This PDF file contains the front matter associated with SPIE Proceedings Volume 11906, including the Title Page, Copyright information, and Table of Contents.

All-dielectric metasurfaces exhibit high damage thresholds and strong enhancement of the driving field, making them attractive platforms to Terahertz generation at the nanoscale. Here we report terahertz electric field from lithium niobate metasurface is enhanced by more than one order of magnitude compared to unpatterned samples. The enhanced terahertz are highly anisotropic with respect to the excitation polarization and are selective by the excitation wavelength due to its resonant features. By combining nanofabrication technology and ultrafast optics, our work paves the way for the design of new compact terahertz photonic devices that operate under high intensities and at short wavelengths.

Electro-optic (EO) sampling is a powerful non-destructive technique for measuring terahertz (THz) electric field with high temporal resolution. EO sampling is a popular method for monitoring the electron beams in accelerators because the measurement of the electric field pulse width directly corresponds to the width of the electron beam. Previous researches mainly focused on measuring the temporal profile. In this work, we also measure the spatial (radial) profile that is perpendicular to the propagation (longitudinal/temporal direction). The measurement of the electric field profiles in both time and space paves the way to access the spatio-temporal electron beam profile, which makes this method promising for a high-order harmonic generation-seeded free-electron laser (FEL). In this research, we investigated the electric field strength profile and the pulse broadening in the radial direction based on the spatio-temporal electric field around a picosecond relativistic electron beam with an energy of 35 MeV. Special relativity predicts that the electric field contracts in the propagation direction and becomes like a disk with a uniform thickness. With this postulate, the Gauss’ theorem in cylindrical coordinates can be applied to the experimental results to deduce the electron beam size from the electric field profile.

Terahertz (THz) photoconductive antennas (PCA’s) from SI-GaAs substrates having one-dimensional (1D) and two-dimensional (2D) micron-size metal line arrays (MLA's) were fabricated. Photolithography and electron beam deposition of Ni/Au were used to fabricate spiral PCA’s and 1D/2D MLA's on the transmission side of the PCA. Compared to a reference bare PCA, the THz time-domain signal enhanced ~6x for 1DMLA and ~11x for 2DMLA, with their corresponding bandwidths broadened. The origin of the enhancement is being investigated but is currently attributed to spoof surface plasmon phenomena. Integrating MLA’s with PCA’s demonstrates a more cost-effective alternative to nanostructure fabrication within the PCA gap.

Terahertz nondestructive testing technology is a technology widely used in samples evaluation with the merit of on-site and surface-damage free. Terahertz waves have low quantum energy and are transparent to most non-polar substances, thus, terahertz nondestructive testing has gradually become a research hotspot over the years. However when a femtosecond laser in used as a terahertz excitation source, the excess pump light in the system may cause radiation damage to the measured object. In order to eliminate the impact of residual pump light in the system on the measured object, we designed the spectral filter elements of reflect and terahertz band transmission in the wave bands of 0.80 μm and 1.56 μm, by using Nb₂O₅ and SiO₂ as high and low refractive index materials, respectively. Then we prepared the filter membrane system electron beam evaporation through an ion beam assisted deposition. The influence of annealing process on the growth structure of the films was analyzed according to the AFM test results of Nb₂O₅ monolayer films under different process conditions, and the annealing process parameters were optimized to reduce the influence of surface roughness on the film spectrum. Based on the KIM vibration model (Lorentz extension model), the optical constant dispersion distributions of NB2O5 thin flims prepared under different processing conditions were accurately fitted. The influence of temperature on the refractive index and absorption of Nb₂O₅ thin films were analyzed. The deposition temperature was optimized to reduce the absorption of the filter in the wave bands of 0.8 μm and 1.560 μm. By using PE Lambda 1050, the reflectivity of the prepared filter in the wave bands of 0.8 μm and 1.560 μm were tested. The prepared filter was scanned by focusing point by point using a built-up transmission terahertz time domain spectral system. The transmittance of the quartz coated sample relative to the reference signal was calculated by analyzing signals in the frequency domain. The average reflectivity of the prepared spectral filter was 99.9% in both the band of 0.9 μm and 1.560 μm. The transmittance of terahertz frequency 2 THz was 68%, which was almost consistent with the theoretical design values, and could meet the requirements for the terahertz nondestructive testing system's application.

