Band-gap properties of two-dimensional low-index photonic crystals

We study the band-gap properties of two-dimensional photonic crystals created by a lattice of rods or holes conformed in a symmetric or asymmetric triangular structure. Using numerical plane-wave method, we calculate a minimum value of the refractive-index contrast for opening both partial and full two-dimensional spectral gaps for both TM- and TE-polarized waves. We also analyze the effect of ellipticity of rods and holes and their orientation on the threshold value and the relative size of the band gaps.