Three-dimensional terahertz wave imaging

Pulsed terahertz (THz) wave sensing and imaging is a coherent measurement technology. Like radar, based on the phase and amplitude of the THz pulse at each frequency, THz waves provide temporal and spectroscopic information that allows us to develop various three-dimensional (3D) terahertz tomographic imaging modalities. The 3D THz tomographic imaging methods we investigated include THz time-of-flight tomography, THz computed tomography (CT) and THz binary lens tomography. THz time-of-flight uses the THz pulses as a probe beam to temporally mark the target, and then constructs a 3D image of the target using the THz waves scattered by the target. THz CT is based on geometrical optics and inspired from X-ray CT. THz binary lens tomography uses the frequency-dependent focal-length property of binary lenses to obtain tomographic images of an object. Three-dimensional THz imaging has potential in such applications as non-destructive inspection. The interaction between a coherent THz pulse and an object provides rich information about the object under study; therefore, 3D THz imaging can be used to inspect or characterize dielectric and semiconductor objects. For example, 3D THz imaging has been used to detect and identify the defects inside a Space Shuttle insulation tile.     

多盲孔缺陷的超声兰姆波时域拓扑能量成像方法研究

传统超声成像方法受瑞利准则的约束，难以对缺陷间距小于成像分辨率阈值的多缺陷进行成像。提出了一种基于时域拓扑能量的超声兰姆波成像方法，将逆散射拓扑成像方法中的拓扑渐进过程转换成求解直接声场和伴随声场。然后通过将伴随声场进行时间反转，两个声场将具有在缺陷处聚焦，在非缺陷处不聚焦的特性。将直接声场和伴随声场进行融合，以时域拓扑能量值作为像素值进行成像，从而使表征缺陷的精度较高。建立了缺陷间距小于分辨率阈值的多盲孔缺陷有限元模型，通过“一发多收”的方式激发S0模式和采集缺陷散射信号，并进行时域拓扑能量成像。仿真结果表明：对于多盲孔缺陷，时域拓扑能量成像法能够获得比延时叠加法和时间反转成像法更高的分辨率，并且能在缺陷间距小于成像分辨率阈值时进行成像。     

Terahertz quasi-time-domain spectroscopy imaging

We demonstrate terahertz (THz) imaging with a quasi-time-domain spectrometer. This type of THz system is inexpensive, compact, and relatively easy to set up. Beating the simultaneously emitted equidistant modes of a compact diode laser allows for analysis of samples at multiple frequencies with a single measurement. Thus, this technique merges the potential of terahertz time-domain spectrometers with the simplicity of continuous wave lasers. Multiple imaging applications and stability issues are discussed.     

Terahertz polarization real-time imaging based on balanced electro-optic detection

A terahertz (THz) polarization real-time imaging system that can effectively reduce experimental time consumption for acquiring a sample's polarization information is achieved. An alternative THz polarization measurement method is proposed. In this method, a <110> zinc-blende crystal is used as the sensor, and the probe polarization is adjusted to detect THz electric fields on the two orthogonal polarization components. The relative sensitivity of the imaging system to the THz polarization angle is estimated to be less than 0.5°. To illustrate the ability of the system, two samples are designed and measured by using the system. From their THz polarization real-time images, each region of these samples can be precisely presented. Experimental results clearly show the special influences of different materials on the THz polarization. This work effectively extends the information content obtained by THz real-time imaging and improves the feasibility of the imaging technique.     

太赫兹脉冲焦平面成像技术的发展与应用

作为太赫兹技术中的重要组成部分,太赫兹脉冲焦平面成像一经问世就引起了行业内的广泛关注,人们引入了各种方法去提升此成像技术的测量性能,同时也尝试将此成像技术应用于不同的工业和基础研究领域。本文综述了近年来人们对太赫兹脉冲焦平面成像的技术改良和应用研究,包括提升成像系统的空间分辨率、信噪比、信息获取能力,以及将此成像技术应用于光谱识别检测、超表面器件功能验证、太赫兹特殊光束测量、太赫兹表面波观测等,希望该综述能够推动太赫兹脉冲焦平面成像的进一步技术革新和应用拓展。     

Terahertz response of ferroelectric nanofibers

Far-infrared optical and dielectric properties of ferroelectric SrTiO3 and BaTiO3 nanofibers, prepared by hydrothermal syntheses, were studied using terahertz time-domain spectroscopy. The power absorption, refractive index, and complex dielectric function were characterized in the frequency range from 0.2 to 1.0 THz. The measured results are well reproduced by theoretical fittings based on the dielectric models and the effective medium model. The study reveals that the low-frequency dielectric properties of the ferroelectric SrTiO3 nanofibers are associated with the lowest transverse optical (TO) soft mode TO1 at 2.70 THz (90.0 cm(-1)), and that of the ferroelectric BaTiO3 nanofibers are related to the lowest pair of transverse optical (TO) and longitudinal optical (LO) modes near 5.35 THz, which are both consistent with their bulk single-crystal and thin-film counterparts.     

