Porous anodic alumina (PAA) photonic crystals with a photonic stop-band (PSB) placed in the mid-infrared (MIR) spectral region represent a promising approach for increasing of gas sensors sensitivity. An onion-like layered distribution of anionic impurities is a hallmark of PAA, and its presence is generally considered to demarcate the boundary between transparent and opaque ranges in the infrared spectral region. Here, we study the effect of annealing in the temperature range of 450 °C–1 100 °C on the structural stability and optical properties in photonic crystals based on PAA fabricated by pulse anodization in oxalic acid. Pulse sequences were selected in a way to obtain photonic crystals of different periodic structures with a PSB located in visible and MIR spectral regions. The first photonic crystal was composed of layers with gradually changing porosity, whereas the second photonic crystal consisted of a sequentially repeated double-layer unit with an abrupt change in porosity. We investigated the response of alumina with rationally designed porosities and different arrangements of porous layers for high-temperature treatment. The microstructure (scanning electron microscopy), phase composition (x-ray diffraction), and optical properties (optical spectroscopy) were analysed to track possible changes after annealing. Both photonic crystals demonstrated an excellent structural stability after 24 h annealing up to 950 °C. At the same time, the evaporation of the anionic impurities from PAA walls caused a shift of the PSB towards the shorter wavelengths. Furthermore, the annealing at 1 100 °C induced a high transparency (up to 90%) of alumina in MIR spectral region. It was shown thus that properly selected electrochemical and annealing conditions enable the fabrication of porous photonic crystals with the high transparency spanning the spectral range up to around 10 μm.

A robust and reliable method for fabricating porous anodic alumina (PAA)-based distributed Bragg reflectors (DBRs), operating in mid-infrared (MIR) spectral region, is presented. The method relies on application of high (U_H) and low (U_L) voltage pulse sequence repeated in cycles. PAA-based DBR consists of alternating high-(d_H) and low-porosity (d_L) layers translated directly into periodically varied refractive index. Two anodization modes were used: time- and charge density-controlled mode. The former generated d_H + d_L pairs with non-uniform thickness (∆d) and effective refractive index (∆n_eff). It is supposed, that owing to a compensation effect between the ∆d and ∆n_eff, the photonic stopbands (PSBs) were symmetrical and intensive (transmittance close to zero). Under the charge density-controlled mode d_H + d_L pairs of uniform thickness were formed. However, the remaining ∆n_eff provided an asymmetrical broadening of PSBs. Furthermore, it is demonstrated that the spectral position of the PSBs can be precisely tuned in the 3500–5500 nm range by changing duration of voltage pulses, the amount of charge passing under subsequent U_H and U_L pulses, and by pore broadening after the electrochemical synthesis. The material can be considered to be used as one-dimensional transparent photonic crystal heat mirrors for solar thermal applications.