Remote sensing of explosives using mid-infrared quantum cascade lasers

The spectroscopic detection of complex molecules, such as explosives, requires a much broader spectral tuning range of the employed laser compared to the well-established tunable diode laser spectroscopy of light molecules. In this paper we demonstrate a new type of spectrally-tunable laser modules, which enables an increase of the tuning range by at least one order of magnitude in comparison to the established current-temperature tuning techniques. This was achieved by increasing the operating temperature range through the use of a temperature-variable diamond submount. The module comprises a quantum cascade (QC) laser mounted on the diamond submount with an integrated heater element and temperature sensor, enabling a controlled temperature change of the laser between 77 K and 400 K at a rate of temperature change of up to 2500 K/s. The favorable temperature characteristics allow us to easily tune the laser wavelength on- and off-resonance with the characteristic absorption bands of the material to be detected. With the QC laser module we demonstrate the detection of surface contaminations of around 10 μg/cm2 TNT performing imaging backscattering spectroscopy.