From gradient-index to step-index filters: A switch between the two types of photonic crystals induced by the amplitude and period of sinusoidal function applied during high-temperature anodisation of aluminium

Abstract

Porous anodic alumina (PAA) holds promise for the realisation of high-quality optical filters with first-order photonic stop-bands (PSBλ1) located in the infrared spectral region, thereby expanding the possibility of its application in photonics and light-based industries. High-temperature (30 °C) sinusoidal pulse anodisation (SPA) of aluminium was applied to fabricate PAA with PSBλ1 in the range of 800–2500 nm. The systematic increase in the anodisation period (tp) and amplitude (A) caused a significant change in the optical properties of the PAAs, which were monitored by both transmittance and reflectance measurements. A switch from gradient-index to step-index optical filters was observed when A and tp were increased from the lowest (5 V and 100 s, respectively) to the highest (15 V and 300 s, respectively) values. The switch was ascribed to the transition between different self-ordering regimes. The finest filtering properties (PSBλ1 reflectance close to 100 %) were achieved when the SPA proceeded between diffusion and kinetic regimes, providing a better porosity/refractive index contrast within the periodic structure. The analysis under polarised light confirmed both the excellent filtering properties of the PAAs and the asymmetrical light reflectance produced through the robust and cost-effective SPA process.