This PDF file contains the front matter associated with SPIE Proceedings Volume 11196, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

A typical plasmon-induced transmission metamaterial and its corresponding complementary structure is designed and explored. The results show that the transmission spectra of the two structures are complementary, but the reflection spectra of the complementary structure and the transmission spectra of the ordinary structure are in good agreement. Furthermore, this work integrate photosensitive silicon into the complementary metal-based metamaterials, realizing the optical active control of the plasmon-induced reflection and transmission. This research strategy provides a new way for the study of reflective structures. Moreover, the active control of the reflection and transmission play a role in slow light devices and terahertz filter.

Fourier phase gratings play a vital role in the multi-beam heterodyne receiver in sub-millimeter astronomical instruments. In this study, a 1×4 beam grating at 660 GHz is developed, by which the surface structure is generated with an iterative algorithm. Far-field beam pattern is simulated with FEKO, where a relative high efficiency of 91% as well as a uniformity of power distribution among 4 beams of less than 1% are obtained. The grating was manufactured in aluminum material by a micro-milling machine. A PC-controlled scanning stage is employed for the beam pattern measurement. Despite the discrepancy from the manufacture of less than 6 μm, measurement results exhibit a good agreement with simulation in both power efficiency and far-field spatial distribution.

Here, we propose an effective classification strategy for THz pulsed signals of breast tissues based on wavelet packet energy (WPE) feature exaction and machine learning classifiers. The parafin-embedded breast tissue samples were adopted in this study and identified as tumor (226 samples), healthy fibrous tissue (233 samples) or adipose tissue (178 samples) based on the histological results. Firstly, the THz pulsed signals of tissue samples were acquired using a standard transmission THz time-domain spectrometer. Then, the signals were decomposed by the wavelet packet transform (WPT) and the features of the WPE were extracted. To reduce the dimensionality of extracted features, the principal components analysis (PCA) method was employed. Six different machine learning classifiers were then performed and compared for automatic classification of different tissue samples. The highest classification accuracy is up to 97% using the fine Gaussian support vector machine (SVM) approach. The results indicate that the WPE feature exaction combined with machine learning classifier can be used for automatic evaluation of biological tissue THz signals with good accuracy.

We show experimental results on the generation of terahertz radiation in at jets of modified water and various liquids. We compare the change in the efficiency of the THz waves generation with a change in the pH and kH values, as well as the salt concentration in water and the fat level in milk. Dependences of the terahertz radiation energy on the pump pulse energy are demonstrated.

Liquid water was not considered as a favorable terahertz (THz) source due to its strongly absorption in THz frequency range. Recently, it has been experimentally demonstrated that broadband THz wave can be generated from liquid water under the excitation of ultrashort laser pulses. Here, we present the measured result of liquid lines as THz emitters. Selected liquids with different polarity, salinity and temperature on THz wave generation are investigated. Comparing with water with high polarity, lower polarity liquids produce stronger THz radiation with a broader bandwidth. α-pinene, a non-polar liquid produces strongest THz signals among all liquids we tested. Saline solutions produce weaker THz radiation compare to pure water. The THz signals from temperature with 8 ℃, 18℃, and 40℃ show marginal difference under our experiment’s conditions. In addition, sugar, acid, alkali solutions and a variety of milks were also tested. Our observations show that non-polar liquids are in favor than polar liquid as alternative THz sources.

Region of interest segmentation is essential for computer aided application of THz imaging. However, THz images is severely degraded by motion blur, poor resolution and noise. A robust, accurate and time-saving algorithm is in dire need for the ROI segmentation of THz images. Recently, ROI segmentation of THz-TDS images and passive THz images has been widely studied. While the ROI segmentation of THz continuous wave (CW) image is still in its infancy. In this paper, we proposed a hybrid ROI segmentation method for THz CW images. The hybrid method combines block match 3D denoising, fuzzy c-means clustering, morphology operation and canny edge detection. The hybrid method is implemented to two images acquired with a THz CW reflection imaging system. To evaluate the performance of our algorithm, we calculated the accuracy, sensitivity and specificity. As the result indicates, this hybrid ROI segmentation method performs well for THz images.

The terahertz time-domain spectra of sildenafil, metformin hydrochloride and phenolphthalein samples were obtained by terahertz time-domain spectroscopy. The refractive index spectra and absorption spectra of these additives were obtained according to the formula. The experimental results show that these three health supplements have obvious characteristic absorption peaks in the terahertz band. The illegal additives were classified based on support vector machine (SVM). The experimental results show that the established SVM model can accurately identify the check set and identify the additive within a certain range. The feasibility of terahertz time-domain spectroscopy for food ingredient detection was verified, which provided a new experimental method for the detection of health product additives.

