Porous Nanostructured Optical Filters Rendered Insensitive to Humidity by Vapor‐Phase Functionalization

Water adsorption in many porous optical coatings can cause a detrimental red‐shift in the optical properties. In this study, a porous nanostructured rugate filter is fabricated using the glancing‐angle deposition (GLAD) technique, and rendered insensitive to large changes in ambient humidity by post‐deposition vapor‐phase functionalization. A central defect mode is added to the stop band of the filter by integrating a phase shift into the sinusoidal refractive‐index profile of the film. By monitoring the wavelength of the defect mode under variable humidity conditions, a six‐times reduction in sensitivity to water vapor is observed upon functionalization with 3,3,3‐trifluoropropyltrichlorosilane. Electrical characterization and advancing aqueous contact‐angle measurements are used to verify the hydrophobic properties of the functionalized thin films.