Terahertz (THz) technology has been demonstrated as a promising tool for detection of explosives and is being
developed for aviation screening and sensing of improvised explosive devices. THz radiation is attractive for many
applications due to its ability to penetrate through a wide range of dielectric materials including clothing, paper,
cardboard, plastics, and wood. Of course, metals block THz waves as is the case for microwave, IR, and visible light.
Our work has involved investigating the reflection spectroscopy of a variety of materials including explosives such as
RDX and PETN, plastic explosive taggants such as DMDNB, and other organic materials. We have also investigated the
changes of the reflection spectra in varying grades of sucrose. Spectral differences are observed between three grades of
crystalline sugar in the region from 0.1 to 1 THz. By exploiting the unique spectral features, the discrimination
capabilities of THz reflection spectroscopy points to the broad applicability of identifying a wide variety of materials.
It is well-known that many explosives have characteristic terahertz (THz) absorption features, and that THz waves can
penetrate many dielectric materials. However security applications generally prohibit using THz technology for
transmission measurements, either because of standoff distances, thick targets, or opaque targets (metals). As a result,
we focus our attention on THz reflection spectroscopy. We have measured the THz reflectivity signature of RDX
residues on smooth metal surfaces that contain about 0.4 mg of RDX. We discuss our efforts in detecting trace
explosives in reflection as well as our recent results including THz spectroscopy of four explosives from 1 to 6 THz,
and measurement of the absolute absorption cross-section of explosives.
We report on the application of poled electro-optic (EO) polymer films in a gap-free, broadband terahertz (THz) system. Using polymer films consisting of 40% Lemke/60% APC (LAPC) as an emitter-sensor pair and a Ti:sapphire regenerative laser pulse amplifier operated at 800-nm-wavelength, we generated and detected transient THz waves, via the optical rectification and EO effect, respectively. We obtained ~12-THz bandwidth from this system with no absorption gaps. The absence of resonant absorption gaps normally seen in THz systems based on crystalline EO materials is attributed to the amorphous form of the polymer films, making our EO polymer emitter-sensor pair advantageous over EO crystals in a gap-free, broadband THz time-domain-spectroscopy (THz-TDS) system. A model has been developed to simulate the spectrum from THz systems and the simulation results were compared with the experimental results. We also report our experiments and simulations for the pulsed THz waves generated by a EO polymer film consisting of 40% DCDHF-6-V/60% APC (DAPC) and detected either by an 80-μm ZnCdTe or a 2-mm ZnTe sensor, with 1300-nm-wavelength pulses from an optical parametric amplifier (OPA). In addition, with the help of our model, we propose employing a wavelength tuning technique to achieve good phase-matching for polymer emitter/sensor pairs, which should lead to very broad bandwidth.
Electro-optic (EO) polymers are promising materials to be used as THz emitters and sensors due to their high nonlinear coefficients and good phase-matching conditions. We demonstrate efficient THz generation from an 80 μm thick EO polymer emitter which is equivalent to that of a 1000 μm thick ZnTe standard. Also, this kind of EO polymer allows a generation up to 20 THz with ultra-short laser pulses. We have observed resonance-enhanced THz generation in another kind of EO polymer composite near its absorption maximum. Due to a sharp resonance of the EO coefficient near the absorption maximum of the material, the amplitude of THz field generated from a 3.1 μm thick film of this composite is 15% larger than that from a 1000 μm thick ZnTe standard. The estimated EO coefficient of this composite at 800 nm is over 1200 pm/V.