Microwave signal with high frequency and low phase noise is generated based on a parity-time (PT) symmetric optoelectronic oscillator (OEO). The PT-symmetric OEO based on normal and reversed traveling-wave modulation is formed by normal and reversed modulation in a travelling-wave intensity modulator (IM) using the radio-fiber (RF) input and termination ports. Gain and loss balance can be realized to achieve PT symmetry thanks to the different modulation efficiencies for microwave signals applied via different RF ports. The operation of the proposed OEO has been experimentally verified. A microwave signal is generated at 10 GHz with a phase noise of -110.7 dBc/Hz at an offset frequency of 10 kHz and a sideband suppression ratio of 46.46 dB. The approach has potential applications in high-quality microwave signal generation and simplify the structure of PT-symmetric OEO system.

Here we describe a novel reflective confocal scanning method for super-resolution terahertz (THz) imaging. The traditional continuous-wave THz confocal method is difficult to break through the diffraction limit owing to the Abbe limit. So, it’s difficult to achieve super-resolution images, especially in low frequency THz wavebands. Besides, the traditional terahertz lens system has a large loss, so it cannot achieve long-distance transmission and imaging. In this paper, low-cost and low-loss terahertz waveguides are printed to replace conventional lenses. Through the reasonable design of the waveguide, we can breaks through the diffraction limit and achieve sub-wavelength focusing, so as to achieve super-resolution terahertz imaging. The final experimental resolution is less than half of the wavelength.

Phased array feeds (PAF) are playing an increasingly important role in low-frequency radio astronomy, but less explored at THz wavelengths. Here we present the design of a 0.35-THz planar array antenna, which consists of an array of twinslot antennas, a coplanar waveguide (CPW) feed network and airbridges. The planar array antenna is based on a silicon substrate without any lens as well as anti-reflection coating. The airbridges are used to improve the performance of the CPW T-junctions and the transmission efficiency of the feed network. Simulation results show that the substrate thickness can change the power distribution on both sides of the substrate. With an appropriate substrate thickness and some airbridges, the total gain of a 16-element array can reach 15.3 dB and the main-beam efficiency is about 54%. This kind of planar array antenna can be easily integrated with a terahertz detector such as hot electron bolometer (HEB) and kinetic inductance detector (KID), based on photolithographic fabrication.

High energy and widely tunable terahertz (THz) generation was demonstrated theoretically based on a semiconductor material 4H-SiC via difference frequency generation (DFG) process. Compared with the conventional THz nonlinear optical (NLO) crystals, 4H-SiC has the main advantages of extremely high optical damage threshold and wide optical transparent range, which implies the potential THz generation with high output energy and broadband tunability. Based on the basic NLO theories, the phase-matching (PM) characteristics, effective nonlinear coefficients, walk-off angles, and PM tolerance of DFG in 4H-SiC were calculated in the 2–15 THz range with different pumping wavelength. The output characteristics of THz generation were simulated in relation with the optical interaction length and the intensities of dual-wavelength pump beams via large-signal analysis among three coupled wave equations, which reveal that efficient and high energy THz generation based on 4H-SiC crystal could be achieved with appropriate crystal length and intensity ratio of dual-wavelength intense pumps, despite of a relatively low nonlinearity of the material.

Terahertz spectroscopy, with the advantages of label-free and non-ionizing has been considered as a potential method for biomolecule detection. Hereby we present a promising approach for the label-free protein sensing in liquid environment. In this work, we designed a grapheme-based device to increase the accumulated amount of protein molecules in the solution. The sensor demonstrates a quick and sensitive response to HER2 proteins and gave a linearly response to the concentrations of HER2 from 63 ng/ml to 4.0 μg/mL. The results demonstrated that once HER2 molecules targeting on the surface of graphene, the conductivity of graphene was significantly changed. This provided us a potential method to detect trace biomarker within liquid phase by using terahertz spectroscopy.

Conventional coherent and non-coherent techniques such as quasi-optical vector network analyze (VNA) and Fourier transform spectroscopy (FTS) can be employed to measure the exhaustive properties of dielectrics in the terahertz band. However, the VNA can only cover a narrow frequency range, and the FTS takes a relatively long period of time for measurement. By contrast, the terahertz time domain spectroscopy (TDS) allows the measurement of material properties such as dielectric constant and loss tangent in a wide frequency range and in a short period of time. Using a terahertz TDS, we characterize the complex properties of some materials commonly used in terahertz superconducting receivers, including high density polyethylene (HDPE), single crystal magnesium oxide (MgO), single crystal quartz, single crystal sapphire, single crystal silicon (S.C. silicon), high resistance silicon (H.R. silicon), and ultra-high molecular weight polyethylene (UHMWPE). The measurements at room temperature have finished yet. The measurements at cryostat temperature are in progress and will be published later.

