Analysis of absolute photonic band gaps in two-dimensional photonic crystals based on CdSe rods embedded in TiO2 matrix

Using the plane-wave numerical expansion method, we investigate the band gap spectra of two-dimensional photonic crystals created by square and hexagonal lattices consisting of coating rods with a thin air layer in a titanium dioxide (TiO₂)matrix. Our results show that in a square lattice a coating alone does not help to create an absolute PBG. However, we showed that adding air hole into the center of each square unit cell can produce the absolute PBG. The optimum gap is achieved by tuning the thickness of the shell layer in the two types of lattices. We believe that the present results are significant to increase the possibilities for experimentalist to design a larger absolute PBG despite the very low contrast index between the rods and the matrix.