Michael R. Papantonakis

Stand-off detection of trace explosives via resonant infrared photothermal imaging

We describe a technique for rapid stand-off detection of trace explosives and other analytes of interest. An infrared (IR) laser is directed to a surface of interest, which is viewed using a thermal imager. Resonant absorption by the analyte at specific IR wavelengths selectively heats the analyte, providing a thermal contrast with the substrate. The concept is demonstrated using trinitrotoluene and cyclotrimethylenetrinitramine on transparent, absorbing, and reflecting substrates. Trace explosives have been detected from particles as small as 10 μm.

Resonant infrared pulsed-laser deposition of polymer films using a free-electron laser

Thin films of polyethylene glycol (MW 1500) have been prepared by pulsed-laser deposition (PLD) using both a tunable infrared (λ=2.9 μm, 3.4 μm) and an ultraviolet laser (λ=193 nm). A comparison of the physicochemical properties of the films by means of Fourier transform infrared spectroscopy, electrospray ionization mass spectrometry, and matrix-assisted laser desorption and ionization shows that when the IR laser is tuned to a resonant absorption in the polymer, the IR PLD thin films are identical to the starting material, whereas the UV PLD show significant structural modification. These results are important for several biomedical applications of organic and polymeric thin films.

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.

Infrared photothermal imaging of trace explosives on relevant substrates

We are developing a technique for the stand-off detection of trace explosives on relevant substrate surfaces using photo-thermal infrared (IR) imaging spectroscopy (PT-IRIS). This approach leverages one or more compact IR quantum cascade lasers, tuned to strong absorption bands in the analytes and directed to illuminate an area on a surface of interest. An IR focal plane array is used to image the surface and detect small increases in thermal emission upon laser illumination. The PT-IRIS signal is processed as a hyperspectral image cube comprised of spatial, spectral and temporal dimensions as vectors within a detection algorithm. The ability to detect trace analytes on relevant substrates is critical for stand-off applications, but is complicated by the optical and thermal analyte/substrate interactions. This manuscript describes recent PT-IRIS experimental results and analysis for traces of RDX, TNT, ammonium nitrate (AN) and sucrose on relevant substrates (steel, polyethylene, glass and painted steel panels). We demonstrate that these analytes can be detected on these substrates at relevant surface mass loadings (10 μg/cm2 to 100 μg/cm2) even at the single pixel level.

Advances in standoff detection of trace explosives by infrared photo-thermal imaging

A technique for stand-off detection of trace explosives using infrared (IR) photo-thermal (PT) imaging, remote explosives detection (RED), is under development at the Naval Research Laboratory. In this approach, compact IR quantum cascade lasers (QCLs) tuned to strong absorption bands of trace explosives illuminate a surface of interest. An IR focal plane array is used to image the surface and detect any small increase in the thermal emission upon laser illumination. The technique has been previously demonstrated at several meters of stand-off distance indoors and in field tests with sensitivity to explosive traces as small as a single grain (~1 ng), while operating the lasers below the eye-safe intensity limit (100 mW/cm2) at the tested wavelengths. By varying the incident wavelength slightly, selectivity between TNT and RDX has been achieved. A complete test and analysis can be performed in less than 1 second. This manuscript critically examines components used with RED and demonstrates several improvements. These include QCL drive electronics for narrower spectral emission linewidth, fixed wavelength QCL packaging that optimizes spectral and spatial output, fiber-optic coupling for QCL beam steering and spatial filtering, cooled IR sensors that increase sensitivity and speed, tunable QCL sources that increase selectivity and extend the library of possible analytes, and dynamic PT signal processing that can increase sensitivity and speed. When considered in combination with the capabilities previously demonstrated for RED, and its capability to operate within eye-safety limits, this technology offers the potential for a wide area of applications relating to the detection of trace explosives on surfaces in both non-contact and stand-off configurations.

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.

