Chemical Detection and Sensing Using Optical Interferometry
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Chemical detection, including analysis of gases and liquids, is a large field in environmental research and industry. It requires sensitive, rapid, and inexpensive chemical sensors. Many industrial materials such as coatings and adhesives readily absorb chemical analytes, which may result in changes of their chemical, mechanical, and optical properties. This uptake of volatile organic compounds either from the gas phase or from an aqueous solution into a thin film is frequently accompanied by a change in material refractive index and film thickness. While the undesired swelling of thin film coatings and their refractive index changes affect their use in harsh environments, the sensitivity of some polymers to solvent vapours can also be exploited for sensing applications. In this project, a method is reported for real-time monitoring of vapour uptake by simultaneous detection of the refractive index, n, and thickness, d, of thin transparent films with a precision of 10-4 for refractive index and 100 nm for thickness. The setup combines a total internal reflection refractometer with an interferometric imaging method. Two setups using 1550 nm and 635 nm measurement wavelengths were developed, with a detection rate of 1 second per measurement. Two processing methods using a fast Fourier transform algorithm to calculate n and d are applied to the experimental results and compared. Both methods could extract n and d simultaneously from each image captured by the refractometer. The results show that the setup is capable of monitoring film RI and thickness change in real-time. The partitioning of volatile organic compound vapours into polydimethylsiloxane (PDMS) and PDMS-polydiphenylsiloxane (PDPS) copolymers is described. The system is also suited for characterization of other solid and liquid films like SU-8 photoresist and crude oil. It shows great potential in commercial applications of thin film characterization.