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

在过去的几年里，太赫兹波（位于红外线和微波之间，即，大致在0.1 -10x10^12 Hz或3 mm至30 µm波长范围内），已成为对各种材料进行实时光谱分析的非常有前途的工具。在太赫兹时域光谱中，一个周期的太赫兹波被发射到目标材料上，并且在材料结构中主要激发分子间的振动。散射信号测量作为时间的函数，然后spectralresolved。在太赫兹波段，许多感兴趣的材料，包括爆炸物、污染物和生化材料，都具有独特而详细的光谱指纹。太赫兹波具有被许多日常包装材料（塑料、纸板、衣服）微弱吸收的显著优势。此外，THz-TDS能够对多层样品进行深度映射，因为随时间变化的反射信号包含各种界面的厚度和反射率信息。虽然THz-TDS在透射模式和精心准备的实验室背向散射模式中已经证明了其对光学薄样品进行化学分析的潜力，但由于实际情况中背向散射模式固有的问题，该技术远未达到其全部强度。在太赫兹TDS研究的主要挑战，以及本项目的目标，是能够在实际情况下，在后向散射模式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，从样品到检测器，（2）解释由不规则表面反向散射的信号，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2家加拿大公司，VOTI（安全）和Pharma Laser（制药行业），这两家公司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.