High-speed and large-area scanning of surfaces for trace chemicals using wavelength-tunable quantum cascade lasers

A standoff chemical detection system is being developed to detect and identify a wide range of trace chemicals on a variety of natural and artificial surfaces. The system is based on active mid-infrared (MIR) hyperspectral imaging in which the target surface is illuminated using miniature, rapidly tunable, external-cavity quantum cascade lasers (ECQCLs). These lasers are tuned across the wavelength range of 7.7 – 11.8 μm while a HgCdTe camera captures images of the reflected light. Hypercubes with 128x128 pixels and more than 130 wavelengths are captured within 0.1 s. By operating the camera in sub-window mode, hypercubes with 16x96 pixels and 138 frames are captured in only 14 ms. To the best of our knowledge, these represent the world’s fastest acquisition of active MIR hypercubes. Raster-scanning of the laser beam is used to scan large regions. In this talk, we will present results for detecting traces of solid chemicals (with loadings on the order of 100 μg) on natural outdoor surfaces such as roofing shingles, concrete, sand, and asphalt at a standoff distance of 5 m. The measured spectra are found to correlate very well with those of reference measurements made of pure chemicals after accounting for the substrate reflectance.