A Transparent Hard Coating with Tunable Refractive Index

We demonstrate a novel method to completely anodize metal films applied to insulators, so that no metal remains and is transparent, except for an arbitrarily small connection line at an edge of the work piece. Normally the anodization results in an opaque metal barrier layer. The transparent anodization results in a hard metal-oxide film with ordered nanoporosity, up to 70% porous in hexagonal close packing, with pore diameters < 30 nm, pore lengths ~200x diameters (thickness ~several optical l). The formation of arrays of high aspect ratio nanopores under anodic oxidation of pure aluminium is a well-known phenomenon. Edge Anodization utilized a controlled immersion of a workpiece into an anodization bath, controlling current density and speed of immersion. Films of Al ~1-5 µm thick anodized to transparent Al₂O₃ resulted in uniform pores with diameter ~10–30 nm and density 8-9 × 10¹⁰ pores cm⁻². The pores are far smaller than visible wavelengths and resultant heterogenous films are considered uniform for the optical properties of visible light with index of refraction n ~ f n_p + (1-f) n_o, where f is the porous fraction of the film, n_p is the pore index, equal to that of air (i.e. n_p = 1) unless filled, and n_o is the oxide matrix index. For the form of Aluminum Oxide here (boehmite, γ-alumina), for example, n_p ~1.6, and so for f = 60% porosity, easily achieved, the index of the film n~ 1.1, and for f = 40%, n~ 1.4, low enough to capture more light in fibers with such a cladding. At n=1.1 and core index n=1.6, ~47% of light is transmitted.