Efficient devices for control properties of electromagnetic waves are essential for the development of terahertz (THz) technologies. But despite the great progress achieved in a study of graphene, the influence of the number of graphene layers on its properties in the THz frequency range has not yet been sufficiently studied. In this work, we experimentally studied properties of multilayer graphene (MLG) films in the frequency range 0.2–0.8 THz, at a room temperature, and a relative humidity of 40%. Using our custom-made THz time-domain spectroscopic polarimetry system, we obtained spectra of the complex relative permittivity and the electrical conductance of the chemical vapor deposition graphene with ∼14, ∼40, and ∼76 layers of graphene on glass substrates. It is shown that the conductance increases nonlinearly with an increase in the graphene layer number and reaches, for ∼76 layers, 0.06 S for the real, and 0.03 S for the imaginary part, respectively.

Thermal imaging is considered as one of the most powerful and easier method that is used in observation applications. Since it can provide users with images that contain a lot of details about the observed scenarios in day and night also during bad atmospheric conditions. But by the fact of aging during the operation of Thermal Imaging Systems (TIS), the TIS main parameters and characteristics’ nominal values are deviated which has a high impact on the TIS tactical performance. These aged TIS are still working but in a lower efficiency, and consequently the observation capability will be decreased. Field performance evaluation of the TIS is one of the powerful measuring methods that is used to determine the actual discrimination ranges under different atmospheric conditions. This paper is devoted to offer a methodology for evaluating the aged TIS at the field. An experimental determination of TIS’s Minimum Resolvable Temperature Difference (MRTD) is performed by using an Inframet DT-150 test station. The discrimination range process (Detection, Recognition and Identification) is executed with the aid of MODTRAN software to simulate the real conditions for the atmospheric scenarios. Our presented methodology has enabled us to stand on the aged TIS actual performance to detect, recognize and identify different standard NATO (vehicle and human) targets. Moreover, redefining the aged TIS new missions according to their actual abilities. Finally, this paper was concluded with a real evaluation for an aged TIS (Bidentifier 100) as a case study. This method can be applied to any TIS whether it operates in mid or long wave Infrared (IR) spectral band.

Infrared character of aircraft is one important factor to estimate the ability of breaching the defense of enemy, this paper firstly analyzed the method of infrared calibration with collimator and energy factors composing the sky background, propose one method of deleting energy of background within current image; furthermore, the paper points out that the parameters of calibration with collimator can’t be directly applied to computing Infrared character of flying target, and based on that propose the method of deleting energy of background with another background image measured before mission.

Based on a current modulated semiconductor laser (SL) subject to optical injection, tunable and broadband microwave frequency comb (MFC) generation is theoretically investigated. For a SL under only current modulation, a seed MFC with relatively narrow bandwidth can be generated. Then, an optical injection is introduced to improve the quality of seed MFC. Furthermore, the influences of operation parameters on the performance of MFC are also discussed. The numerical results show that after introducing optical injection with injection coefficient k = 1.8×10^-4 and detuning frequency Δf = 6.0 GHz, the MFC bandwidth can be further increased by 15.6 GHz and the signal-to-noise ratio can be improved by more than 20 dB.

Based on Gaussian optical system theory and vector method, an unobstructed off axis zoom system in terahertz band is designed. The field angle of view of the system is 3 ° × 3°~0.6° × 0.6 °, the MTF of the system at 10 LP / mm ≥ 0.4. By using the designer search and intelligent algorithm automatically to find the optimal solutions, two different methods have compared, and achieved the two design results. These have been seen that because the reflective zoom system is different from the relatively mature transmission continuous zoom systems fully, that these limitations in increasing the field of view and the energy utilization of the detection surface have been solved. Therefore, there are still much more space for research and improvement in engineering application.

