Pump-probe photothermal spectroscopy using quantum cascade lasers

Obtaining compositional information for objects from a distance remains a major challenge in chemical and biological sensing. Capitalizing on mid-infrared (IR) excitation of molecules by using quantum cascade lasers (QCLs) and invoking a pump-probe technique, we present a variation of the photothermal process that can provide spectral fingerprints of substances from a variable standoff distance. We have evaluated the modal variations of the QCL beam that must be taken into account when applying QCLs for photothermal measurements. The results compare well with spectra obtained from conventional IR spectroscopy. Guided by the results, the potential of the measurements to be extended such that each point within a target region may be spectrally interrogated to form a hyperspectral image is discussed.