Coupling Schemes for Low-Group Velocity Photonic Crystal Devices

A new concept for coupling light into a slow light mode of a photonic crystal is presented and discussed in this paper. The concept is based on a slight modification of the coupling facets of the photonic crystal structure. A thin layer of the bulk material forms the interface between photonic crystal and air, which is called anti-reflection layer. The working principle of the anti-reflection layer is the efficient coupling of impinging light to surface modes of the PhC. Without any coupling layer, surface modes can only be excited for frequencies a/2c > 0.7. By adding the anti-reflection layer, the effective k-vector inside the layer is increased and surface modes with significantly lower frequencies can be excited. The impinging light can couple very efficiently via this surface states to a slow light mode, which leads to a high transmission.