Fast evanescent wave spectroscopy in the long wave infrared

We report evanescent wave long wave infrared (LWIR) spectroscopy on a microsecond wavelength switching time scale using the recently developed acousto-optic modulator (AOM) tuned quantum cascade laser. We have examined several liquids to demonstrate the technique and they include isopropyl alcohol (IPA), ethanol, water, alcoholic beverage such as vodka, gin and whiskey, 2,2,2-trifluroethanol and Epsom salt dissolved in water. We also carried out dilution studies of absolute ethanol with water. We find that the commercially available vodka, gin and scotch alcohol concentrations fit well with our calibration. In each of these cases, we were able to collect spectral data from similar to 8.6 mu m to similar to 9.5 mu m in a spectral scan time of <500 mu s. The fast data collection capability point to applications of the AOM tuned QCL system as a real time process control tool where flowing fluids are mixed for production of new chemical and biological end products. (C) 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement