Direct observation of surface mode excitation and slow light coupling in photonic crystal waveguides.

A scanning near-field optical microscope (SNOM) is used to systematically study the properties of guided modes in linear and slow-light regimes of silicon-on-insulator (SOI)-based photonic crystal waveguides (PhCWs) with different terminations of the photonic lattice. High quality SNOM images are obtained for light at telecom wavelengths propagating in the PhCW, demonstrating directly, for the first time to our knowledge, drastic widening of the PhCW guided mode in the slow-light regime and excitation of surface waves at the PhCW interface along with their feeding into the guided mode for the lattice terminations corresponding to significantly reduced coupling loss.