We demonstrated that the total reflection spectroscopy with the Si container attached on the prism was proved to be a powerful investigative technique. In addition, a new theory was induced to calculate the dielectric responses of several living glial-like cells based on the combination of the single-interface and double-interface ATR model. For the double interface ATR model, the sample was composed of the cell monolayer and the culture medium. For the single-interface ATR model, the sample was composed of the cell monolayer and the air medium after removing all the culture medium. The results showed the cell structure could impact on the dielectric responses in THz region significantly. Furthermore, these promising results suggest that the new method has great potential for the cancer detection in biomedical field.

Real-time detection for living cells in vitro is essential for cell physiology, leading to a strong requirement of low cost and label free biosensors. At present, the terahertz plasmonic metamaterials (TPMMs) are an especially attractive application for biosensing owing to their sharp resonances respond. Compared with traditional biosensors, such as flow cytomertry, the TPMMs biosensors have many unique advantages, containing real-time monitoring, free label and high sensitivity. In this paper, we proposed a TPMMs which is designed by digging out periodically arranged regular hexagonal holes on the metal plate with the thickness of 200 nm. The samples of the TPMMs is used as a platform for detecting liver cancer cell GEP2 concentration at five levels (1 × 10⁴, 5 × 10⁴, 1 × 10⁵, 3 × 10⁵ and 5 × 10⁵cells/ml). The results show that The THz PMMs biosensor cannot distinguish cell concentrations within the orders of magnitude between 1 × 10⁴ and 5 × 10⁴ cells/ml, however, it can distinguish cell concentrations within the orders of magnitude between 1 × 10⁴ and 1 × 10⁵ cells/ml based on the x-polarized reflection spectrum TPMMs biosensor. On the other hand, the transmission spectrum TPMMs biosensor has a significant detectability of the orders of magnitude cell concentration between 10⁴ and 10⁵ cells/ml. The proposed TPMMs biosensor paves a fascinating platform for have been widely applied for cell detection, biotechnology.

To enhance the modulation of the terahertz wave in graphene based modulators, the hollow metallic structures are proposed in our works. With the field localization and enhancement raised by the resonant of the hollow metallic structures, the interaction between the graphene and the terahertz wave is enhanced. It leads to the modulation enhanced when the conductance of graphene is tuned by applied voltage. The samples with hollow square ring array structure and hollow H-cross array structure is discussed with simulations and experiments. The results indicate that the maximum modulation enhancement is achieved near the resonant frequencies.

Here we report a tunable polarization-independent broadband absorber in the terahertz regime. The proposed structure consists of periodic all-dielectric array on a gold substrate, sandwiched by a monolayer graphene and an epsilon-near-zero layer. Simulation results show that the absorption that is independent from the incident polarization remains above 90% over a broadband spectral range from 1.6 THz to 4.1 THz, corresponding to a bandwidth of 2.5 THz and a relative bandwidth of 87.7%. By varying the graphenes Fermi energy from 0.2 eV to 0.5 eV, the absorption bandwidth can be turned from 1.5 THz to 2.5 THz. We expect this polarization- independent absorber with dynamically tunable bandwidth can be used a filters in applications such as terahertz detectors.

PC materials have been widely used in medical equipment, automotive manufacturing, aerospace, electronics and other fields, and its thickness detection has become an important part of its quality control. In this paper, based on the thickness detection requirements of PC materials, the propagation characteristics of THz waves in PC materials are simulated, and a pulsed terahertz reflection system is built. The PC materials of different thicknesses are sampled and their single-point waveforms are obtained as actual detection signals. Using the deconvoluted air signal as input, the optical parameters of PC material extracted by transmissive method, the propagation simulation model of THz wave in different thickness PC materials is established, and the simulation waveform is simulated from three aspects: flight time, waveform and amplitude. Compare and analyze the actual detected waveform.

We propose a novel type of bowtie terahertz antenna based on hyperbolic metamaterials that are composed of multilayers of Indium Antimonide (InSb) and SiO₂. The InSb-SiO₂ multilayers have hyperbolic dispersion at terahertz frequency range. Compared with the conventional bowtie antenna composed of gold, fully-vectorial simulations show that, the localized field enhancement in the proposed structure is 16 times of that for the gold antenna. We further reveal that this great field enhancement attributes to the significantly enhanced out-of-plane electric field component in the hyperbolic metamaterial antenna. We expect this work will find applications in terahertz sensors, detectors, and nonlinear devices.

An efficient terahertz (THz) metamaterial linear polarization converter is proposed. The structure consists of three layers of metal-dielectric-metal, including an anisotropic super-surface based on bimetallic arc structure as the top layer and a dielectric layer and a metal plate as middle layer and bottom layer, separately. The numerical analysis results show that broadband polarization conversion of a linearly polarized wave in the terahertz band is realized using the structure proposed. The polarization conversion ratio of the linearly polarized wave is more than 90% across a wide frequency range from 1.21 THz to 2.82 THz. The ratio of bandwidth to central frequency can reach 80%. The mechanism of high polarization conversion rate and broadband is analyzed. At the same time, the dependence of the polarization conversion rate of the structure on the incident angle is studied. The results show that the structure has good polarization conversion performance in the incident angle range of 0-60°. Compared with previous designs, the polarization converter has not only simple structure, but also wide bandwidth. It has potential application value in polarization modulation of terahertz wave.

