Simulation and experiment of terahertz stand-off detection

Abstract

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

Terahertz Time‐Domain Spectroscopy of Gases, Liquids, and Solids

“…[20] This fact can be used for trace gas detection, but also causes problems for freespace measurement of THz transients in ambient air. [21] As water molecules are polar, absorption lines of water vapor are seen in the measurement even without inserted sample. In Figure 4 this is exemplarily shown for a 50-cm-long free-space propagation in dry air in comparison to standard laboratory atmosphere (31 % humidity).…”

Mail Inspection Based on Terahertz Time-Domain Spectroscopy

“…One of the main reasons why stand-off detection of explosives over several tens of meters is still a challenge, is the signal absorption due to water vapor in the environmental air [26]. Within a relatively tiny mail inspection system, the overall beam-path length is much lower and hence the influence of water vapor is less.…”

Study of atmospheric attenuation characteristics of terahertz wave based on line‐by‐line integration

“…With the development of new technologies and materials, a series of researches and experiments in the terahertz band have been carried out in recent years. In the field of accurate calculation method of atmospheric mean transmittance, researchers from all over the world have proposed various algorithms to integrate the wavenumbers, including the line‐by‐line integrating method, the theoretical band model method, and the correlated k distribution method …”

“…In the field of accurate calculation method of atmospheric mean transmittance, researchers from all over the world have proposed various algorithms to integrate the wavenumbers, including the line-by-line integrating method, the theoretical band model method, and the correlated k distribution method. [6][7][8] The analysis of the molecular transport in the terahertz atmosphere demands an accurate and effective atmospheric propagation model. In 1989, Liebe put forward the atmospheric millimeter wave propagation model (MPM) by taking the effects of water vapor, clouds, fog, haze, and rain water suspension into consideration.…”