On the role of evanescent modes and group index tapering in slow light photonic crystal waveguide coupling efficiency

We investigate effects of different mechanisms on coupling efficiency between strip waveguides and the slow light mode in photonic crystal waveguides (PCWs). Both numerical simulations and experimental results show that group index (n(g)) tapering improves strip-PCW butt-coupling efficiency when compared to a direct coupling between a strip waveguide and a high-n(g) PCW. However, coupling efficiency is even higher when an intermediate low-n(g) PCW is used to couple from a strip waveguide to a high-n(g) PCW without an n(g) tapering. Our results suggest that the role of evanescent mode is more dominant in efficient coupling between two PCWs with large n(g) mismatch. (C) 2011 American Institute of Physics. [doi:10.1063/1.3548557]