Due to the limitation of high sampling rate and large coding matrix, it often takes a long time to decode and reconstruct millimeter-wave radiometric images, which becomes a difficult problem for Compressed Sensing (CS) theory in the field of millimeter-wave radiometric imaging. In order to effectively reconstruct millimeter-wave images within the framework of block-based CS, the adaptive sampling methods based on visual saliency and block information weight are proposed in this paper. In view of the irrationality of allocating the same amount of encoded data to different image blocks in the traditional block-based CS, both the adaptive sampling method based on visual saliency and local variance weighted adaptive allocation, and the adaptive sampling method based on visual saliency and two-dimensional information entropy weighted adaptive allocation, are put forward and compared with other allocation methods. Some experimental results demonstrate that the proposed method can effectively improve the PSNR value of the reconstructed image. In addition, the reconstructed images with a low sampling rate are also in conformity with human vision.

Microwave Kinetic Inductance Detectors (MKIDs), with high sensitivity and relatively simple frequency multiplexing, are emerging as a kind of large-array detectors in the terahertz (THz) band. An MKID, like a superconducting resonator, is composed of a planar capacitor and inductor that are made from superconducting films. The energy gap of the superconducting film for the planar inductor (at least part of it) is lower than the energy of photons to be detected. The kinetic inductance of the superconducting film irradiated by the photons is increased, shifting the resonance frequency toward a lower frequency. The frequency shift is proportional to the kinetic inductance fraction, i.e., the ratio of the kinetic inductance to the total inductance of the planar superconducting inductor. In this paper, we thoroughly investigate the kinetic inductance fraction for lumped-element and antenna-coupled MKIDs, which are both made from NbTiN. The detailed simulation, calculation, and measurement results will be presented.

Metamaterials are artificial materials with unique electromagnetic properties and could exhibit a strong electromagnetic resonance. Resonant metamaterials are widely used in the field of biosensors because they are sensitive to the changes of surface dielectric environment. In this study, we demonstrated a highly sensitive detection method of 4-Mercaptohydrocinnamic acid using terahertz metamaterials fabricated on silicon substrate with metal arrays of the three bars structure. We measured the transmission spectra of the metamaterial to detect the 4-Mercaptohydrocinnamic acid at three different densities by the terahertz time-domain spectroscopy system. With the density of 4-Mercaptohydrocinnamic acid increasing, resonance dips exhibit the red-shift phenomena and transmission at the resonance frequency simultaneously reduce. The movement of resonance dips and the change of transmission show that the three bars resonators are sensitive to the density of analyte. The finite-difference time-domain simulation shows a good agreement with the experimental data, and the simulation of surface current and electric field distributions at resonance dips can further understand the resonance modes in transmission spectra. Our study provides new prospect into the application of terahertz metamaterials in biosensing.

Terahertz (THz) communication is a research hotspot in the future communication field. However, limited by the power of the THz source and various particles in the air, indoor THz wireless communication with short distance transmission has practical research value. Due to the strong directivity of THz beam, the line of sight (LOS) path occupies most of the energy of the signal. However, when the LOS path is blocked, the not line of sight (NLOS) path can be used as a supplement to ensure the stability of the communication link. In this paper, a 3D transmission model combining LOS path, primary reflection path and secondary reflection path was established by ray tracing method for indoor laboratory scenes with high demand for communication rate. The carrier frequency range is 220-330GHz. Through the results of power delay profile (PDP) and power angle profile (PAP) at the receiver, the correlation characteristics of important channel parameters such as Rician K-factor, root mean square (RMS) delay spread with different frequency points and different paths are analyzed. The results show that all the channel parameters are strongly correlated with frequency and transmission distance. These theoretical results lay a foundation for the subsequent communication experiments in real experimental condition.

In recent years, with the continuous development of terahertz detection technology, the use of terahertz spectroscopy to study chemical samples has become one of the indispensable means in the field of biochemistry. However, the biological activity of most biomolecules can only be expressed in water solution, while water as polar molecules has strong absorption properties of THz wave. Therefore, it is difficult to study the activity of biological samples in water solution by using terahertz technology. In this study, a sandwich terahertz microfluidic chip with high transmission to terahertz wave was designed. The detection area of THz microfluidic chip is COC material, which is colorless and transparent, has no obvious absorption peak in THz frequency range, and has high THz wave transmission. The THz transmission of 2 mm thick COC can reach more than 90%. A valveless micropump was designed on the chip surface to realize automatic injection. The terahertz transmission spectra of distilled water, 0.9 mol/L NH₄Cl, (NH₄)₂SO₄, (NH₄)₂CO₃ and CH₃COONH₄ were measured by combining the chip with terahertz technology in the frequency range of 0.1–0.7 THz. The experimental results show that at the same concentration, the THz spectra of different ammonium salt solutions are obviously different and the spectral intensity is lower than that of distilled water, which indicates that the hydration process of ions has an effect on the hydrogen bond between water molecules, and is reflected in the terahertz transmission spectrum.

