Fast broadband QCL-based chemical detection: an optical alternative for road surface reconnaissance

Fastly detecting hazardous, non-volatile chemical substances on paved roads and streets is a topic of utmost military importance in an area denial scenario. Since the 1980s, inherently slow manual sampling has been avoided on armored vehicles using a small silicone wheel that continuously accumulates surface contaminations. After a given sampling period, collected (and potentially hazardous) contaminants on the wheel are thermally desorbed and analyzed by mass spectrometry. This approach led to further technological advancements, including implementing a double-wheel sampling system for automated, uninterrupted operation. Suspicious areas are examined at low driving speeds (approximating a fast-walking speed) with comparatively low spatial resolution, as the silicone wheels can only be rolled comparatively slowly to ensure continuous surface contact. Incremental improvements may further optimize the double-wheel sampling system. In that context, we are currently investigating laser desorption technology to achieve a more targeted heat treatment of the complete silicone wheel and increase spatial resolution and sensitivity. In addition, we also contribute to the development of advanced ion mobility spectrometers, which are both fast scanning and highly sensitive, as a viable alternative to cumbersome mass spectrometers. As a radically different approach, we report here on a measurement system using back-scattering IR-spectroscopy to optically interrogate samples at a standoff distance and process the information without delay. The used IR light source consists of three coupled broadband quantum cascade laser modules, each with an integrated micro-opto-electro-mechanical grating scanner (MOEMS EC-QCL). The elaborate coupling of three such pulsed laser modules provides an ultra-broadband spectral scan within the IR-fingerprint area (covered by those three MOEMS EC-QCLs) at a repetition rate of almost one kilohertz, thus resulting in measurement times of as short as one millisecond per (ultra-broadband) spectrum. We found that even minor contaminations of hazardous substances are identified using this setup. Furthermore, preliminary laboratory tests revealed a successful detection after the measurements on a fast-moving contaminated object. The experiments were performed at different observation angles with a considerable focal depth. The proof of concept shows that this novel QCL-based chemical detection approach is fast enough and promising to continuously monitor the ground with sufficient geometric resolution at cruise speeds on uneven and textured surfaces.