Terahertz time-domain spectroscopy (THz-TDS) combining with chemometrics methods was proposed for the identification of hepatic tumors. Two linear compression methods, principle component analysis and locality preserving projections (LPPs), and a nonlinear method, Isomap, were used to reduce the dimensionality of the measured dataset. Comparing two-dimensional (2-D) data reduced by these three dimensionality reduction techniques, only 2-D Isomap plot could separate the distances between two classes for the THz time-domain data and LPP had capacity of distinguishing two types of samples building on frequency-domain data. The best classification accuracies from 2-D time-domain data were ${\text{99.81}}\,\pm \,{\text{0.30}}\% $ and ${\text{99.69}}\,\pm \,{\text{0.61}}\% $ given by Isomap probabilistic neural network (PNN) and Isomap support vector machine (SVM), respectively, while the best classification results of 2-D frequency-domain data were ${\text{100.00}}\,\pm \,{\text{0.00}}\% $, ${\text{99.75}}\,\pm \,{\text{0.32}}\% $ provided by LPP-PNN, LPP-SVM. The results showed that Isomap and LPP are appropriate techniques to reflect the nonlinear manifold of the THz data. The THz technology either in time-domain or frequency-domain coupled with Isomap-PNN or LPP-PNN could offer a potential procedure to identify hepatic tumors.

In terahertz imaging systems based on Gaussian beam active illumination and focal plane array detectors, severe image distortion has been observed, which significantly reduces the resolving power of the imaging system. To solve this problem, a novel computational method, Light Field Imaging (LFI), has been introduced for terahertz imaging. A conventional transmission-type terahertz imaging system based on a gas-pumped terahertz source and terahertz Focal Plane Array Detectors (FPA) arrays is established to analyze the problem of image distortion. An experimental virtual camera array terahertz LFI system is also established. With the acquisition and reconstruction of synthetic aperture terahertz light fields, the improvement on resolving power and SNR performance have been validated.

In recent years, tremendous advancements have been made in various technologies such as far-infrared, low-frequency Raman, and two-dimensional (2D) Raman terahertz (THz) spectroscopies. A coherent method has emerged from numerous experimental and theoretical investigations of molecular dynamics in liquids by comparing linear and non-linear spectroscopic techniques. Intermolecular hydrogen bond vibration, molecular reorientation motion, and interaction between molecule/ionic solute and hydrogen bonds have been demonstrated to occur in the THz region, which are closely related to their physical/chemical properties and structural dynamics. However, precise probing of various modes of motion is difficult because of the complexity of the collective and cooperative motion of molecules and spectral overlap of related modes. With the development of THz science and technology, current state-of-the-art THz sources can generate pulsed electric fields with peak intensities of the order of microvolts per centimeter (MV/cm). Such strong fields enable the use of THz waves as the light source for non-linear polarization of the medium and in turn leads to the development of the emerging THz Kerr effect (TKE) technique. Many low-frequency molecular motions, such as the collective directional motion of molecules and cooperative motion under the constraint of weak intermolecular interactions, are resonantly excited by an intense THz electric field. Thus, the TKE technique provides an interesting prospect for investigating low-frequency dynamics of different media. In view of this, this paper first summarizes the research work on TKE spectroscopy by taking a solid material without low-frequency molecular motions as an example. Starting from the principle of TKE technology and its application in investigating the properties of solid matter, we have explored the low-frequency molecular dynamics of liquid water and aqueous solutions using TKE. Liquid water is a core of life and possesses many extraordinary physical and biochemical properties. The hydrogen bond network plays a crucial role in these properties and is the main reason for its various kinetic and thermodynamic properties, which differ from those of other liquids. However, the structure of the hydrogen bond network between water and solutes is not well known. Therefore, evaluating the hydrogen bond-related kinetic properties of liquid water is important.

Terahertz (THz) waves were generated using a two-color femtosecond laser with a fundamental wavelength of 1500 nm; the waves were modulated using two-color modulation plasma with a fundamental wavelength of 800 nm in a collinear configuration. The properties of the THz waves including intensity, polarization, and spectrum, were systematically investigated by changing the energy ratio, relative phase, and the polarization direction of the pump and modulation beams. The results demonstrate that the THz intensity has a superimposed effect when the two beams simultaneously ionize the ambient air. When the energy ratio of the beams was changed, the intensity ratio of the high- and low-frequency components of the THz spectrum changed continuously, and the THz polarization gradually transitioned from elliptical to linear. Moreover, the center frequency of the THz spectrum can be blue shifted by shortening the optical length of the modulation beam. Besides, an arbitrary elliptically polarized THz wave can be obtained by controlling the relative polarization angle of the two-color modulation lasers. The findings of this article will facilitate practical applications involving the generation and selection of specific THz radiation.

目前太赫兹辐射信号的功率不高, 辐射带宽也较窄, 这些对于生化、含能材料的太赫兹检测应用领域来说是一大限制因素, 因此如何获得宽谱高功率的太赫兹源对于太赫兹时域光谱系统的发展是非常重要的; 另一方面, 常规的太赫兹系统是在自由空间传输探测的, 测量过程需要在氮气或者干燥空气环境中进行, 以克服空气中水的吸收干扰, 同时自由空间中的光场与物质相互作用的模式又降低了物质检测的灵敏度, 这对于痕量物质检测运用来说构成了一个挑战. 面对这一问题, 太赫兹片上系统利用微纳结构中的局域场效应实现对物质的低浓度检测, 这一方案可以有助于解决这一应用难题. 综上所述, 本文分成以下两部分阐述: 首先阐述了纳米金属薄膜作为新的太赫兹源, 它可以同时产生非相干的和相干的太赫兹信号, 其输出为超过100 THz的太赫兹-红外辐射, 功率能高达10 mW, 这超宽谱和高功率现象主要是由于非相干的热辐射效应引起的; 第二, 阐述了基于不同传输线结构、不同基底材料的太赫兹片上系统结构设计和光谱应用. 基于共面带状线结构和聚合物材料基底的太赫兹片上系统有着较低的损耗, 能够实现超过2 THz带宽的测量和生化应用.