Terahertz communication technology has attracted wide attention from academic circles and industry, and is regarded as the key wireless technology to meet the real-time traffic demand of mobile heterogeneous network system, which can alleviate the capacity bottleneck of the current wireless system and realize ultra-high-speed wireless communication. As a key content of Beam-forming Techniques, indoor channel modeling of terahertz band in NLOS (non-line-of-sight) has become an urgent task. In this paper, according to the position of the receiver and the surrounding scenes, the possible path from the transmitter to the receiver is traced back through geometric optics. The influence of diffraction on the transmission channel of 300GHz indoor communication system is evaluated. Ray tracing technology is combined with knife edge diffraction (KED) and double knife edge (DKE) to simulate the diffraction of wedge-shaped object and human body respectively. The results show that when the wedge-shaped object diffracts, the diffraction coefficient decreases with the increase of the diffraction angle. Diffraction in the wedge area can be ignored in most areas of the room's line of sight, but diffraction power is dominant in the incident boundary and reflection boundary area. Due to the shadow effect caused by human movement, the path attenuation increases significantly. The results provide a research basis for the development of indoor terahertz communication.

The vibrational and rotational energy levels of many biomolecules are in the terahertz band, so terahertz technology can be used to detect biomolecules. In addition, because the photon energy of terahertz wave is low and will not destroy biological samples in the detection process, terahertz technology has a very wide application prospect in the future research fields such as biochemical detection and information communication. As we all know, most biomolecules need to be in liquid environment to give full play to their biological activity. However, the hydrogen bond in aqueous solution will produce strong absorption in terahertz band. In addition, water molecules are polar molecules, and terahertz wave has strong resonance absorption to polar molecules, which makes it very difficult to detect active biomolecules in liquid environment by terahertz technology. Therefore, many research teams combine terahertz spectroscopy with microfluidic technology to reduce the impact of various factors on biomolecular detection. In this study, the THz microfluidic chip was prepared with cycloolefin copolymer (COC). The five potassium salt solutions in a constant electric field at different times were studied, which provided a basis for further strengthening the application of THz technology in biochemistry. What’s more, the electrolyte solution contains a large number of anions and cations, which will move under the action of electric field. And it provides technical support for the study of the dynamic characteristics of electrolyte solution by terahertz technology.

Because the vibrational and rotational energy levels of many biomolecules are in the terahertz range, they can interact with terahertz. In addition, the photon energy of terahertz wave is very low, only meV level, which will not cause damage to the material, so terahertz technology can be used for non-destructive detection of biological molecules. As we all know, many biomolecules need to maintain their biological activity in aqueous solution, so it is of practical significance to study the biological characteristics in aqueous solution. The combination of terahertz and microfluidic technology can greatly reduce the absorption of liquid samples to terahertz, so it can detect more accurate signals. Therefore, terahertz time domain spectroscopy technology has a good application prospect in biological detection. The terahertz transmission characteristics of different sodium salts standing in electric field for different time are studied. Double layer terahertz microfluidic chips were prepared by using cycloolefin copolymer (COC) materials with high transmission rate for terahertz wave. The terahertz transmission characteristics of four kinds of sodium salts under the action of electric field were tested by using microfluidic chips with strong sealing performance. In order to avoid the influence of ionic impurities in aqueous solution, deionized water was used to prepare the solution. NaCl, CH₃COONa, NaHCO₃ and Na₂CO₃ electrolyte solutions with concentration of 0.9 mol/L are selected. The four solutions were injected into the microfluidic chip in turn, and then put into the external electric field device. The electric field treatment time is 0 min, 5 min, 10 min, 15 min and 20 min respectively. The transmission intensity is measured by terahertz time domain spectroscopy system every five minutes, and the corresponding spectrum can be obtained. The results show that with the increase of electric field time, the THz transmission intensity of the four sodium salt solutions increases.