A novel nested anti-resonant hollow core fiber (NAHF), based on Topas, with low loss and flattened dispersion is proposed for efficient transmission of terahertz wave. Finite element method with an ideally matched layer (PML) boundary condition is used to investigate its guiding properties. A cladding structure of nested anti-resonant elliptical rings is introduced to reduce mode power leakage. The NAHF shows a low confinement loss (< 0.29 cm^-1 ) and a small effective material loss (< 0.019 cm^-1 ) in the frequency range of 0.9-1.5 THz. An ultra-flatted near zero dispersion profile of ±0.029 ps/THz/cm is obtained within a broad frequency range of 0.6-1.5 THz. Furthermore, optimizing the structure parameters in NAHF, higher core power fraction over 80 %, higher effective mode area of ~10^-6 μm² and the bending loss of 3.05×10^-5 cm^-1 at the bending radius of 10 cm are also achieved.

We propose and demonstrate a low-cost single-pixel terahertz imaging method based on near-field photomodulation and compressed sensing. By using monolayer graphene on a silicon substrate as the photomodulator, and a low-cost continuous-wave laser and digital micromirror device for effective patterned photomodulation, we achieve fast single-pixel terahertz imaging based on the compressed sensing algorithm. We further show that adopting a graphene on silicon substrate leads to deeper modulation depth and thus better image quality than a high-resistance silicon substrate. We expect this work will advance the development of low-cost single-pixel terahertz imaging and promote this technique into practical applications.

Terahertz quantum cascade laser sources with intra-cavity difference frequency generation are currently the only electrically-pumped monolithic semiconductor light sources operating at room temperature in the 1-6 THz spectral range. These devices demonstrated drastic improvements in performance in the past several years and can produce broadband and narrow-linewidth single-mode terahertz emission with power output sufficient for spectroscopic applications. Recent efforts in the wavefunction engineering using an active region design based on a dual-upper-state concept led to a significant enhancement of the optical nonlinearity of the active region for efficient terahertz generation. Dual-upper-state terahertz quantum cascade laser sources exhibit the power of >0.3 mW. Here, we report low frequency generation from terahertz quantum cascade laser sources based on intra-cavity nonlinear frequency mixing. In order to achieve higher nonlinear susceptibility in low frequency region, we design a long wavelength dual-upper-state active region in which transition dipole moments are increased. A fabricated device with distributed feedback grating demonstrates a THz peak output power of 40 μW at room temperature, with multi-mode THz emission at a frequency of 1.4 THz. Besides, a device produces THz output power of >250 μW at 110 K, which is higher output power, compared to low- frequency THz-QCLs at liquid helium temperature.

Microwave Kinetic Inductance Detectors (MKID) are a promising low temperature superconducting detector because of high sensitivity, easy frequency-domain multiplexing and simple structure for large-format arrays. To develop large-format THz detectors for China’s Antarctic THz telescope, we have preliminarily designed an aluminum 64-pixel MKID array operating at the 350 GHz band. In this paper, the characteristics of the MKID array are thoroughly measured.

The inevitable noises and oscillation of generated terahertz (THz) signal subject to mechanism of hardware could distort and affect optical properties of the measured materials. Several well-known techniques used to attenuate oscillation and noises could achieve expectation. In this work, a piece of fractal artwork was chosen as the sample. Double Gaussian filtered inverse filtering (DGIF) and Hanning window inverse filtering (HIF) deconvolution techniques were employed to remove the oscillation of raw signal and ripples of THz B-scan image. A technique of composite multiscale entropy (CMSE) on complexity assessment of time series was introduced to compare the entropy profiles of the raw signal and two deconvovled signals.

Superconductor-insulator-superconductor (SIS) mixers, with nearly quantum-limited sensitivity, have been playing an important role in Terahertz astronomy. For practical THz SIS receivers, however, the measured noise temperatures are sometimes higher than the expected value. The extra noise is mainly due to considerable RF noise contribution from the receiver components such as beam splitter, Dewar window, and infrared filter. In this paper, we mainly present the simulation and measurement results of the three components with different materials and thicknesses. Their noise contributions are also analyzed.

