracted considerable attention in recent years because it involves a complex plasma process. The theories used to explain the intrinsic mechanism of laser ionization of air to produce THz wave are not particularly numerous, further exploration of its mechanism in the air is necessary. We report the THz wave generation from single color scheme modulated by pre-ionized air plasma via an orthogonal pumping geometry. It is found that the amplitude of the THz signal generated by the pump beam tends to decrease gradually with the increase of the modulation power. We believe that the ponderomotive force plays an important role in the process of the interaction between the pump beam and pre-ionization beam. The hydrostatic state of the electrostatic separation field caused by modulation beam will directly affect the generation efficiency of the THz wave. Our results contribute to further understanding of the theoretical mechanism and expanding of the practical applications of THz wave generation and modulation.

In recent years, the technology of terahertz (THz) source has developed rapidly. Due to the advantages of long-distance generation and non-damage thresholds, THz wave generation from the air plasma excited by femtosecond laser attracted global attention. Since the first time the method was proposed, it has experienced many tremendous developments and leaps. In this letter, we propose a new method to generate THz wave with higher intensity. We demonstrate that the energy of THz wave can be effectively enhanced when the air plasma excited by a vortex beam. The energy of THz wave induced by two-color laser field generated through a 100-mm-thick BBO crystal is recorded by using a Golay detector. By comparing the intensity and spatial distribution of THz wave generated by different orders of vortex beams, we find that the energy of THz wave generated from low-order vortex beams is stronger than that from Gaussian beams with the invariant laser energy. To understand the effect of the vortex beam on the generation of THz wave, we compared the distribution of the focal plane between the Gaussian beam and the vortex beam. We believe this method can pave the way for finding the type of laser beam which can produce a stronger THz wave.

The loss characteristics of the low-frequency terahertz band (0.325THz-0.50 THz) were tracked during sample aging from perspective of loss tangent, amplitude and phase. The attenuation of signal not only increases with the frequency but also with the aging time. Phase decrease with frequency and is also related to aging time. But the change of rate is almost same for all samples. Besides, it is observed that the loss tangent increases with the frequency, which will also increase with the aging time. Thus, investigators may get a deep understanding of rubber’s remaining life by analyzing optical parameter in terahertz range.

The transmittance of chemical vapor deposition (CVD) ZnS can be effectively improved through the processing of hot isostatic pressing (HIP) in inert gas, especially in the waveband of visible light and near infrared. In general, the size of the particles of the polycrystalline ZnS bulk material can be increased after the processing of HIP. This change can definitely affect the optical properties and mechanical properties of the material. In this paper, a two-step method was applied to improve preparation of transparent polycrystalline ZnS bulk materials. The first step was the growth of polycrystalline ZnS through dynamic CVD. Then the grown polycrystalline ZnS bulk materials was annealed in inert gas at 800~900°C for 10～50 h. Experimental results showed that the optical properties and mechanical properties of the materials have been significantly improved. The average transmission rate of the materials was over 74% in the wavebands 8~12 μm and was over 70% in the mid-infrared 3~5 μm. The absorption peak of the materials at 6 μm attributed to Zn-H complex and S vacancy was obviously decreased. Furthermore, results of imaging experiments showed that lenses made of the annealed transparent polycrystalline ZnS bulk materials has excellent performance in optical properties and modulation transfer function (MTF).

A spectrally wide-band terahertz modulator based on monolayer graphene on germanium (GOG) is proposed. Utilizing a homemade THz-TDS (Terahertz-time domain spectroscopy) system, we experimentally demonstrated that the THz modulator can be tuned by a 1550 nm pump beam in a frequency range from 0.2 to 1.5 THz. The average transmittance of THz decreases from 40% to 22% when the pump power is increased to 250 mW, while the absorption coefficient averaged increases from 19 cm^-1 to 44 cm^-1. The maximum modulation depth of the GOG modulator can reach as high as 62% at 0.38 THz and in a frequency range from 0.2 to 0.7 THz, the modulation depth is over 50%. Compared with bare Ge, it was proved that the modulation performance can be moderately enhanced by introducing monolayer graphene. This novel optically controlled graphene based THz modulator provides a feasible method for terahertz applications in communication and imaging.