In order to realize multi-function, the radar has wide instantaneous bandwidth and working bandwidth. Unfortunately, the traditional phased array radar antenna by microwave phase shifter or direct digital synthesis in the way of phasing. There are two limitations after increasing the instantaneous bandwidth: "beam squinting" and "aperture traverse delay", which cause the beam divergence and the beam broadening. In this paper, the integrated optical beamforming network technologies are discussed analyzed, which provide certain theoretical support for the research of the future multi-band broadband reconfigurable radar and even the engineering of microwave photonic radar.

With the continuous progress of modern industry, the traditional nondestructive testing methods cannot meet the growing demand. At present, as a novel approach, the thermography non-destructive testing (TNDT) technology has been widely used in various industrial fields. In this paper, we present the study of a three-layer "metal-air-metal" structure, firstly through the modeling by finite element analysis software COMSOL Multiphysics, and then under the laboratory conditions using flash lamp as excitation methods for TNDT. Based on the preliminary simulation and experimental results , it is proved that it is possible to detect the bottom layer defects with a certain aspect ratio in the multilayer structure. It has practical significance in many scenarios, such as the locating the rivet holes under aircraft skin, the detection of defects below insulation layer and tomographic inspection of post-impact multilayer composites.

We experimentally demonstrated an optical controlled terahertz modulator based on the 3D Dirac semimetal Cd₃As₂ thin film (~100 nm, λ/3000). Utilizing the time-resolved terahertz spectroscopy, the photoactive characters of Cd₃As₂ thin film are investigated. The measurements reveal that Cd₃As₂ modulator exhibits a low optical threshold (< 63.5 μJ/cm²) and ultrafast (< 20 ps) broadband (0.2-1.5 THz) modulation of terahertz waves. Moreover, integrated with the metamaterials structures, Cd₃As₂ could potentially be used as photo-sensitive and ultrafast reconfigurable terahertz resonant plasmonic/meta-device.

Emissivity is one of the important parameters to reflect the thermal characteristics of the surface of an object. It plays a great role in the research of infrared coatings with high or low emissivity. In this paper, a novel non-contact experimental method is proposed to measure the coating’s emissivity using the dual-temperature method, and the error of the measured infrared emissivity with different base board materials is analyzed.

In order to measure the thickness of the rusted bronze layer quickly and accurately, an active infrared thermal imaging technique is developed. Pulse flash lamp was used to excite the surface of the specimen, and infrared thermal imager was used to record the temperature decay of the specimen surface before and after flash. This paper presents a pulse thermal imaging multi-layer analysis method (PTI-MLA) which consists of a pulsed thermal imaging (PTI) system to acquire experimental data and a multilayer analysis (MLA) data-processing algorithm to derive parameters of the corroded bronze. This paper studies a bronze pot fragment from the China’s Spring and Autumn period excavated in Sujialong site, Hubei province. The corrosion thickness map of the fragment is calculated and analyzed by PTI-MLA method. Comparing the measured values with the CT scanning results, the error is less than 10%. The final result indicates that the PTI-MLA method can be used to measure the thickness of rust layer on the surface of bronze ware and obtain high-precision thickness images.

Pulse phase thermography (PPT), principal component thermography (PCT) method and Thermal signal reconstruction (TSR) analysis including Logarithmic First-Derivative (LFD) and Logarithmic Second-Derivative (LSD) method have been widely used to improve detection sensitivity and quantitative measurement accuracy in infrared thermal imaging. In this paper, the difference of pulsed thermography results of aluminum specimens with flat bottom holes (FBHs) under PPT method, PCT method and LFD and LSD method was compared. With the result of the above methods, the multi-image average method is proposed to further improve the signal-to-noise ratio at the defect of images. Finally, the difference between the original result and the improved result are compared.

This paper mainly explores the application of thermal signals generated by sequential laser pulses in the detection of metal material defects. Here we use flat bottom holes with different sizes on stainless steel as our sample. Sequential laser pulses are used as thermal excitation source and the surface temperature field is recorded by infrared thermal imager.By analyzing the relationship between the surface temperature of the defect area under sequential pulse excitation, the defect aspect ratio can be obtained due to different 3D thermal diffusion process. The experimental scheme and data processing method described in this paper provide a new method and theoretical basis for the measurement of metal defects size by sequential laser pulses, and have certain reference value for the detection and research of other metal material defects

Thermal barrier coatings (TBCs) have been widely used in hot end components of engines, such as blades and combustors, due to their high temperature corrosion resistance, heat insulation and oxidation resistance. The thermal conductivity, constant pressure heat capacity and coating thickness are important indexes in the design process of TBCs. A reasonable coating thickness is an important criterion for its quality evaluation. In this paper, pulsed thermal imaging multilayer analysis method is used to measure the coating thickness of thermal barrier coating samples. Firstly, a two-layer model is constructed by finite element simulation to obtain the measurement results under ideal conditions, and the method is evaluated. Then, the accuracy of the method is further verified by experiments with five TBC samples with different coating thickness.

