Group velocity enhancement for guided lightwave by band-pulling effect in single-mode silicon comb photonic crystal wires

Photonic crystal waveguides (PCWs), which usually exhibit the slow-light effect near the band edge frequency, may in fact be used to induce higher group velocity (v_g). The presence of the band edge, due to periodically carving a strip waveguide (SW) into a PCW of a comb-shape dielectric wire, results in the band-pulling effect. The band-pulling effect, which lifts the fundamental photonic band toward the band gap, not only enhances v_g by reducing the effective index but also prevents the band from bending toward the light cone of the core material, leading to additional v_g enhancement. The maximum PCW v_g (0.43c; c = vacuum light speed) found from numerical modeling was higher than SW v_g at all widths and thicknesses, except for extremely low thickness below 80 nm. We found by numerical modeling and experiments that up to 75% enhancement at the 1550 nm wavelength may be achieved in a fabricated single-mode PCW (0.38c) compared with an SW (0.21c) of the same width and thickness. The higher v_g was confirmed by measuring the PCW's group index using ring resonators. The PCW propagation loss of 2.1 dB mm⁻¹ and the bandwidth-distance product of 100 Gb/s-cm were deduced from experimental results. Our findings seemed encouraging for employing PCWs in optical interconnect applications. (paper)