Chris Kendziora

Stand-off detection of trace explosives by infrared photothermal imaging

We have developed a technique for the stand-off detection of trace explosives using infrared photothermal imaging. In this approach, infrared quantum cascade lasers tuned to strong vibrational absorption bands of the explosive particles illuminate a surface of interest, preferentially heating the explosives material. An infrared focal plane array is used to image the surface and detect a small increase in the thermal intensity upon laser illumination. We have demonstrated the technique using TNT and RDX residues at several meters of stand-off distance under laboratory conditions, while operating the lasers below the eye-safe intensity limit. Sensitivity to explosives traces as small as a single grain (~100 ng) of TNT has been demonstrated using an uncooled bolometer array. We show the viability of this approach on a variety of surfaces which transmit, reflect or absorb the infrared laser light and have a range of thermal conductivities. By varying the incident wavelength slightly, we demonstrate selectivity between TNT and RDX. Using a sequence of lasers at different wavelengths, we increase both sensitivity and selectivity while reducing the false alarm rate. At higher energy levels we also show it is possible to generate vapor from solid materials with inherently low vapor pressures.

Stand-off detection of trace explosives by infrared photo-thermal spectroscopy

We have developed a technique for stand-off detection of trace explosives using infrared photo-thermal imaging. Compact infrared quantum cascade lasers tuned to strong absorption bands in the explosive traces illuminate a surface of interest while an infrared camera detects the small increase in thermal signal. We have demonstrated the technique at several meters of stand-off distance under laboratory conditions using TNT and RDX traces, while operating the lasers below the eye-safe limit (100 mW/cm2). Sensitivity to explosive traces as small as 1ng has been demonstrated, using a micro-bolometer array. We show the viability of this approach on a variety of surfaces which transmit, reflect or absorb the infrared laser light. By varying the incident wavelength slightly, we show selectivity between TNT and RDX. Using several laser wavelengths, we increase both sensitivity and selectivity while reducing the false alarm rate. We have developed a prototype system for outdoor testing at longer stand-offs.

Advances in photo-thermal infrared imaging microspectroscopy

There is a growing need for chemical imaging techniques in many fields of science and technology: forensics, materials science, pharmaceutical and chemical industries, just to name a few. While FTIR micro-spectroscopy is commonly used, its practical resolution limit of about 20 microns or more is often insufficient. Raman micro-spectroscopy provides better spatial resolution (~1 micron), but is not always practical because of samples exhibiting fluorescence or low Raman scattering efficiency. We are developing a non-contact and non-destructive technique we call photo-thermal infrared imaging spectroscopy (PT-IRIS). It involves photo-thermal heating of the sample with a tunable quantum cascade laser and measuring the resulting increase in thermal emission with an infrared detector. Photo-thermal emission spectra resemble FTIR absorbance spectra and can be acquired in both stand-off and microscopy configurations. Furthermore, PT-IRIS allows the acquisition of absorbance-like photo-thermal spectra in a reflected geometry, suitable for field applications and for in-situ study of samples on optically IR-opaque substrates (metals, fabrics, paint, glass etc.). Conventional FTIR microscopes in reflection mode measure the reflectance spectra which are different from absorbance spectra and are usually not catalogued in FTIR spectral libraries. In this paper, we continue developing this new technique. We perform a series of numerical simulations of the laser heating of samples during photo-thermal microscopy. We develop parameterized formulas to help the user pick the appropriate laser illumination power. We also examine the influence of sample geometry on spectral signatures. Finally, we measure and compare photo-thermal and reflectance spectra for two test samples.

Raman spectroscopy of Bi2Sr2CaCu2O8+δ with varying oxygen stoichiometry

Abstract Measurements of Raman scattering from the electronic continuum are reported for crystals of the cuprate superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (Bi 2212) post-annealed at both high and low oxygen partial pressures to adjust T c . For temperatures below T c , a peak forms in the continuum at an energy shift which depends upon the polarizations of the incident and collected light as well as oxygen concentration. The results are discussed in terms of an anisotropic superconducting energy gap.

Advances in sublimation studies for particles of explosives

When handling explosives, or related surfaces, the hands routinely become contaminated with particles of explosives and related materials. Subsequent contact with a solid surface results in particle crushing and deposition. These particles provide an evidentiary trail which is useful for security applications. As such, the opto-physico-chemical characteristics of these particles are critical to trace explosives detection applications in DOD or DHS arenas. As the persistence of these particles is vital to their forensic exploitation, it is important to understand which factors influence their persistence. The longevity or stability of explosives particles on a substrate is a function of several environmental parameters or particle properties including: Vapor pressure, particle geometry, airflow, particle field size, substrate topography, humidity, reactivity, adlayers, admixtures, particle areal density, and temperature. In this work we deposited particles of 2,4-dinitrotoluene on standard microscopy glass slides by particle sieving and studied their sublimation as a function of airflow velocity, areal particle density and particle field size. Analysis of 2D microscopic images was used to compute and track particle size and geometrical characteristics. The humidity, temperature and substrate type were kept constant for each experiment. A custom airflow cell, using standard microscopy glass slide, allowed in-situ photomicroscopy. Areal particle densities and airflow velocities were selected to provide relevant loadings and flow velocities for a range of potential applications. For a chemical of interest, we define the radial sublimation velocity (RSV) for the equivalent sphere of a particle as the parameter to characterize the sublimation rate. The RSV is a useful parameter because it is independent of particle size. The sublimation rate for an ensemble of particles was found to significantly depend on airflow velocity, the areal density of the particles, and the particle field size. To compare sublimation studies these parameters must be known.

Electronic Raman scattering as a function of doping in high-{Tc} superconductors

We report the results of Raman scattering from the electronic continuum in Bi2Sr2CaCu2O8+(delta ) (Bi 2212) and Tl2Ba2CuO6+(delta ) (Tl 2201) high temperature superconductors with variations in the oxygen content, (delta) . Below Tc, a peak develops in the Raman continuum associated with the opening of a superconducting gap, (Delta) (k). By selecting the polarizations of incident and scattered light, we are sensitive to possible anisotropy of the gap within the a-b plane. Near optimal doping, both materials show gap anisotropy, with 2(Delta) /kBTc values of 7.2 (B1g) vs. 5.8 (A1g) in Tl 2201 and 8.5 (B1g) vs. 6.2 (A1g) in Bi 2212. In contrast, both show an isotropic gap at much lower energy shifts when the carrier concentration is raised: 2(Delta) /kBTc equals 3.9 (5.5) for Tl 2201 (Bi 2212) with Tc equals 37 K (57 K). We compare the observed spectra with calculations based on order parameters with d-wave as well as isotropic s-wave symmetry and conclude that raising the doping level reduces the gap anisotropy to near zero.