Enhancing forbidden light propagation in nanoporous anodic alumina gradient-index filters by alcohol additives

Abstract

A comprehensive study on structural and optical engineering of high-quality nanoporous anodic alumina gradient-index filters (NAA-GIFs) by selective addition of alcohols to the anodizing acid electrolyte is presented. Analysis of the combined effect of type (methanol, ethanol, isopropanol, and ethylene glycol) and concentration (from 10 to 70 vol %) of alcohol additives in sulfuric acid electrolytes on the optical features of the characteristic photonic stopband of NAA-GIFs—central wavelength, full width at half maximum, intensity, and quality factor—allows us to elucidate the most optimal fabrication conditions to produce high-quality NAA-GIFs by sinusoidal pulse anodization under mild conditions. Comprehensive electrochemical, structural, optical, and chemical analyses demonstrate that a suitable selection of type and concentration of the alcohol additive is critical in controlling the quality of forbidden light propagation within these photonic crystal (PC) structures. Particularly, NAA-GIFs produced by sinusoidal pulse anodization in the sulfuric acid electrolyte modified with 40 vol % methanol achieve an outstanding quality factor of ∼58 (32–54% superior than that of other alcohol additives). Our findings indicate that a combination of anodic oxide growth rate and suppressing dissolution efficiencies, mildly branched nanoporous structure, and incorporation of carbon-containing impurities into the structure of NAA-GIFs are critical factors in enhancing light-forbidding quality of these NAA-based PCs. Our results advance the understanding of structural engineering of NAA-PCs by dynamic modification of the input anodization profile under optimal electrolyte conditions.