Design of adiabatic matching stages for efficient light coupling to propagating modes of photonic crystals

We propose a general, robust, and easy-to-implement scheme to design adiabatic buffer stages to couple light into and out of arbitrary photonic crystal (PC) structures. This buffer layer can be used as an intermediate layer to reduce the reflection for coupling the incident beam to PCs, extracting the light from PCs, or coupling light from one PC to another PC. The design is based on gradually changing the structure from the incident region to the PC in order to minimize the reflection due to modal mismatch. The results are compared with those with no matching stage used, and show considerable reflection reduction in a broad range of frequencies. We show that the resulting adiabatic design is insensitive to the frequency and angle of incident light, and is therefore superior to other schemes proposed to date that are designed for a specific incident angle and frequency.