Since its first operational demonstration in 1994, the quantum cascade laser (QCL) advanced to a powerful and reliable spectroscopic source of coherent light covering the mid-infrared (MIR) and terahertz spectral region for sensitive detection of molecular species on their fundamental vibrational bands and rendered laser-based absorption spectroscopy a powerful tool for industrial gas sensing, atmospheric science, environmental monitoring, combustion research, and fundamental physics, enabling detailed investigations and advancements.

Direct and indirect measurement principles are applied in order to quantify the light-matter-interaction. 

Direct laser-based gas phase spectroscopy techniques including direct absorption (DA), wavelength modulation (WM) and heterodyne phase shift dispersion (HPSD) provide information about molecular energy levels and transitions by evaluating the primary properties of optical transmission after the interaction of light with gas molecules. These include transmission intensity (and inferred absorption), dispersion and polarization.

 In contrast, indirect techniques, evaluate secondary macroscopic effects caused by the absorption of energy. These effects mainly include contributions of photoacoustic (pressure waves), photothermal (temperature, refractive index change) and photoconductive nature.

Johannes Paul Waclawek
Harald Moser
Davide Pinto
Giulia Malvicini
Savda Sam
Iskander Gazizov