The terahertz (THz) regime (0.1-10 THz) is rich with emerging possibilities in sensing, imaging and communications, with unique applications to screening for weapons, explosives and biohazards, imaging of concealed objects, water content and skin. Here we present initial surveys to evaluate the possibility of sensing plastic explosives and bacterial spores using field-deployable electronic THz techniques based on short-pulse generation and coherent detection using nonlinear transmission lines and diode sampling bridges. We also review the barriers and approaches to achieving greater sensing-at-a-distance (stand-off) capabilities for THz sensing systems. We have made several reflection measurements of metallic and non-metallic targets in our laboratory, and have observed high contrast relative to reflection from skin. In particular, we have taken small quantities of energetic materials such as plastic explosives and a variety of Bacillus spores, and measured them in transmission and in reflection using a broadband pulsed electronic THz reflectometer. The pattern of reflection versus frequency gives rise to signatures that are remarkably specific to the composition of the target, even though the target's morphology and position is varied. Although more work needs to be done to reduce the effects of standing waves through time-gating or attenuators, the possibility of mapping out this contrast for imaging and detection is very attractive.

By illuminating the sample with a broadband 10-300 GHz stimulus and coherently detecting the response, we obtain reflection and transmission spectra of common powdered substances, and compare them as a starting point for distinguishing concealed threats in envelopes and on personnel. Because these samples are irregular and their dielectric properties cannot be modulated, however, the spectral information we obtain is largely qualitative. To show how to gain quantitative information on biological species at micro- and millimetre-wave frequencies, we introduce thermal modulation of a globular protein in solution, and show that changes in single-wavelength microwave reflections coincide with accepted visible absorption spectra, pointing the way towards gaining quantitative chemical and biological spectra from broadband terahertz systems.

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The terahertz regime (0.1 to 10 THz) is rich with emerging possibilities in sensing, imaging and communications, with unique applications to screening for weapons, explosives and biohazards, imaging of concealed objects, water content and skin. Here we present initial surveys to evaluate the possibility of sensing bacterial spores and chemical material using field-deployable electronic terahertz techniques that use short-pulse generation and coherent detection based on nonlinear transmission lines and diode sampling bridges. We also review the barriers and approaches to achieving greater sensing-at-a-distance (stand-off) capabilities for THz sensing systems. We have made several reflection measurements of metallic and non-metallic targets in our laboratory, and have observed high contrast relative to reflection from skin. In particular, we have taken small quantities of materials such as dimethyl methylphosphonate (DMMP) and several variants of Bacillus spores, and measured them in transmission and in reflection using a broadband pulsed electronic THz reflectometer. The pattern of reflection versus frequency gives rise to signatures that indicate specificity of the target. Although more work needs to be done to reduce the effects of standing waves through time gating or attenuators, the possibility of mapping out this contrast for imaging and detection is very attractive.

No abstract