A quantitative analysis of the optical reflection properties of self-assembled opal films

The reflection spectra of self-assembled opal films on glass substrates are analyzed in detail, with an eye to employing the film as a tunable filter, using the photonic band diagram and the simulation on the reflectance corresponding to the real structure. The reflection peak commonly thought to be due to the photonic pseudogap formed by the (111) planes is found to be actually formed by two different pseudogaps by the (111) planes and the (200) planes. The small value for the photonic pseudogap in the <111> direction makes the corresponding light frequency propagate more than 40 SiO2 layers in the <111> direction. Therefore, a reflectivity of larger than 65% is difficult to achieve with an opal film whose thickness is smaller than 25 SiO2 layers in any direction with incident angles between 5° and 75°. The reflectivity of annealed opal film is found to be lower than that of the ideal film due to the reduction of the photonic pseudogaps and a microscopic wavy surface that is believed to be due to the variation of the SiO2 sphere size and the deformation of the SiO2 spheres caused by the contraction of the SiO2 spheres during the annealing process.

Research highlights
► Artificial opal films as tunable filter.
► Quantitative analysis of optical reflection.
► Photonic psedugap in <111> and <200> directions.
► Angle-dependent reflection peak heights.