Anodizing aluminum in a viscous electrolyte to produce one-dimensional photonic crystals

Abstract

In the paper, the possibility to produce anodic aluminum oxide (AAO) featuring one-dimensional photonic crystal along the normal to the surface is shown. The AAO structure is represented by alternating layers of different porosity and is formed in a viscous electrolyte based on sulfuric acid and ethylene glycol at the periodically varying from high (1.8 mA/cm2) to low (0.4 mA/cm2) current density with a rectangular pulse shape. The pore sizes and interpore distance, pore density and porosity, thickness and period of the AAO structure have been determined. The specular reflection spectra features for single layers that make up the AAO structure and for one-dimensional photonic crystals structures consisting of 165 periods have been studied. An increase in the porosity of the upper layers of the structure due to chemical etching of the pores during the oxide growth is noted. It is shown that the invariance of the spectral position of the photonic band gap for AAO structures is achieved by a 0.1 % decrease in charge at each subsequent anodizing cycle during their formation, which leads to a decrease in the period of the structure in the lower layers, compensating for the increase in the upper layers porosity. The reflection spectra have been analyzed for the incidence angles of 10° and 30° and used to calculate the period of the structure and the effective refractive index. The effective refractive index of the single layers that make up the AAO structure is calculated using the optical Fabry–Perot oscillations. For AAO with the properties of one-dimensional photonic crystal, a green color is observed at normal light incidence, and an iridescent color is observed when the angle changes. AAO can be used as a decorative coating on the housings of electronic devices (tablets, laptops, phones, etc.) and when creating design objects made of aluminum and its alloys.