Coupling of a 2D photonic crystal-metal surface wave to photonic crystal waveguide modes

Photonic crystal waveguides are being used for different applications in sensors, lasers, interferometers, and optical circuits. The process of coupling a light beam to a waveguide presents an inherent loss of energy, particularly for monomode waveguides. In this work, we study the coupling of an interface mode in a metal-2D photonic crystal (2DPC) and a photonic crystal waveguide mode. The coupling produces a hybrid mode that presents an avoided crossing inside the photonic crystal band gap. The avoided crossing produces a mini-stop band whose frequency width is larger when the waveguide is closer to the interface. Using the finite difference time domain method (FDTD) we show that the hybrid mode can be excited by incident light normal to the interface for an appropriate thickness of the metal. For an incident light beam with a given frequency such that two hybrid modes can be excited simultaneously, the energy goes from the interface to the waveguide and vice versa, oscillating with a period given by the coupling distance, as described by coupled mode theory (CMT).