The rapid detection of cancer cells is crucial for clinical diagnosis in biomedical field. The traditional flow cytometry (FC) in visible band, a fluorescence-labelling detection, gives rise to the complicated sample preparation and the irrecoverable antibody consumption; it blocks the development toward a convenient detection platform with fast, inexpensive and non-labelling. Here, a specifically designed metamaterial based on split ring resonators (SRRs) is proposed. Such metamaterial operating in terahertz (THz) range exhibits polarization-dependent resonances, which are observed both in experiments and simulations. Additionally, the biosensing property of the metamaterial is investigated. On metamaterial surfaces, the lung cancer cells A549 are cultured. Under the irradiation of x-polarized THz waves, it is found that for the cell concentrations from 1×10⁵ cells/ml to 5×10⁵ cells/ml, the maximum frequency shift Δf (the frequency difference between measured sample and bare one) at 2.24 THz increases from 15 GHz to 137 GHz, respectively. Such results also imply that a larger cell concentration leads to a higher frequency shift. Subsequently, the samples are further measured at different polarization angles. The results show that for cell concentration of 5×10⁵ cells/ml, the Δf exhibits the same value of 130 GHz when polarization angle equals 30° and 150°, and 15 GHz when polarization angle equals 60° and 120°. Our proposed metamaterial may supply a potential biosensing method for the detection of cancer cells, exhibiting a new insight toward the cancer cell biosensing with certain information of polarization response.

High-density Polyethylene (HDPE), with a density above 0.95 g/cm3, has been widely used in terahertz systems. The advantages of low absorption loss, low refractive index and high rigidity make HDPE an ideal material for cryostat window, focus lens and substrate. HDPE can be machined easily and be used as a substrate material for components such as metal mesh filters and polarizers. What’s more, it is quite inert and can be used at cryogenic temperatures. On account of these applications, we need to characterize the dielectric property of HDPE precisely in a wide frequency range. In this paper, we present the transmittance measurements of a 2 mm thick HDPE sheet from 0.1 THz to 15 THz. Three kinds of measurement methods are employed to cover the whole frequency range. A vector network analyzer (VNA) combined with a quasi-optical transmissometer has been used to measure the transmittance and dielectric constant of HDPE from 0.16 THz to 0.18 THz at 300 K and 4 K. A Time Domain Spectrometer (TDS) is employed to cover the frequency range from 0.2 THz to 3 THz since the VNA can’t work upon 1 THz. A Fourier Transform Spectroscopy (FTS) has been used for the measurement from 3 THz to 15 THz since the TDS can’t achieve broad band and fast scan speed. The measured transmittance of HDPE is nearly 0.93 below 1 THz and decrease to 0.3 when the frequency increase to 15 THz. A rather elusive absorption band at 2.2 THz has also been observed. The dielectric constant of HDPE has been measured by VNA and TDS, showing a frequency independency from 0.1 THz to 3 THz.

Besides the sensitivity nearly approaching the quantum limit, the intermediate-frequency (IF) bandwidth is of particular interesting for Superconductor-insulator-superconductor (SIS) mixers for radio astronomy research. In this paper, we are going to present the characteristic of IF bandwidth of two type of NbN SIS mixers, Long distributed junctions and Parallel-connected twin junctions. Firstly, the relative mixer gain are measured with different IF load impedance (25Ω, 50Ω, 75Ω) for both two SIS mixers. And also the mixers gain with different IF load impedance is simulated to get the optimum IF load impedance over a relative large IF bandwidth (2-15GHz). Finally, an IF matching circuit is designed and measured associating with SIS mixers, the measurement results show that the mixers gain are flatter over a large IF bandwidth than with 50Ω IF load impedance.

The BlackBody Unit (BBU) mounted in front of the IR optical system is located off the optical path to avoid any thermal noise induced by the unit. To perform a thermal calibration of the optical system, a mirror is inserted in the optical path to refract the optical path toward the BBU during the calibration mode. To maximize the performance of the BBU, it is required to fill the Focal Plane Array (FPA) with the thermal irradiance as even as possible. To verify the performance of the BBU during the calibration mode, a ray tracing is performed. As a result, it is confirmed the uniformity of the thermal signal to be smaller than 0.01K when the BBU has the thermal distribution of 1K.

The designed infra-red optical system requires the thermal compensation of 3.5 um according to the environmental thermal variation. To compensate the depth of focus, the mechanical barrel should move not only the amount of movement required but also the appropriate direction. However, the current single shell structure cannot compensate the amount and direction required. To achieve the goal, a multi-layer shell structure is required. However, it is highly limited to select the material appropriate for the space craft. As a result, the goal of 3.5 um and required direction is achieved with multi-material shell structure.

A high sensitive terahertz metamaterial sensor is realized by electromagnetically induced transparency (EIT) analogue effect. This sensor is composed of a metal a split ring resonator (SRR) and a double bars array structure on a flexible polyimide substrate. The physical mechanism of this sensor has been further discussed. Moreover, the resonant peak exhibits red-shift from 1.26 to 1.14 THz as the concentration of the injected ionic solution changes, which makes it a possible application in liquid sensing in terahertz region.

