Abstract
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.
Highlights
Since the first demonstrations of the near-field scanning optical microscope (NSOM) in 1984 [1,2], the technique has proven to be an important tool for subwavelength resolution observation of various optical field configurations [3,4,5], including evanescent and other nonpropagating fields [6,7,8,9,10]
Fourier analysis of the experimentally obtained optical amplitude and phase data permits identification of the propagating waveguide modes, including the direction of propagation. This analysis reveals the presence of two superposed propagating modes in the waveguide
As nanostructure-based photonic devices and systems continue to grow in complexity and scale, the heterodyne near-field scanning optical microscope (H-NSOM) technique will likely prove to be an invaluable tool in observing the local optical fields in such devices, understanding the optical interactions between integrated components, and optimizing the performance of photonic systems based on this technology
Summary
Since the first demonstrations of the near-field scanning optical microscope (NSOM) in 1984 [1,2], the technique has proven to be an important tool for subwavelength resolution observation of various optical field configurations [3,4,5], including evanescent and other nonpropagating fields [6,7,8,9,10]. A more recent innovation, the heterodyne NSOM (H-NSOM) [11], permits the near-field measurement of both amplitude and phase, providing previously inaccessible information about the optical fields under investigation This technique has been applied to a large number of studies, in particular involving subwavelength-scale structures, localized optical field phenomena, and photonic crystals (PhCs) in the visible range [12,13,14]. By comparing measurement results obtained for a range of optical wavelengths around 1550 nm, we determine a number of important characteristics concerning the propagating modes in the waveguide, including the number of propagating modes, their effective wavelength, their relative amplitudes, the position of the band edge, and the approximate input coupling efficiency as a function of wavelength This information supports a much better understanding of the propagation of light within such a structure, enabling a detailed comparison of the experimental results with theoretical predictions.
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