Robert Furstenberg

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.

Trace gas absorption spectroscopy using functionalized microring resonators

We detect trace gases at parts-per-billion levels using evanescent-field absorption spectroscopy in silicon nitride microring resonators coated with a functionalized sorbent polymer. An analysis of the microring resonance line shapes enables a measurement of the differential absorption spectra for a number of vapor-phase analytes. The spectra are obtained at the near-infrared overtone of OH-stretch resonance, which provides information about the toxicity of the analyte vapor.

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.

Towards Enhanced Detection of Chemical Agents: Design and Development of a Microfabricated Preconcentrator

Cascade Avalanche Sorbent Plate ARray (CASPAR), a micromachined hotplate coated with sorbent polymer, has been demonstrated earlier as a selective preconcentrator for chemical agents and explosives. Up to two orders of magnitude increase in sensitivity has been established by incorporating CASPAR as a front end-modification to commercial detectors. Experimental evidence obtained via thermal imaging and fluorescent particle flow tagging suggests that an 8.5 mm times 8.5 mm current hotplate design is sub-optimal due to non-uniformities in the heating and vapor/particle collection profiles. In this study, we discuss the CASPAR design optimization with the aim to enhance the collection efficiency of hazardous chemical vapor while maintaining thermal stability and precise control before injection of a focused pulse of analyte into a detector.

Infrared microthermography of microfabricated devices.

We report a new experimental apparatus for infrared microthermography applicable to a wide class of samples including semitransparent ones and perforated devices. This setup is particularly well suited for the thermography of microfabricated devices. Traditionally, temperature calibration is performed using calibration hot plates, but this is not applicable to transmissive samples. In this work a custom designed miniature calibration oven in conjunction with spatial filtering is used to obtain accurate static and transient temperature maps of actively heated devices. The procedure does not require prior knowledge of the emissivity. Calibration and image processing algorithms are discussed and analyzed. We show that relatively inexpensive uncooled bolometer arrays can be a suitable detector choice in certain radiometric applications. As an example, we apply this method in the analysis of temperature profiles of an actively heated microfabricated preconcentrator device that incorporates a perforated membrane and is used in trace detection of illicit substances.

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.