Self-referenced composite Fabry-Pérot cavity vapor sensors

We develop a versatile, self-referenced composite Fabry-Pérot (FP) sensor and the corresponding detection scheme for rapid and precise measurement of vapors. The composite FP vapor sensor is formed by etching two juxtaposed micron-deep wells, with a precisely controlled offset in depth, on a silicon wafer. The wells are then coated with a vapor sensitive polymer and the reflected light from each well is detected by a CMOS imager. Due to its self-referenced nature, the composite FP sensor is able to extract the change in thickness and refractive index of the polymer layer upon exposure to analyte vapors, thus allowing for accurate vapor quantitation regardless of the polymer thickness, refractive index, and light incident angle and wavelength. Theoretical analysis is first performed to elucidate the underlying detection principle, followed by experimental demonstration at two different incident angles showing rapid and consistent measurement of the polymer changes when the polymer is exposed to three different analytes at various concentrations. The vapor detection limit is found to be on the order of a few pico-grams (~100 ppb).