In this paper, the S-parameters of 2mm silicon wafer, 14.345mm wood I, 14.215mm wood II and 13.80mm wood III were measured based on 220-325 GHz quasi optical Vector Network Analyzer measurement system. Basic theory based on free space electromagnetic component equations, boundary conditions and S parameters. The electromagnetic field of "air-MUT-air" three-layer structure between calibrated reference planes is analyzed in detail, and the formula of complex permittivity is derived. The results of complex permittivity of silicon wafer extracted by this method are basically consistent with those in the literature. The measurement of woods can accurately distinguish heartwood from sapwood. This study is helpful for us to better understand the interaction mechanism between terahertz waves and wood media, and can lay a foundation for the identification of woods.

In this paper, the coating thickness of thermal barrier coating (TBC) on gas turbine blades is measured by terahertz time domain spectroscopy system. At present, the most mature and widely used ceramic layer (TC) material is yttria partially stabilized zirconia (YSZ). In order to measure the coating thickness on TBC surface by terahertz spectrum, it is necessary to determine the optical properties (refractive index and absorption coefficient) of coating material YSZ in terahertz band. The refractive indexes of YSZ samples in terahertz band under different preparation conditions were obtained by transmission THz-TDS. The refractive indexes of these samples were different under the influence of different process parameters and spraying conditions. Then the time domain signal of TBC sample is obtained by reflective THz-TDS, and the coating thickness is obtained according to the time delay of terahertz wave transmission in the sample. When THz wave propagates at different depths of the sample, the dispersion is different, resulting in waveform broadening. We qualitatively analyze the influence of waveform broadening on thickness measurement. The results show that, within the error range, the coating thickness obtained from the time delay between the negative peak of surface reflection signal and the negative peak of interface reflection signal is in good agreement with the thickness measurement results of metallographic method. The optical properties of YSZ material and the thickness of thermal barrier coating are nondestructive tested in THz band.

The ps-laser micro-machining technology for substrate-removed HgCdTe IRFPAs is researched in this paper. By analysis the microstructure morphologies under different pulse energy, the ablation threshold is fitted. Based on the analysis of ablation threshold, we optimize the laser parameters and etch grooves on the HgCdTe chip which is substrate removed and thinned to 5μm. The FPAs experiment test results show that thermal effected area was about 10μm away from the grooves, and the laser has no effect on the ROIC under the HgCdTe chip. The results are important to verify the optimal strategy of ps-pulsed laser for robust operation of substrate-removed HgCdTe IRFPAs and for laser micromachining industrial applications.

In this contribution, we discuss the features of design and polarimetric inspection of terahertz achromatic waveplates. The design of the crystalline quartz half-wave plate was performed taking into account the ellipticity and the introduced phase difference between the orthogonal components of the output radiation polarization vector. The designed waveplate are relatively thin, work equally efficiently in the frequency range from 0.4 to 1.4 THz, and, most importantly, are cheap to manufacture. The modification of the experimental terahertz time-domain spectroscopy polarimetric setup involving direct waveform detection is proposed. The proposed polarimetry THz time-domain spectrometer with electro-optic detection shows parasitical signals absence and easier measurement procedure.

We demonstrate that titanium-doped lithium niobate strip optical waveguides can act as terahertz emitters. Terahertz pulses were produced by pumping with a femtosecond laser at a wavelength of 1.55 μm via the prism-coupled Cherenkov-phase matching scheme. Near-single cycle time-domain pulses were generated and the terahertz time-domain peak signal from the 40-μm waveguide was found to be ~18% higher than that observed with bulk crystal. The THz power spectra revealed a broad bandwidth and high signal-to-noise ratio. These findings signify the feasibility of titanium-in-diffusion to produce strong terahertz radiation from titanium-doped lithium niobate waveguides.