Near-field characterization of low-loss photonic crystal waveguides

A scanning near-field optical microscope is used to directly map the propagation of light in the wavelength range of 1500-1630 nm along straight photonic crystal waveguides (PCWs) fabricated on silicon-on-insulator wafers. The PVWs were formed by removing a single row of holes in the triangular 428-nm-period lattices with different filling factors (0.76 and 0.82) and connected to access ridge waveguides. Using the near-field optical images we investigate the light propagation along PCWs for TM and TE polarization (the electric field is perpendicular/parallel to the sample surface). Efficient guiding (for both samples) of the TM-polarized radiation is observed in the whole range of laser tunability. For TE polarization, the efficient guiding is limited to the wavelengths shorter than 1552 or 1570 nm for the PCW with the filling factor of 0.76 or 0.82, respectively. For longer wavelengths, we observe drastic and rapid deterioration of the waveguiding and PCW mode confinement, ending up with the complete disappearance of the PCW mode once the wavelength exceeds the cutoff value by merely 2 nm. Using averaged cross sections of the intensity distributions along the PCW axis, the propagation loss is evaluated and found to be in good agreement with the corresponding transmission spectra. Considering spatial frequency spectra of the intensity variations along the PCW axis, we determine (for both polarizations) the dispersion of the PCW mode propagation constant and, thereby, the mode group and phase velocities.