Variation of the refractive index by means of sulfate anion incorporation into nanoporous anodic aluminum oxide films

• Sulfur incorporation into nanoporous AAO films varies from 6.3 up to 11.9%.
• Refractive index is calculated by means of an iterative method based on the Snell's law.
• The incorporated sulfate anions vary the refractive index of the nanoporous AAO film.
• The variation of the refractive index Δneff = 0.08 is constant within 400–1200 nm range.
• Refractive index values increase as decrease the amount of sulfates into AAO films.

The variation in the refractive index of nanoporous anodic aluminum oxide (AAO) films regarding the sulfur anion incorporation is studied in this work. For this purpose, different samples are grown under potentiostatic conditions at different voltages and concentrations of sulfuric acid in the electrolyte. The samples are analyzed by Rutherford backscattering spectroscopy and infrared spectrometry, confirming the presence of sulfate anions and water embedded into the nanoporous AAO films. The incorporation of sulfate ions into the alumina matrix varies from 6.3 up to 11.7% regarding aluminum content. We have studied Fabry–Pérot optical interferences by shining incident monochromatic light in specular reflectance conditions. The reflected monochromatic light waves interact in internal reflectivity generating constructive and destructive interferences known as Fabry–Pérot optical interferences. An iterative method based on the equation for constructive interferences conditions in thin films and Snell equations is developed in order to calculate the refractive index of the nanoporous AAO films as a function of the wavelength. The calculated refractive indices increase when the sulfur content of the nanoporous AAO films decreases. The variation of the calculated refractive index is 0.08 and remains constant in the wavelength range 400–1200 nm.

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