Near-field imaging of light propagation in photonic crystal waveguides: Explicit role of Bloch harmonics

We employ a collection scanning near-field optical microscope (SNOM) to image the propagation of light at telecommunication wavelengths along straight and bent regions of silicon-on-insulator photonic crystal waveguides (PCWs) formed by removing a single row of holes in the triangular 410-nm-period lattice along the GammaM direction of the irreducible Brillouin zone. We obtain high quality SNOM images of PCWs excited in the wavelength range of 1520-1570 nm, which indicate good PCW mode confinement and low propagation loss. Using averaged cross sections of the intensity distributions before and after PCW bends, bend loss is evaluated and found to noticeably increase with the increase of the light wavelength from similar to1 dB at 1520 nm to similar to6 dB at 1570 nm. We analyze light intensity variations along PCWs measured with the SNOM at different distances from the sample surface. Considering the interference between a quasihomogeneous background field and Bloch harmonics of the PCW mode, we account for spatial frequency spectra of the intensity variations and determine the propagation constant of the PCW mode at 1520 nm. The possibilities and limitations of SNOM imaging for the characterization of PCWs are discussed.