Near-field characterization of planar photonic-crystal-waveguide structures

Characterization of photonic-crystal-waveguide (PCW) structures at telecommunication wavelengths with a collection scanning near-field optical microscope (SNOM) is considered. The propagation of light in silicon-on-insulator PCWs, formed by removing a single row of holes in the triangular lattice and connecting to access ridge waveguides, is imaged with the SNOM. High-contrast and high-resolution SNOM images of PCW structures containing straight PCWs as well as 90degrees gradual and 60degrees sharp PCW bends are obtained in the wavelength range of 1520-1570 nm, allowing for the determination of PCW mode characteristics and bend loss. We relate optical-signal variations along straight PCWs to the interference between a quasi-homogeneous background field and Bloch harmonics of the PCW mode, and use spatial-frequency spectra of the intensity variations to determine the dispersion of the PCW mode propagation constant. The bend loss is evaluated using averaged signal cross-sections and taking into account the fact that the signal is proportional to the PCW mode amplitude. It is directly shown that the bend loss for 60degrees bends is significantly decreased for smoothened bends having one or three holes moved from the inner side of the bend corner to the outer one. The possibilities and limitations of SNOM imaging for the characterization of PCW structures are discussed.