In recent years, terahertz metamaterials have attracted great attention due to their widely application in frequency selector, filter, and other optical devices. Vanadium dioxide as the promising thermal memory material, has been used in making terahertz modulation device because of its well-known insulator-to-metal transition triggered by thermal, optical, and electrical. Here, we designed a composite metamaterial with three rings structure, where outer and inner rings are set to be ideal metallic gold and the middle ring is replaced by VO₂. Using finite-difference time-domain method, we theoretically studied the variety of THz transmitted spectra during the insulator-to-metal transition of VO₂ and its influence on the resonance coupling effect. When VO₂ is in insulating phase, there are two resonant dips in spectrum. However, as VO₂ undergoes an insulator-to-metal transition, the low-frequency resonance gradually moves to high frequency and finally splits into two. Our obtained results indicate the reversible terahertz modulation can be achieved by combining metamaterials with phase transition material.

Metamaterials are artificial materials with unique electromagnetic properties not found in nature and could exhibit a strong electromagnetic resonance with frequency. These characteristics depending on the geometry, dimensions, compositions of the structure, and even the symmetry. In this study, we demonstrated a highly sensitive detection method of glycerol-water solutions using terahertz metamaterials fabricated on silicon substrate with metal arrays of the asymmetric double split-ring resonator structure. We measured the transmitted spectra of the metamaterial to detect the water-glycerol solutions at different concentrations by the terahertz time-domain spectroscopy system. With the increasing of glycerol concentration, resonant dips of metamaterial exhibit the blue-shift phenomena and transmittance at the resonance frequency simultaneously enhance. The movement of resonant dips and the change of transmittance shows that these resonators are sensitive to the variation of glycerol concentration. The finite-difference time-domain simulation agree well with the experimental data, and the simulation of surface current and electric field distribution at resonance dips can further understand the resonance modes in transmission spectra. Our study provides new prospect into the application of terahertz metamaterials in detecting glycerol proportions of solution.

The hydrogen bond (H-bond) in organic-water molecules is essential in nature. Combining with the charge - transfer analyses, we investigated the penetrating molecular-orbitals in glycine-water clusters, which give evidences of the covalent-like characteristics of H-bonds in this system. Besides, the infrared spectral features provide a rare opportunity to discover the exceedingly-evident redshifts of symmetric stretching modes (Symst) in water on forming H-bond, in contrast to the slightly-redshifted asymmetric stretching modes (Asyst) in water. To explain these intriguing behaviors, we further analyzed the nuclear vibrating patterns, which clearly reveal that H-bond retains two unexpected effects on nuclear motions in water: (i) Intensifying donor Symst, and (ii) Inhibiting donor Asyst. Furthermore, we also quantified the impact of anharmonic quantum fluctuations on each hydrogen bond. For the stretching modes involved in H-bonds, red shifts up to more than one hundred wave numbers are observed under anharmonic vibration, explicitly indicating the increased ‘covalency’ of H-bonds. These finds shed light on the essential understanding of H-bonding comprehensively, and should provide incentives for future experimental studies.

In recent years, terahertz metamaterials have attracted extensive attention because of their high sensitivity to electromagnetic waves in biosensing application. In order to obtain high-quality factor resonance, we designed and fabricated single split-ring structures with the radius of 25.5 μm. In the experiment, we measured the terahertz transmission spectra when the angle between the open gap direction and the incident wave horizontal polarization is set to be 0, 30, 45, 60 and 90 degrees, respectively, and compared the effects of different angles on the transmission characteristics. With the increase of the rotation angle, the frequency of sharpest resonance dip in the terahertz transmission spectra has a remarkable blue-shift. Furthermore, we further simulated the polarization conversion characteristics of the single splitting rings with different rotation angles. It is found that there is no polarization conversion effect when the sample is placed at 0 and 90 degrees. When the sample is rotated by 30 degrees, 45 degrees and 60 degrees, polarization conversion is observed, with the highest conversion efficiency at 45 degrees. Our obtained results indicate that the terahertz metamaterials have great application prospects in biosensors.

The spectral reflectivity of the visible light bands of a non-self-illuminating object is one of the root causes of the colour formation of an object, and its uniqueness can be used to distinguish objects. Under low illumination conditions, the ability of the visible light image sensor to capture the colour of an object is greatly reduced. However, the object still has near-infrared radiation. The near-infrared radiation of the object collected by the near-infrared image sensor is combined with the visible light information of the object to improve the colour quality of the acquired object. Since the spectral reflectivity information of an object requires professional equipment to collect, it is inconvenient to use. Aim to more convenient use of spectral information data for colour restoration under low illumination conditions. In this paper, a plant’s spectral reflectivity database in visible and near-infrared bands was proposed. Firstly, 50 plants samples such as ginkgo were collected from 400 nm to 1000 nm spectral reflectivity data by a full-bands hyperspectral sorter and processed by Minimum Noise Fraction (MNF). Secondly, according to the requirements of this database, the business process, function modules and architecture of the database are designed in detail. Finally, according to the design, use C# and SQL Server to establish the database. The establishment of this database provides some data support for colour restoration and other applications.