连续太赫兹波数字全息相衬成像

太赫兹波具有独特的低能性、高穿透性、惧水性等成像特性,将其应用于相衬成像能够反映物体的内部结构和更加丰富全面的生物信息,在生物医学检测等领域具有重要的应用。其中,太赫兹波数字全息成像是一种可以给出定量的振幅和相位信息的非接触、全场相衬成像方法,是太赫兹成像技术领域的重要研究方向之一。本文基于连续太赫兹源,从离轴式和同轴式数字全息成像的相衬成像原理、光路系统和再现算法多个方面,介绍了相关技术的研究现状,分析了太赫兹源、再现算法等因素对成像分辨率的影响,并对太赫兹数字全息的发展趋势进行了展望。     

Low-loss polymers for terahertz applications

We have performed high-precision terahertz time-domain spectroscopy measurements on polymers (cross-linked polystyrene, TPX, Zeonor) from 0.2 to 4.2 THz. They show very interesting terahertz and visible transparency. We also investigated the terahertz characteristics of PDMS, a polymer extensively used in microfluidics, which showed absorption compatible with terahertz experiments. The thermoplastic properties of these polymers make them suitable for use as lens, window, waveguide, or support materials in such applications as biological imaging or microfluidics necessitating a constant visual control not provided by conventional silicon- or teflon-based devices.     

Material parameter estimation with terahertz time-domain spectroscopy.

Imaging systems based on terahertz (THz) time-domain spectroscopy offer a range of unique modalities owing to the broad bandwidth, subpicosecond duration, and phase-sensitive detection of the THz pulses. Furthermore, the possibility exists for combining spectroscopic characterization or identification with imaging because the radiation is broadband in nature. To achieve this, we require novel methods for real-time analysis of THz waveforms. This paper describes a robust algorithm for extracting material parameters from measured THz waveforms. Our algorithm simultaneously obtains both the thickness and the complex refractive index of an unknown sample under certain conditions. In contrast, most spectroscopic transmission measurements require knowledge of the sample's thickness for an accurate determination of its optical parameters. Our approach relies on a model-based estimation, a gradient descent search, and the total variation measure. We explore the limits of this technique and compare the results with literature data for optical parameters of several different materials.     

Terahertz wave propagation in one-dimensional periodic dielectrics

We demonstrate the temporal evolution of terahertz (THz) wave propagation in one-dimensional periodic dielectrics. Distributed Bragg reflectors and a resonant cavity are investigated: The structures involve air gaps interleaved between polymer films. Transmitted and reflected broadband THz waves are measured by means of THz time-domain spectroscopy. The experimental results agree well with transfer matrix calculations.     

Integrated planar terahertz resonators for femtomolar sensitivity label-free detection of DNA hybridization

A promising label-free approach for the analysis of genetic material by means of detecting the hybridization of polynucleotides with electromagnetic waves at terahertz (THz) frequencies is presented. Using an integrated waveguide approach, incorporating resonant THz structures as sample carriers and transducers for the analysis of the DNA molecules, we achieve a sensitivity down to femtomolar levels. The approach is demonstrated with time-domain ultrafast techniques based on femtosecond laser pulses for generating and electro-optically detecting broadband THz signals, although the principle can certainly be transferred to other THz technologies.     

A study on the resolution of a terahertz spectrometer for the assessment of the porosity of pharmaceutical tablets

In this study, the resolution of a time-domain terahertz spectrometer for tablet porosity measurements was estimated using two sets of pharmaceutical tablets. One set consisted of tablets with constant thickness and with porosity as a variable. The other set consisted of tablets with variable thickness and only a relatively small change in porosity. The set with constant thickness was used for the calibration of the terahertz (THz) spectrometer for the porosity measurements, and the resolution of the terahertz spectrometer was estimated using the set with different thicknesses and slightly different porosities. It was shown that a time-domain THz spectrometer is a sensitive device that allows monitoring of minute changes in the porosity of pharmaceutical tablets by detecting the time delay of the THz pulse, which is attributable to different refractive indices within the tablet.     