A system for generating ultra-broadband microwave frequency combs (MFCs) is proposed and investigated. In such a system, a current modulated distributed feedback semiconductor laser (DFB-SL) is utilized to generate a seed resource of MFC, whose comb space can be tuned but bandwidth is relatively narrow. Then, the seed resource of MFC is injected into another DFB-SL for enhancing the bandwidth. The results demonstrate that, after being injected into another DFB-SL, the bandwidth of seed resource of the MFC can be enhanced greatly, and the MFC with bandwidth over 70 GHz can be obtained under suitable injection parameters.

In recent years, THz modulators have been improved by 2D materials, yet facing a conflict between the demand for high modulation depth and the limitation of low pump power. Previously, by pumping continuous-wave laser on a highresistivity silicon wafer, we achieved modulation depth >95% in 0.3-1.5 THz, demanding continuous-wave pump power of 11.8 W. In this work, we added a mono-layer WSe₂ on the high-resistivity silicon wafer by mechanical exfoliation, raising the modulation depth of THz pulse from 20% to 58% under 0.05 W femtosecond laser pump. The modulation depth can be further enhanced by raising pump power. The modulation behavior is most significant from 1.3 THz to >1.5 THz. This modulation enhancement is due to the interface state between WSe₂ and silicon, as well as the direct bandgap of mono-layer WSe₂. If exposed to the air, WSe₂ starts oxidization at a low exciting power of <1 mW. Experiments shows that even if WSe2 is partly oxidized to WO₃, the modulation depth is only slightly weakened, yet still better than the bare silicon substrate. Also, even if WSe₂ does not fully cover the range of THz wave, or if the pump laser beam does not fully cover the range of THz wave, the modulation enhancement is also partly effective. Our work suggests a lowcost way to improve the efficiency of all-optical THz modulators.

Tilted pulse front technique in lithium niobate has been widely used for strong-field terahertz generation in laboratories and with this method, lots of strong terahertz field induced phenomena have been observed. However, for mJ-level pulse energy, focused electric field >10 MV/cm solid state terahertz sources, there are still many scientific and technical challenges waiting to be explored. For real applications, the properties of intense terahertz source is very important, such as spatial chirp effect. In this work, we systematically investigate the spatial dispersion of intense terahertz generation process in lithium niobate. We also observe obvious non-uniform spatial terahertz frequency distribution with respect to the emission plane using a knife-edge measurement. Higher frequency generation is obtained when the emission spot is far away from the cutting edge of the crystal, while lower frequency emission is detected when the emission spot is close to the crystal edge. This phenomenon is contrary to the original predicts, of which higher frequencies will experience longer propagation distance resulting in weak contribution. The possible mechanism is the nonlinear distortion effect caused by high energy laser pumping. Our study is very important and useful for building intense terahertz systems with the applications in extreme terahertz science, and time-resolved nonlinear spectroscopy.

The spectral characteristics of the asymmetric double-wire structures have been investigated in the terahertz (THz) range. Based on the THz time-domain spectroscopy system, the spectral of the samples has been measured. It is shown that a transparent peak appears in the transmission spectrum which indicate that a resonance coupling exist between the two metal bars. Furthermore, we study the optical modulation properties when the pump light irradiates on the sample by using optical pump-terahertz probe technique (OPTP). The measured results indicate that the pump light can realize an optical switch effect to modulate the transmittance of THz wave. When adjusting the time delay between the optical pump and the terahertz probe pulses, the transmittance of THz wave varies, indicating the photoexcited carriers in the substrate have great influence on the resonance characteristics of the structure. Our obtained results indicate optical modulation method could provide the functionally potential applications in the terahertz modulators and filters.