Graphene, made of carbon atoms arranged in a honeycomb lattice, has already attracted intense research and commercial interest in recent years. Early research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The unique properties of graphene have also attracted various researches on carrier dynamics using THz spectroscopy. Here we present an experimental demonstration of monolayer CVD grown graphene via THz time-domain spectroscopy, as well as optical pump terahertz probe system. We observe that the maximum transmittance of the graphene is nearly 96% compared to the ambient signal. However, under the excitation of different optical pump fluences, it is found that unlike the semiconductor material, its transmitted amplitude is enhanced accordingly. We observed a wide-band modulation of the terahertz transmission at the range of 0.3-1.6 THz and a large modulation depth of 16.4% with a certain optical excitation. We attributed it to suppression of the air-adsorbed graphene photoconductivity due to an increase in the carrier scattering rate induced by the increase in the free-carrier concentration by photoexcitation. The obtained results not only highlight the influence of air conditions on how THz characterizations would guide the design and fabrication of graphene-based terahertz modulators and optoelectronic devices, but also show that graphene exhibits the potential for terahertz broadband transmission enhancement with photoexcitation.

A kind of terahertz amplitude modulation structure has been proposed in this work, which could achieve ~75% modulation depth of transmittance under 10V gate voltage. It is attributed to two side factors, one is making use of metallic metamaterial array which excites terahertz resonance and enhances the interaction between the terahertz wave and graphene. Therefore, the modulation of terahertz transmission could be enhanced. The other factor is adopting polyethylene oxide-based electrolytes, as a kind of high capacitance material to effectively tuned chemical potential of graphene layer which acts as the active material modulating the transmitted energy.

The Hyperspectral infrared imaging technique is an optical detection technology which integrates spectral measurement technology and infrared imaging technology. It can not only obtain the spectral information of the target, but also obtain the image information, which provides a powerful guarantee for the accurate recognition of the target. Due to the spectral characteristics of the mid-wave infrared, mid-infrared hyperspectral imaging system is widely used in civilian and military areas, such as geographical remote sensing, target detection and thermal fluent analysis. With the development of related technologies, imaging spectroscopy technology has diversified. In this paper, the push-type mid-wave infrared reflection planar grating hyperspectral imaging system is selected as the design target. The system consists of: front optical system, slit, collimation system, grating spectroscopic system, imaging optical system, imaging electrical component and control module.

Terahertz light field imaging is also known as computational imaging, and its acquisition is limited to terahertz camera scanning. Camera scanning light field imaging systems are limited by the performance of the device, and there are problems such as large image noise and small field of view. This paper proposes a single-camera terahertz light field imaging, using Matlab software for system simulation and image processing, reducing output image noise by adjusting simulation parameters, improving image quality, and browning the image to make the outline clearer. Further analysis of the improvement in imaging quality lays the foundation for the development of terahertz light field imaging in refocusing and depth estimation. Finally, it is pointed out that the terahertz band has strong penetrating power in imaging and can play a unique advantage in biometrics, military detection and security.

In this paper, we demonstrated a quasi-optical method for extracting electromagnetic parameters of materials in free space. This contactless and non-destructive method shows the great value and significance in detecting the materials that is hard to process. The system contains a high-precision vector network analyzer and a quasi-optical system, a pair of high-gain corrugated horn antennas and an off-axis parabolic mirror system for beam shaping and focus controlling of transmitting and receiving electromagnetic waves. The measurement frequency can reach the range of 220GHz~325GHz as the frequency conversion module ZVA-Z325 matches with the vector network analyzer. The scattering parameter matrix of Teflon and a porous ceramics are measured. Fourier transform of the measured signal and time domain gating are used to eliminate multiple reflections and diffraction interference. The complex permittivity of the materials, including the complex permeability and complex refractive index, are derived from classical Fresnel formula. The differences between the results in this experiment and classical THz-TDS measurements in the same frequency range are compared to analyze the errors and make further explanations.

Light field microscopic imaging technology is achieved by inserting a micro-lens array capable of capturing light field information on the relay image plane of a conventional optical microscope. The multi-focus surface can be extracted by the refocusing technique to increase the geometric depth of field. The terahertz wave has many advantages, such as strong penetrating power, low photon energy , good coherence and so on. Therefore, It can be used for microscopic imaging to obtain more comprehensive information, and can also improve the depth of field and resolution of imaging, and is also suitable for the detection of living organisms. Based on the principle of light field microscopic imaging and terahertz microscopic imaging technology, this paper analyzed the depth of field is related to the wavelength of the light source and the size of the micro-lens array. It has been found that the depth of terahertz light field microscopy technology can be increased by at least one to two orders of magnitude when the wavelength range is extended to the terahertz band.

