Defect modes in multisection helical photonic crystals.

We examine defect modes in four-section helical hotonic crystals (HPCs) that comprise three twist defects located at the intersections. The three twist defects are introduced by a single angle phit , but they are quantified differently by the jump angles across the successive sections. The two defect modes are localized at the different defect sites and can be either coupled or uncoupled to each other, depending on the value of phit . The two defect modes are excited by normally incident plane waves of different circular polarization states as the HPC thickness increases. When the two defect modes are uncoupled to each other, two co-handed reflection holes are present in the Bragg regime for small thickness, but they evolve into two stable cross-handed transmission holes for sufficiently large thickness. When the two defect modes are coupled to each other, however, three co-handed reflection holes appear around the center of the Bragg regime for small thickness, and they evolve into three cross-handed transmission holes as thickness increases, and eventually all three co-handed transmission holes coalesce into one stable cross-handed transmission hole for sufficiently large thickness. Finally, the simultaneous occurrence of the two types of spectral holes at a single resonance wavelength can be realized for specific values of sample thickness when the two defect modes are uncoupled to each other.