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We analyze the propagating optical modes in a Silicon membrane photonic crystal waveguide, based on subwavelength-resolution amplitude and phase measurements of the optical fields using a heterodyne near-field scanning optical microscope (H-NSOM). Fourier analysis of the experimentally obtained optical amplitude and phase data permits identification of the propagating waveguide modes, including the direction of propagation (in contrast to intensity-only measurement techniques). This analysis reveals the presence of two superposed propagating modes in the waveguide. The characteristics of each mode are determined and found to be consistent with theoretical predictions within the limits of fabrication tolerances. An analysis of the relative amplitudes of these two modes as a function of wavelength show periodic oscillation with a period of approximately 3.3 nm. The coupling efficiency between the ridge waveguide and the photonic crystal waveguide is also estimated and found to be consistent with the internal propagating mode characteristics. The combination of high-sensitivity amplitude and phase measurements, subwavelength spatial resolution, and appropriate interpretive techniques permits the in-situ observation of the optical properties of the device with an unprecedented level of detail, and facilitates the characterization and optimization of nanostructure-based photonic devices and systems.

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We apply heterodyne scanning near-field optical microscopy (SNOM) to observe with subwavelength resolution the amplitude and phase of optical fields propagating in several microfabricated waveguide devices operating around the 1.55 m wavelength. Good agreement between the SNOM measurements and predicted optical mode propagation characteristics in standard ridge waveguides demonstrates the validity of the method. In situ observation of the subwavelength-scale distribution and propagation of optical fields in straight and 90°bend photonic crystal waveguides facilitates a more detailed understanding of the optical performance characteristics of these devices and illustrates the usefulness of the technique for investigating nanostructured photonic devices. © 2005 Optical Society of America OCIS codes: 040.2840, 120.5050, 180.5810, 230.7370. Photonic nanostructures, and in particular photonic crystals (PCs), provide a number of possibilities for developing novel devices based on near-field optical phenomena, as well as for monolithic integration of large-scale photonic systems. 1-3 However, as many of these devices depend on the subwavelength-scale localization of optical fields, traditional far-field characterization techniques are incapable of fully capturing the propagation characteristics of light in such structures. Scanning near-field optical microscopy 4,5 (SNOM) provides the ability to observe directly the optical fields propagating in a photonic structure with subwavelength resolution and has been applied to investigate PC resonators and various other devices. 6,7 In heterodyne SNOM, light collected by the scanning probe is combined with a frequencyshifted reference beam to permit simultaneous measurement of the local amplitude and phase of the probed field. [8][9][10] This technique has been used to characterize several devices, including periodically structured waveguides 11 and structured optical fibers, 12 demonstrating the usefulness of SNOM for the detailed characterization of optical field propagation inside microstructured waveguides. In this Letter, we apply this technique to investigate microfabricated ridge and PC waveguides and demonstrate the measurement of the phase and amplitude of the propagating fields in silicon-based photonic devices at the 1.55 m wavelength. 13 The obtained results are in good agreement with predicted values, illustrating the viability of the technique as a method for investigating the complex amplitude of optical fields as they propagate inside the waveguides, and could have a significant effect on the understanding and development of nanostructured optical devices.In the present setup, the source is a semiconductor cw laser operating in a wavelength band around 1.55 m. The cantilevered tapered fiber probe has an aperture of 200 nm and is scanned in tapping mode. This scanning technique reduces lateral interaction forces and is particularly advantageous for the investigation of silicon membranes or other fragile samples having abrupt topographical changes.First, we c...