Simulation and experiment of terahertz stand-off detection
Terahertz (THz) spectroscopy is a promising technique for the stand-off detection and characterization of hidden objects. The THz band is particularly well suited firstly because THz radiation penetrates many dielectrics like clothes and secondly because many potentially hazardous substances have characteristic signatures in the THz spectral region. In order to utilize the full potential of THz radiation for detecting possible hazards and recognizing characteristic signatures, disturbing influences must be accounted for. We have performed experiments and simulations in order to investigate the limits of terahertz stand-off detection. A special emphasis is paid on humidity in ambient air and properties of the sample like surface roughness, alignment and interfaces. Water vapor absorption st rongly affects the THz spectra. Since the absorption lines are strong and narrow, the calculation must be precise. We have checked various models well-known in meteorology covering the infrared and the microwave region of the electromagnetic spectrum. By extending the models into the THz region, an accurate description of the measured spectral absorption is achieved. In our studies transmission spectra for different substances were tested. In a reflection scheme metallized sandpaper of various grit sizes was used to determine the influences of different surface properties. Further measurements were performed for different tilt angles to analyze the influence of the surface roughness. We are currently creating a look-up table to show which parts of the THz spectrum can be used for THz stand-off detection.