Materials whose structure is characterized by a periodic change in the refractive index at a scale comparable to the wavelength of light are called photonic crystals. When the porous alumina films are formed by electrochemical method, the material porosity can be modulated by periodical variation of the anodization voltage [1]. When a refractive index changes only along one direction, a photonic crystal is called a one-dimensional or Bragg mirror.
One-dimensional photonic crystals based on anodic aluminium oxide have great prospects for practical use in great number of areas: from optoelectronics to decorative coatings. Today, many modes of anodizing aluminium are suggested, which allow periodic modulation of porosity. But, in the literature, there is a lack of data on the dependence of the optical properties of anodic aluminium oxide-based photonic crystals on the number of periods in the film structure.
In our group, a systematic study of the optical properties of photonic crystals based on anodic aluminum oxide with a number of periods from 1 to 300 has been performed [2]. Bright saturated color, which necessary for decorative coatings on the surface of aluminum parts, is achieved for 150 or more periods of structure. At the same time, 75 periods of the structure are enough to obtain a narrow and intense photonic band gap.
[1] B. Wang, G.T. Fei, M. Wang, M.G. Kong, L.D. Zhang, Preparation of photonic crystals made of air pores in anodic alumina, Nanotechnology. 18 (2007) 365601. DOI: 10.1088/0957-4484/18/36/365601.
[2] S.E. Kushnir, K.S. Napolskii, Thickness-dependent iridescence of one-dimensional photonic crystals based on anodic alumina, Mater. Des. 144 (2018) 140–150. DOI: 10.1016/j.matdes.2018.02.012.