With the advent of the 5G era, the demand for wireless communication rates is increasing, making high-speed wireless communication become a hot spot and focus. Due to the wider frequency bandwidth and good confidentiality, the terahertz band has attracted wide attention in the field of high-speed wireless communication above Gbps. However, terahertz wave is easily absorbed by water and attenuates seriously when it is transmitted in the atmosphere, which seriously restricts the development of communication in the terahertz band. However, the spread spectrum communication has attracted much attention in the field of terahertz communication because of its strong anti-fading, anti-interference and good confidentiality. In this paper, the simulation model of direct sequence spread spectrum (DSSS) communication system is established by using MATLAB, and the baseband signal and m-sequence spreading code of different rates are set, and 310GHz is used as the carrier frequency to transmit in additive white Gaussian noise channels with signal-to-noise ratio (SNR) of 17dB to -1dB. Medium transmission, by checking the bit error rate (BER), to verify the anti-interference of the DSSS technology applied in the terahertz band. The simulation results show that when the spread spectrum gain is large, the DSSS can be used in terahertz communication under the same BER to transmit in channels with a SNR of 10dB worse. Therefore, the application of spread spectrum communication technology to terahertz communication has a good development scenario and will effectively promote the development of terahertz communication.

With the rapid development of 5G communication technology, terahertz wave has attracted extensive attention due to its large capacity, good directivity and strong anti-interference ability. However, in the terahertz communication system based on the subharmonic mixing method, in the process of generating the terahertz signal, due to the high frequency multiplication, the floor and phase noise of the original frequency source pass through the frequency doubling and amplification links, and the spectrum is realized. The shifting and amplification are degraded, while the circuitry of the link itself also produces an in-band noise superposition. In order to reduce noise, this paper studied on the basis of the principle of harmonic mixing terahertz communication system of the high harmonics and the vibration noise impact on the quality of communication, this paper proposes a method of harmonic interference suppression method this method by selecting the required harmonic band-pass filtering, set the appropriate frequency range can make other harmonic component to fall outside the frequency band, so as to achieve effect of reducing the noise of a terahertz communication system.

Terahertz spectroscopic characteristics are related to the large amplitude vibration of molecules and their interaction, so it can be used for molecular detection. Sodium nitrate and sodium nitrite are two very important salts in people's lives. Sodium nitrate is usually used as quick-acting fertilizer in acidic soil, but excessive use will cause certain pollution to the environment. If sodium nitrate is heated, it becomes very stable sodium nitrite, which is usually added to food as a preservative and colorant, but excessive consumption of nitrite can lead to food poisoning and even death. Therefore, it is necessary to detect sodium nitrate and sodium nitrite. Based on THz-TDS system, the spectral characteristics of sodium nitrate and sodium nitrite in 0.1-1 THz band were studied. Polyethylene, sodium nitrate and sodium nitrite were mixed in proportion of 1:1, 1:2, 1:3, 1:4 and 1:5, respectively. The mixture was evenly grinded and stirred, and then pressed. The prepared samples were tested in THz-TDS system, and the time-domain spectra of the samples with different proportions were obtained. The terahertz frequency spectrum and absorption spectra of the two samples at different ratios were obtained by Fourier transform. After comparing spectra of the two samples, it is found that they have their own absorption peaks, and their peak positions are obviously different. Therefore, it is very important to study these two substances by terahertz technology.

Compared with traditional imaging, optical field imaging records the four-dimensional position information and direction information of the object. It has two degrees of freedom more than the two-dimensional traditional image, so it has more abundant image information in the later processing. According to Levoy's theory of light field rendering, light field can be parameterized by two parallel planes. Compared with visible light, terahertz has the characteristics of long wavelength, strong penetration and low energy. Therefore, terahertz optical field imaging technology will have the advantages of wide dynamic range and deep field. This paper summarizes the acquisition methods of light field and describes the post-processing methods of light field, including digital refocusing, super-resolution, depth estimation, synthetic aperture imaging and microscopic imaging technology. Finally, the application of terahertz optical field imaging and the possible problems are summarized.

Terahertz on-chip system integrates the generation of terahertz, detection devices and transmission lines on the same substrate. It has the characteristics of small size, high measurement stability and easy operation. This paper is devoted to the study of the electrode of the photoconductive antenna necessary for the generation and detection of terahertz. HFSS software is used to simulate seven types of antennas with two types, large aperture and small aperture. The S11 images and VSWR images of each structure are obtained, and the emission efficiency of the optimal antenna electrode structure is obtained by comparison. For small aperture antenna, rectangular antenna has the highest transmitting efficiency; for large aperture antenna, dipole antenna and dish antenna have higher efficiency. But in the butterfly antenna, when the antenna spacing and the antenna length are both 50μm, the reflection power is the smallest and the performance is the best. These simulation results provide a parameter basis for the subsequent fabrication of chips.

