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

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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.