Recently, the terahertz biosensors based on metamaterials have attracted much attention due to the fact that metamaterials are sensitive to the local enhancement of electromagnetic field and the changes of the surrounding dielectric environment. In order to obtain the resonances with the high quality factor for biosensing, here we designed, simulated, and fabricated the metamaterial structures composed of a series of the asymmetric “double” circular arc (DASR) structures. The experimental data show there are three sharp resonance dips named Fano resonance in the terahertz transmission spectra. In the previous study of asymmetric double rings, we studied the effect of different cutting widths on the transmission characteristics of terahertz when the samples were placed at 0 degrees. Here, the spectral characteristics and polarization conversion characteristics of the samples after 90 degrees were studied. We found that when the sample rotated by 90 degrees only two resonance dips exist in the transmission spectrum. As the separation of asymmetric arcs gradually increases, two resonance frequencies also show blue-shift. To further analyze the reasons for the changes in the transmission spectrum at different angles of sample placement, we present the surface currents and the electromagnetic field distributions in those structures. Our obtained results indicate the terahertz metamaterial has great potential in application of biosensing field.

The infrared detection technology of ballistic missile based on near space platform can effectively make up the shortcomings of high-cost of traditional early warning satellites and the limited earth curvature of ground-based early warning radar. To estimate the detection range of near space-based infrared system for boost-phase ballistic missile, the background infrared radiation as well as ballistic missiles is analyzed in detail. As for the lack of applicability and accuracy of the role distance algorithm which based on the performance contrast, the wave number to the radiation flux formula is introduced. The detection ranges of skin, plume and tail nozzle for boost-phase ballistic missile at 4.25 to 4.55μm are simulated under various conditions. The results show that the improved algorithm can provide the certain engineering application value for the design of near space-based infrared system.

Hyperspectral imaging technology is used to study methods for diagnosing and monitoring potato diseases, so as to improve the efficiency and accuracy of disease diagnosis and monitoring, and to solve the information acquisition, processing and analysis of potato diseases. The experiment took potato late blight as the research object, using hyperspectral image acquisition system to collect the hyperspectral image data of potato plants for different days. After disease inoculation, in the hyperspectral image data in the range of 366-976 nm, feature extraction method was used to select the image corresponding to the characteristic wavelength and optimal principal component image. Hyperspectral imagery of potato plants was used to obtain spectral reflectance at different disease stages, and then the disease incidence trend of inoculation of late plague was studied to realize disease monitoring. The research results show that using hyperspectral imaging technology can achieve rapid and accurate diagnosis of potato disease.

BeiDou Navigation Satellite System (BDS) is one of the notably independent Global Navigation Satellite System (GNSS) devised in China. As of the year of 2018,BDS is still operational for regional navigation customers due to its incomplete engineering of global satellites constellation. BDS has a unique function of short message communication. Based on short message communication, low Earth orbiting spacecrafts (LEO S/C) could send imminent information to ground command center and scientific application center as promptly as possible. It is expected that the third phase of BDS will come into being by the end of the year 2020 or so. The third phase of BDS being finished, there will be 19 satellites which have functioning of short message communication; five out of 19 satellites are in geostationary orbits, whose footprints are located in geographic eastern longitude of 58.75deg ,80deg, 110.5deg, 140deg , 160deg; and the other 14 satellites are in middle Earth orbits. With such a modernized GNSS, LEO S/C will find an approach to forward imminent information to ground command center and scientific application center almost in real time, which will greatly improve the observation efficiency of LEO S/C. For LEO in three typical sun-synchronous orbits of 600km,800km,1000km, simulations are presented to demonstrate the advantages of LEO S/C under the co-elaboration with modernized BDS, respectively.

This paper discusses a kind of photonic switch, it’s consist of the WBG LED and photonic diode. Because the light pass through the air and plastic in this kind of photonic switch, so it is difficult to realize in the actual practical application. But we use WBG LED and photonic diode realized this kind of photonic switch. It is very stable and reliable.

This paper discusses a kind of Nuclear Fusion Reactor, the 4 NFR is equal to 600MWatt or 1,000MWatt generator set. It’s consist of the two spherical gas storage tanks and metal and concrete shell and electronic control system which has three subsystems, the IR detectors are used to measure temperature, and the main electronic control system use the photodiodes and fiber to communicate with desktop computer. The ratio of the two spherical gas storage tanks volume is exactly, so the main spherical gas storage tank will have the enough pressure to occur the nuclear fusion reaction by charging the pure helium. The electronic valve which connect the main spherical gas storage tank and the sub spherical gas storage tank is controlled by the electronic system with several needed temperature and pressure sensors. The sub spherical gas storage tank will charge one time to the main spherical gas storage tank by the instruct of electronic control system, and the size of the liquid hydrogen that is the nuclear fusion reaction material will be sized exactly to excite the nuclear fusion reaction occurring. We will discuss details below.

