Anodized aluminum oxide membranes of tunable porosity with platinum nanoscale-coating for photonic application

We fabricate anodized aluminum oxide (AAO) membranes with platinum (Pt) coating of various thickness ranging from 0 nm to 12 nm. We also demonstrate the tunability of membrane porosity, to add a degree of freedom in addition to the Pt coating, for optical modulation that can be used in photonic application. The reflectance measurement and its analysis are provided based on Fabry–Perot etalon optics that takes into account spectral dependence of optical absorption and loss in the AAO membrane. It is revealed that the frequency-dependent optical finesse (color purity) enhances nonlinearly with the Pt thickness, by the quantitative estimation, while the effective refractive index is substantially governed by the membrane porosity. These features can allow for the AAO membrane to be used as a dye-free color displaying film with efficient tunability of both displaying color and its color purity. In particular, the strong dependence of an effective refractive index on porosity is demonstrated such that the AAO membrane can have the effective refractive index down to about 1.2, making it a possible candidate for a single layer anti-reflection coating material deposited on optical components including low-index glass. We also fabricate, by facile dipping method, an AAO membrane whose pore size changes gradually along a direction on its surface. This intra-membrane modulation of porosity allows it to find application in an optical grating or band-pass filter whose transmission wavelength varies with position on the device surface.

► We fabricate AAO membranes with Pt coating of thickness ranging from 0 nm to 12 nm.
► We demonstrate the tunability of membrane porosity.
► We analyze the reflectance spectroscopy by Fabry–Perot description.
► We examine the effects of porosity and Pt thickness on membrane effective index.
► AAO membranes can be a candidate for a single layer anti-reflection coating.