Near-field microscopy of light propagation in photonic crystal waveguides

A collection scanning near-field optical microscope (SNOM) is used 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 FM direction of the irreducible Brillouin zone. High quality SNOM images of PCWs and access ridge waveguides excited in the wavelength range of 1520-1570 nm are obtained, demonstrating multimode behavior of ridge waveguides and good PCW (fundamental) mode confinement along with its low propagation loss. We analyze light intensity variations along the ridge and PCW waveguides measured with the SNOM at different distances from the sample surface. Considering the interference between a quasi-homogeneous background field and propagating mode fields and taking into account Bloch harmonics of the PCW modes, we account for spatial frequency spectra of the intensity variations and determine the dispersion of the PCW mode propagation constant. The possibilities and limitations of SNOM imaging for the characterization of PCWs as well as conventional waveguides are discussed.