Near-field observation of anomalous optical propagation in photonic crystal coupled-cavity waveguides

An air-bridged silicon-based photonic crystal coupled-cavity waveguide (PCCCW) connected with an input and output W1 PC waveguide (PCW) was designed and fabricated. We mapped its intensity distributions with a near-field scanning optical microscope (NSOM) at near-infrared wavelengths around 1550 nm. Surprisingly, the intensity distributions demonstrate that the second odd eigenmode dominates in such a PCCCW, even though it possesses a much slower group velocity of light than that of the first even one. Further considering the measured transmission spectrum, we find that the modal profile and impedance matching between the eigenmodes in the PCW and PCCCW plays an important role in the optical propagation efficiency. Mode conversion between the first even and the second odd eigenmode was also detected at the interfaces between the W1 PCW and PCCCW.