Optical near-field study of photonic crystal tapers.

A scanning near-field optical microscopy study is undertaken on photonic crystal tapers. These tapers are used to couple a 3 microm wide multimode dielectric waveguide to a single-line defect monomode photonic crystal waveguide. Two kinds of taper, with or without a localized defect, are compared. Higher transmission efficiency is obtained when a defect is utilized. The near-field study at 1550 nm shows experimentally that this defect prevents leaky resonant states to appear in the taper and thus permits a decrease in out-of-plane losses. These observations are supported by band diagram calculations.