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

在过去的几年里，太赫兹波(位于红外和微波之间，即大致在0.1-10x10^12赫兹或3 mm-30微米波长范围内)已成为一种非常有前途的工具，可用于各种材料的实时光谱分析。在THz-TDS中，一个周期的THz波被发射到目标材料上，并在材料结构中主要激发分子间的振动。然后对作为时间函数测量的散射信号进行光谱解析。许多感兴趣的物质，包括爆炸物、污染物和生化物质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。太赫兹波具有一个显著的优势，即被许多日常包装材料(塑料、纸板、衣服)弱吸收。此外，由于随时间变化的反射信号包含关于各种界面的厚度和反射率的信息，因此THz-TDS能够对多层样品进行深入映射。尽管THz-TDS已经证明了其在传输模式和在后向散射模式下精心准备的实验室实验中用于光学薄样品的化学分析的潜力，但由于实际情况下后向散射模式固有的问题，该技术远未达到其全部强度。太赫兹-TDS研究中的主要挑战，也是本项目的目标，是能够在实际情况下以后向散射模式进行化学和结构分析。具体地说，这一挑战包括(1)有效地将波从源引导到样品，并从样品引导到探测器，(2)解释不规则表面的后向散射信号，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名合作者以及两家加拿大公司，Voti(安全)和Pharma激光(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.