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Near-field microscopy in liquid for accurate high- resolution optical characterization


Near-field scanning optical microscopy (NSOM) is a powerful technique which allows deeply subwavelength imaging by placing a nanoscale aperture in close proximity to a sample where it can collect evanescent fields which contain information about subwavelength features. Additionally, by coupling out these evanescent fields, it has the ability to image light propagation within light- confining guided-wave structures. NSOM can be enhanced by integration into the signal arm of a heterodyne interferometer (H-NSOM), which allows imaging of both amplitude and phase at subwavelength resolution. In this dissertation we apply H-NSOM to characterize novel structures, and introduce a new technique for using H-NSOM in a liquid environment. First, we use the H-NSOM to characterize an asymetric mode converter, which uses a one -way wavevector created by a metallic grating. The asymmetric propagation is visualized directly, then verified by using Fourier analysis to examine the mode content of the waveguide fields. Next, we propose and implement a scheme for H-NSOM measurement of silicon integrated waveguides with liquid cladding. Fourier analysis is used to determine an effective index shift of .08 in the quasi-TM mode between air and water overcladdings. This technique is then succesfully applied to directly image long range surface plasmons for the first time. The liquid cladding enables preservation of the symmetric cladding environment required for long range plasmon propagation. We directly observe the field distribution of the single mode, and show that it matches well to simulations

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