In this paper, we demonstrated that tumors in freshly excised whole brain tissue could be differentiated clearly from normal brain tissue using a homemade continuous terahertz (THz) wave attenuated total reflection (ATR) imaging system. The resolution of this system was about 400μm×450μm at 2.52THz. The terahertz images characters of fresh brains with tumor was studied using this THz-ATR imaging system. Tumor regions could be differentiated clearly from normal brain tissue by THz intensity imaging at different frequencies. The high absorption regions in THz images corresponded well to the tumor regions in the hematoxylin and eosin-stained microscopic images. Moreover, the morphological reconstruction method was applied to restore the blurred imaging results. The noise caused by power fluctuation in THz-ATR image was almost eliminated and the visibility of objects has been successfully enhanced. These promising results suggest that THz-ATR imaging could be used as a tool for label-free and real-time imaging of brain tumors, which would be great potential as an alternative method for the fast diagnosis tumor region during brain surgery.

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. The operating range of the infrared system based on the near space platform is the key index for detecting ballistic missile. In order to analyze the operating range of infrared detection system based on near space platform, an improved range equation from investigating the characteristics of point source target is discussed and deduced. The simulation results show this method has scientific guiding significance for system overall scheme demonstration and design technically.

Transmission characteristics of Fabry-Perot (F-P) filter based on silicon substrate with different transmissivity of high reflection (HR) coatings, incident angle, and interference orders are investigated. The results show that the transmissivity of HR coatings has great effect on full width at half maximum (FWHM) of transmission spectrum, the FWHM of F-P filter reduced from 209 nm to 3.4 nm with the reflectivity of HR coatings increased from 84.7% to 99.6%. The peak wavelength shifts from 1546.3 nm at 0° to 1542.6 nm at 5°, indicating that the FWHM of transmission spectrum broadens as the incident angle increases. The 1st, 2nd, 3rd, and 4th order interference are 3.4 nm, 2.3 nm, 1.8 nm, and 1.5 nm, respectively. Thus, in the applications tuning in a narrow wavelength range, F-P filter can be designed to operate in high-order mode to achieve a narrow transmission spectrum.

After entering the near space, a layer of plasma sheath is formed outside the hypersonic vehicles due to the hightemperature and high-pressure environment. The plasma sheath, which characteristic frequency is similar to microwave, will cause serious impediment to communication signal. This phenomenon is known as the blackout problem. With the rapid development of aerospace industry, plasma sheath blackout has become an urgent problem to be solved. Current research shows that increasing the frequency of electromagnetic wave higher than the plasma characteristic frequency can effectively reduce the shielding effect of plasma. The frequency of terahertz (THz) wave is much higher than microwave, it can propagate through plasma sheath, which provides an effective method to solve the problem of plasma sheath. In this paper, a theoretical model of plasma is established, and the transmission properties of THz wave in plasma is simulated using scattering matrix method. Then a kind of plasma jet is produced in laboratory environment according to dielectric barrier discharge. And the experiments of a broadband THz source and THz time-domain spectrum transmission in this kind of plasma and a 2.52 THz wave reflection imaging of target under plasma shelter are carried out respectively. The transmittance increases with frequency under 0.5 THz and becomes stable at 100% over 0.5 THz, and the result of experiments and simulation are in good agreement. Both theory and experiments show that THz wave has good penetration in plasma jet and can detect targets behind plasma, and this study will lay a theoretical foundation for solving the plasma blackout problem of hypersonic vehicle in near space.

Most DNA methylation occurs in the cytosine of nucleosides. In this study, the absorption coefficients of cytosine and 5-methylcytosine were measured at 0.1-2.0 THz through terahertz time domain spectroscopy (THz-TDS) without being labeled. Our results showed that these important biomolecules had distinct spectral characteristics within this terahertz band. Moreover, structural optimization and lattice dynamics calculations were performed to determine the lattice vibrational motions of both cytosine crystals and 5-methylcytosine crystals using the density functional theory (DFT) and the properly modeled intermolecular potentials. Typically, the lattice vibrational motion simulated the absorption spectrum which indicated that the terahertz technique was able to detect the absorption spectral features of cytosine and 5-methylcytosine. Hopefully, this study provided the molecular background to detect DNA methylation and lay certain foundation for subsequent cancer detection.

A division of aperture medium wave infrared Stokes imaging polarimeter (ISIP) with optimal linear polarization measurements is presented. The focal plane array of the ISIP is divided into four independent imaging channels by a lens array, which is turned into four independent polarimetric analyzing channels by placing four linear polarizers of different orientation angles in front of each channel and a wave plate in one of the channels. The experiments are performed for detecting camouflaged targets by using the optimized ISIP, which demonstrate that the polarimetric images are effective in detecting camouflaged targets form natural background and improving the signal to noise ratio of target images.