Terahertz-frequency Spectroscopy as a Technique for the Remote Detection of Biological Warfare Agents

This paper discusses the potential use of terahertzfrequency spectroscopy as a technique for the remote detection of biological warfare agents. Here, design studies are presented for a differential-absorption-radar (DAR) approach that utilizes the spectral signatures of Bacillus (B.) subtillus spores within the terahertz (THz) regime as the detection mechanism. The signature data used in these studies is taken from laboratory measurements performed on uniform thin films of B. subtillus spores and the system performance is assessed for both incoherent and coherent detector modalities. These studies consider DAR remote sensing of biological (bio) clouds at significant ranges (i.e., 1 km) and include the effects of realistic atmospheric conditions. Specifically, THz spectral signatures were used in a highlevel remote-sensor design to estimate the probabilities of detection (Pd) and false-alarm (Pfa) associated with this general technique. The system design studies suggest useful remote-detection performance can be achieved (i.e., Pd > 0.9 & Pfa << 10 for bio-cloud densities < 10 cm) at 1 km ranges if the THz signature information remains predictably stable under varying atmospheric conditions (e.g., changes in humidity, spore activity state, etc). Hence, this paper will also present recent results from scientific studies on bio-molecules that are exploring the underlying mechanisms responsible for the spectral signatures. DAR system performance is also assessed in a more practical bio-warfare military or civilian defense scenario. Here, the Pd and Pfa statistics for the DAR system are mapped onto the time-dependent evolution of an aerosol bio-cloud release to give a real-time indication of the effectiveness of the detection methodology. All together, these results demonstrate that standoff detection of bio-agents is feasible for threat-level concentrations in practical battlefield environments at sufficient ranges to provide for early warning.