Optical microsensors operating in the infrared for the detection of emerging pollutants in groundwater and marine waters.

Summary The development of optical sensors operating in the mid-infrared is a major challenge to detect (bio-) molecules. Indeed, the mid-infrared (4000 - 400 cm-1) contains a large majority of the absorptions due to the vibrations of organic molecules. The objective of this research is to develop more selective, more sensitive and more compact sensors. The research work presented in this manuscript concerns the development of chalcogenide glass-based optical waveguides sensitive to target molecules via the absorption of the evanescent part of the guided light propagating on the surface of the guide. The synthesis of infrared materials is one of the key steps. Chalcogenide glasses are particularly suitable materials for this application of pollutant detection. These glasses have a wide range of transparency in the infrared (2 - 15 µm for selenium-based glasses) and high refractive indices (between 2 and 3). The development of the waveguide requires the fabrication of thin layers of chalcogenide glass by RF magnetron sputtering. In order to control the development of the infrared microsensor, a design of experiments has been developed to study the influence of the deposition parameters on the characteristics of the thin films. The architecture of the guide has been defined and realized by RIE-ICP (reactive ion etching coupled to the inductively coupled plasma etching process) and the optical loss and injection measurements in the MIR (6.3 µm and 7.8 µm) have been performed. These are the very first guides operating so far in the mid-infrared. The final step is to functionalize the surface of the guide to increase its selectivity. First tests have been performed on a ZnSe ATR crystal with a hydrophobic polymer. They allowed the detection of polluting molecules absorbing at 13.8 µm, present in very low concentration (25 ppb) in hydrocarbon solutions (BTX) or more complex waters (sewage plant waters and groundwater).