Strain Rate Behavior of Closed-Cell Al-Si-Ti Foams: Experiment and Numerical Modeling

We present a combined experimental and numerical study on the strain rate effect of closed-cell Al-Si-Ti foams having different relative densities fabricated using the powder metallurgy foaming technique. The high strain rate tests were conducted with split Hopkinson pressure bar technique at 800 to 2500 s^?1. Two-dimensional mesoscale finite element models were created from tomographic images of the homologous foam. The rate sensitivity of the foam originates mainly from that of its parent material, increasing with increasing relative density. Stress elevation due to other effects, such as micro-inertia, shock wave, and gas pressure in individual cells, is negligible.     

太赫兹波计算鬼成像:原理和展望

本文立足于太赫兹波成像领域近年来备受关注的研究热点—太赫兹波计算鬼成像,首先回顾了鬼成像从量子到经典再到计算的历史过程,然后阐述了计算鬼成像的数学原理,随后综述了计算鬼成像在太赫兹波段的发展历程,及其在超衍射分辨成像、石墨烯光电导成像、太赫兹光谱成像等方面的应用,并在最后展望了太赫兹波计算鬼成像的发展前景:计算鬼成像作为一种成像手段,可以绕开在太赫兹频段缺乏经济高效的焦面阵列式探测器的难题,但目前的成像帧率还难以满足快速成像的应用需求,相信在未来随着器件性能的提升和成像算法的优化,其成像帧率可以得到大幅提升。     

Terahertz time-domain spectroscopy studies of the optical constants of the nematic liquid crystal 5CB

The optical constants of a nematic liquid crystal, 4'-n-pentyl-4-cyanobiphenyl (5CB), in the frequency range 0.3-1.4 THz were determined by terahertz (THz) time-domain spectroscopy. The real parts of the extraordinary refractive index n(e) and the ordinary refractive index n(o) of 5CB varied from 1.74 to 2.04 and from 1.59 to 1.83, respectively. Liquid-crystal 5CB exhibits a relatively small absorption loss in this frequency range. The birefringence of 5CB was found to be as large as 0.21. The experimental results indicate that liquid crystal 5CB is potentially useful for device applications in the THz frequency range.     

复谱频域OCT系统高分辨率图像重建

为了提高复杂多层样品层析图像的分辨率,构建了复谱频域光学相干层析成像(CSOCT)系统.由于其在低亮度和高速成像方面相对于时域OCT具有更高的灵敏度,因此在光学相干层析成像系统中具有重要的作用.本文对测试样品二层盖波片进行成像实验,基于光学相干层析基本理论,采用五帧相移算法,最终获得测试样品的复谱频域OCT图像.实验结果表明,该系统可以消除谱频域OCT图像中的寄生项和镜像,改善和提高层析图像的分辨率.     

Characterization of plasma sprayed coatings using thermal wave infrared video imaging

A pulsed (time-domain) thermal wave infrared video imaging technique, using an IR camera, a video image processor, and a flash lamp, is described. This system is applied to the imaging of interfacial defects in plasma sprayed coatings on metallic substrates. Results are compared with the conventional point-scanned cw infrared thermal wave imaging technique.     

Terahertz birefringence and attenuation properties of wood and paper

The far-infrared properties of spruce wood are examined with a terahertz time-domain spectrometer. The solid wood is shown to exhibit both birefringence and diattenuation. The birefringence properties are sufficient for construction of a quarter-wave plate operating at 0.36 THz, and a half-wave plate operating at 0.71 THz. The origin of the birefringence is attributed to preferential fiber orientation within the wood. Similar birefringence is observed in lens paper in which the fibers are preferentially oriented in one direction.     

太赫兹时域谱技术快速定性检测奶粉中的三聚氰胺

采用太赫兹时域谱(THz-TDS)技术测量了奶粉中三聚氰胺在0.2~1.8 THz频段的吸收谱和折射率谱,同时运用密度泛函理论(DFT)计算了三聚氰胺在太赫兹波段的振动频率,并据此对实验光谱吸收峰进行了指认,计算结果与实验光谱特征峰很好吻合.表明太赫兹时域谱技术对快速测定识别奶粉中三聚氰胺成分具有良好前景.