Broad band tunable quantum cascade lasers for stand-off detection of explosives

We present experimental results on a Quantum cascade laser (QC laser) embedded in an external cavity. These results were obtained with a broadly tunable laser exceeding 80 cm-1 covering a characteristic absorption band of trinitrotoluene (TNT). By combining the laser source with a high performance IR imager a stand-off detection setup based on multi-spectral MIR backscattering spectroscopy has been realized. With this technique TNT surface-contaminations of as low as 10 µg/cm2 could be detected on surfaces such as an aluminum-sheet and standard car paint. The contrast of the detection technique depends on the reflectance of the surface. A surface leading to mirror-like reflectance of the IR laser radiation leads to absorbance-like signatures of the TNT contamination, while surfaces showing high absorbance of the laser light may induce a contrast-reversal in the resulting image of the TNT coverage. This effect can be explained by a theoretical model for thin film coated substrates taking into account differences in the reflectance. Limitations and further work needed to explore the full potential of the IR backscattering technique are also discussed.