In this paper, a mid-/long-wave dual-band detector with N-M-π-B-π-N sturcture was developed based on type-II InAs/GaSb superlattice was fabricated by adopting the dual-band structure. The dual-band detector epi-layer presented high crystalline quality of epi-layers. The two channels, with respective 50% cut-off wavelength at 3.5 μm and 11.8 μm were obtained. The peak quantum efficiency (QE) of mid wavelength infrared (MWIR) band and long wavelength infrared (LWIR) band were 22% at 2.7 μm under no bias voltage and 23% at 9.1 μm under -180 mV, respectively. The resistance under 0 and -180 mV of applied bias were 1.7×104 Ω·cm2 and 97 Ω·cm². Due to the high resistance of long wavelength infrared channel, the specific detectivity of LWIR band maintains above 10¹¹ cm·Hz^1/2/W from 4.5 μm to 12.6 μm under - 180 mV at 77K. Finally, the thermal images of both channel were taken by the fabricated FPA.

The GaAs-based HEMT device is prepared by MBE. The device resistance is 300Ω. The structure size of the bow-tie antenna is simulated and optimized with HFSS. The optimization process uses a lumped source feed to realize the impedance matching of the bow-tie antenna and the two-dimensional electron gas (2DEG) channel. The optimized antenna gain can reach 6dB and the voltage standing wave ratio (VSWR) is 1.35. The VDI company 0.3THz terahertz source is used to measure the device. The measurement results show that the device has good polarization characteristics, the signal to noise ratio (SNR) is 80, the detection response rate is 5.11V/W, and the response rate is in the same order of magnitude as the theoretical calculation.

Near-infrared InAs/GaSb Type-Ⅱ superlattices is widely used in biomimetics, sensing, color-imaging technology and other applications. An antireflection coating(AR coating) can help it perform better, making the infrared photodetector a higher responstivity and also a higher quantum efficiency. We produce a broadband AR coating by plasma-enhanced chemical vapor deposition(PECVD) then using the lift-off technology making no damage without any change in the usual Infrared detector process flow, a 260 nm SiO₂ AR coating is transform onto the surface of the infrared photodetector. After using the AR coating, the antireflection can provide up to 40% light gain, while the average reflectivity of the surface of InAs/GaSb type-Ⅱ superlattice is decreased from 33% to 14%. The responsitivity is increased obviously.

We propose a novel absorber with over 90% absorption and is not sensitive to polarization at mid-infrared frequencies (from 31.37 THz to 34.14 THz, 2.77 THz broadwith). The structure of the graphene absorber is special in that it has MgF₂ and SiO₂ multilayer stacking unit cells and arrayed on an Au film plane, which can be easy to fabricate under current manufacturing technology. We can adjust the position of the absorption bands by tuning the Fermi energy without changing the geometric parameters of the complex three-dimensional structure. The tunability of this metamaterial absorber can be achieved via changing the external gate voltage to modify the Fermi energy of graphene. Simulation results demonstrate that the absorption efficiency of the proposed structure can be as high as more than 90% from 28.7THz to 34.14 THz with variation of the Fermi energy from 1.6 eV to 2.0 eV. At the same time, we can also change the Fermi energy to achieve high absorption or high reflection of the absorber.

In this paper, we are report an avalanche photodiodes (APD) with an InGaAs absorption region and an InAlAs avalanche region. Devices are designed with separate absorption, grading, charge, and multiplication (SAGCM) layers on InP substrates, which are demonstrated to detect 1550 nm wavelength light. The epilayers of the APD devices are grown by a Veeco Gen 930 MBE system. The quality of epilayers is good which shown in the surface morphology characterized by AFM. The root mean square (RMS) of surface morphology is only 1.4Å.Operating at room temperature and in the linear mode, the APD achieved a dark-current level of 2.7ｕA/mm²,and a maximum gain of M>300 is demonstrated.