Band gap effects in asymmetric photonic crystal slabs

It is found that the band gap effect does exist in asymmetric photonic crystal slabs, although there are no separable noninteracting subsets of the modes. This is obtained through band structures analysis, rather than the structures are approximated as symmetric if the modes are sufficiently localized. The band edge of the gap is decided by the fundamental ${\mathrm{TE}}_0$-like and ${\mathrm{TM}}_0$-like modes. It is also shown that waveguides (line defects) in asymmetric photonic crystal slabs could support lossless guided modes inside the band gap region.