Theodore Maiman Seminar: "Advances in mid-IR laser based liquid and gas sensing: New measurement modalities and applications" (Prof. Bernhard Lend)

When interacting with gaseous or liquid samples, laser radiation does more than attenuate light intensity; it also induces a phase shift in the transmitted light and heats the sample containing the analyte. While direct absorption spectroscopy primarily exploits intensity attenuation, these additional effects form the basis for dispersion spectroscopy and indirect methods like photoacoustic and photothermal spectroscopies.
This presentation will review potential measurement modalities, highlight recent instrumental advancements, and showcase selected applications. For liquid sensing, we employ broadly tunable external cavity quantum cascade lasers. We demonstrate their use in measuring protein secondary structures in water, exemplified by in-line detection in liquid chromatography and protein melting experiments. Dispersion spectroscopy will be introduced through measuring ethanol in water, illustrating its suitability for dynamic reaction monitoring. This will include observing the enzyme-catalyzed hydrolysis of sucrose into glucose and fructose.
For gas sensing, the presentation introduces Interferometric Cavity-Assisted Photothermal Spectroscopy (ICAPS) as a novel and compact approach for gas analysis. ICAPS utilizes a distributed feedback quantum cascade laser in the mid-IR range, a Fabry-Pérot (FP) cavity, and a NIR probe laser. The target analyte's periodic absorption of mid-IR radiation leads to a modulated refractive index change in the gas matrix due to temperature fluctuations. This occurs within an FP cavity, causing a shift in its transmission function, detectable by a NIR laser passing through the cavity. This method achieves single-digit ppb sensitivities in a total absorption length of just 1 mm, showcasing its high precision and compactness.