Experimental observation of slow light in photonic crystal coupled waveguides.

We experimentally demonstrate wideband dispersion-free slow light in chirped photonic crystal coupled waveguides (PCCW). In unchirped PCCWs, the zero group velocity can occur at an inflection point of a photonic band of even symmetric mode. The even symmetric mode is selectively excited by connecting the device with input and output waveguides through optimized branch and confluence structures. In the device fabricated on SOI substrate, a large increase in group delay was observed with a maximum group index of 140 and the zero group velocity dispersion at the inflection point. Photonic bands estimated from the group delay characteristics corresponded to calculated ones. In the chirped PCCWs, the group velocity dispersion was internally compensated and the nearly constant group index of 50-60 was obtained in a wavelength bandwidth of 10 nm. The dispersion compensation was also confirmed through the transmission measurement of sub-ps optical pulses.