Terahertz time-domain spectroscopy for the detection and identification of concealed materials in transmission and reflection configurations

The main purpose of this work is to demonstrate the capabilities of terahertz time-domain spectroscopy (THz-TDS) technique for noncontact and noninvasive identification of prohibited materials even they are concealed by ordinary fabrics. We will particularly highlight the interest of carrying out combined measurements in transmission and in reflection, especially in the case when the samples are made by a mixture of several constituents. To emphasize the relevance of our approach, we have used vanillin, which exhibits very clear resonant frequencies in the terahertz range. Vanillin hidden through textiles (wool and cotton) are tested using transmission and reflection THz-TDS configurations in the range from 0.3 to 1.5 THz. For either transmission or reflection measurements, our results demonstrate that concealed materials can be successfully detected and identified using the THz-TDS technique.