Real-world particulate explosives test coupons for optical detection applications

Trace or residue explosives detection typically involves examining explosives found as solid particles on a solid substrate. Different optical spectroscopy techniques are being developed to detect these explosives in situ by probing how light interacts with the surface bound particles of explosives. In order to evaluate these technologies it is important to have available suitable test coupons coated with particles of explosives. When fabricating test coupons to evaluate detection performance or help train a detection algorithm, it is important to use realistic test coupons and consider how the physicochemical properties of the explosives particles, related chemicals, and substrate may affect the spectra produced or signal intensities observed. Specific features of interest include surface fill factor, particle sizes, areal density, degree of particle contact with a substrate and any other chemicals in addition to the explosives and substrate. This level of complexity highlights the need to fabricate test coupons which mimic “real world” particle coated surfaces. With respect to metrics derived from fingerprints, we compare the properties of test coupons fabricated by sieving and inkjetting for ammonium nitrate, TNT, RDX, and sucrose on stainless steel, automotive painted steel, glass and polyethylene substrates. Sieving provides a random distribution of particles, allows fractionation of relevant particle sizes and allows relevant surface fill factors to be achieved. Inkjetting provides precise control of aerial density but because of complications related to solvent-substrate interactions, relevant fill factors and particle sizes are difficult to achieve. In addition, we introduce a custom image analysis technique, NRL ParticleMath, developed to characterize and quantify particle loadings on test coupons.

The challenge of changing signatures in infrared stand-off detection of trace explosives

We report our preliminary results on numerical modeling of IR back-scattering (reflectivity) and absorption (photothermal) IR signatures of micron-size (5-20 μm) particles. We use the Mie scattering theory which is an exact solution of the scattering problem for spherical particles of arbitrary size. In this paper, we approximate the particles as spheres with an equivalent volume. While we expect particle shape to influence IR spectra (albeit to a lesser extent), in this paper, we restrict ourselves to the effect of size (i.e. particle diameter) only. We also study the effect of air, solvent and other additive inclusions on the IR spectra. Finally, we address the effect of particle surface roughness. We show that all these parameters (size, inclusions, roughness) affect the scattering process which results in distortions to the IR spectra as compared to library values for bulk material. The effect of substrate on the IR spectra is not studied due to the limitations of the Mie scattering theory which was developed for isolated particles only. In addition to particle-related effects on IR spectra, the presence of substrate will have additional effects as well and this was studied previously by other research groups. We expect that the results of this study will help improve the performance of various detection algorithms by accounting for changing IR spectra.

Modeling of laser-analyte-substrate interaction in photo-thermal infrared imaging and laser trace vaporization

We are developing two techniques for non-contact detection of explosives and other substances with low vapor pressure. In one approach, quantum cascade lasers (QCLs) at eye-safe power levels heat trace residues on surfaces at stand-off distances and the photo-thermal signal is imaged with an infrared camera. When using wavelengths corresponding to vibrational resonances specific to the trace molecules, the traces can be selectively heated and become visible in the infrared. In a second approach, a QCL or other IR laser of higher power is used to enhance the vapor signature of the analyte, thus facilitating vapor-based (e.g. ion mobility spectrometry) techniques. Details and advances in these techniques will be reported elsewhere. In this paper, we study the laser heating of analytes on substrates using the simulation software COMSOL. A model is validated with experimental results for particles of well characterized shape and size. The heat transfer between particle and substrate is of special interest, but not necessarily the dominant contributor to heat loss. Both air- and substrate-mediated heating of neighboring interferent particles is generally negligible. The presence of neighboring explosives particles affects the thermal kinetics via air-mediated heat transfer.

Laser vaporization of trace explosives for enhanced non-contact detection

Trace explosives contamination is found primarily in the form of solid particulates on surfaces, due to the low vapor pressure of most explosives materials. Today, the standard sampling procedure involves physical removal of particulate matter from surfaces of interest. A variety of collection methods have been used including air-jetting or swabbing surfaces of interest. The sampled particles are typically heated to generate vapor for analysis in hand held, bench top, or portal detection systems. These sampling methods are time-consuming (and hence costly), require a skilled technician for optimal performance, and are inherently non-selective, allowing non-explosives particles to be co-sampled and analyzed. This can adversely affect the sensitivity and selectivity of detectors, especially those with a limited dynamic range. We present a new approach to sampling solid particles on a solid surface that is targeted, non-contact, and which selectively enhances trace explosive signatures thus improving the selectivity and sensitivity of existing detectors. Our method involves the illumination of a surface of interest with infrared laser light with a wavelength that matches a distinctive vibrational mode of an explosive. The resonant coupling of laser energy results in rapid heating of explosive particles and rapid release of a vapor plume. Neighboring particles unrelated to explosives are generally not directly heated as their vibrational modes are not resonant with the laser. As a result, the generated vapor plume includes a higher concentration of explosives than if the particles were heated with a non-selective light source (e.g. heat lamp). We present results with both benchtop infrared lasers as well as miniature quantum cascade lasers.