Chemical and structural analysis by terahertz time domain spectroscopy in the backscattering mode

在过去的几年中，太赫兹波（位于红外和微波之间，即大致在0.1-10x10^12 Hz或3mm至30 μ m波长范围内）已成为一种非常有前途的工具，用于各种材料的实时光谱分析。在太赫兹- tds中，一个单周期的太赫兹波被发射到目标材料上，在材料结构中主要激发分子间振动。作为时间函数测量的散射信号，然后进行频谱分解。许多感兴趣的材料，包括炸药、污染物和生化材料的独特和详细的光谱指纹已经在太赫兹范围内被识别出来。太赫兹波具有被许多日常包装材料（塑料、纸板、衣服）弱吸收的显著优势。此外，由于时间相关的反射信号包含了各种界面的厚度和反射率信息，因此THz-TDS可以对多层样品进行深度映射。虽然太赫兹- tds已经在传输模式和精心准备的后向散射模式的实验室实验中证明了其在光学薄样品化学分析方面的潜力，但由于实际情况下后向散射模式固有的问题，该技术还远远没有达到其全部强度。太赫兹- tds研究的主要挑战，也是本项目的目标，是能够在实际情况下在后向散射模式下进行化学和结构分析。具体来说，这一挑战包括(1)有效地引导波从源到样品和从样品到探测器，(2)解释不规则表面的信号反向散射，以及(3)找到测量参考信号的替代方案。该项目涉及来自一所大学（INRS）的3名合作者以及两家加拿大公司，VOTI（安全）和Pharma Laser（制药工业），他们都参与了复杂分子化合物的鉴定。

Over the last few years, Terahertz waves (which lie between infrared and microwaves, i.e., roughlyin the range0.1-10x10^12 Hz or 3 mm to 30 µm wavelength), have become a highlypromising tool for real-time spectroscopyanalysis of a wide varietyof materials. In THz-TDS, a one-cycle THz wave is emitted onto a target material and there excites mainlyintermolecular vibrations in the material structure. The scattered signal measured as a function of time, is then spectrallyresolved. Unique and detailed spectral fingerprints of many materials of interest, including explosives, pollutants, and biochemical materials have alreadybeen identified in the THz range. THz waves present the striking advantage of being weaklyabsorbed bymanyeveryday packaging materials (plastics, cardboard, clothes). In addition, THz-TDS enables in-depth mapping of multilayer samples since the time-dependent reflected signal contains information on the thickness and reflectivityof the various interfaces. Although THz-TDS has demonstrated its potential for chemical analysis of opticallythin samples in the transmission mode and in carefullyprepared laboratoryexperiments in the backscattering mode, the technique is far from having reached its full strength yet due to problems inherent to the backscattering mode in practical situations. The major challenge in the research on THz-TDS, and the goal of this project, is to be able to perform chemical and structural analysis in the backscattering mode in practical situations. Specificallythis challenge includes (1) guiding efficientlythe wave from the source to the sample and from the sample to the detector, (2) interpreting the signal backscattered byirregular surfaces, and (3) finding alternatives to measuring the reference signal. The project involves 3 collaborators from a university (INRS) as well as 2 Canadian companies, VOTI(security) and Pharma Laser (pharmaceutical industry), which are both involved in the identification of complex molecular compounds.