Optical imaging provides richer information than any other imaging technique based on topographic information. However, the spatial resolution in optical microscopy is restricted by the diffraction limits of the probing light, which is a few hundred nanometers for visible light. This problem can be overcome by combining the plasmonic effects of near-field techniques with optical Raman microscopy. In our previous work [1,2], we have shown that Raman scattering combined with near-field microscopy, i.e., tip-enhanced Raman spectroscopy (TERS), provides super spatial resolution far beyond the diffraction limits of the probing light, along with an enhanced scattering efficiency. This is due to the nano-sized evanescent field created in close proximity of the apex of a nano-metallic tip, which enhances the scattering from the sample molecules directly under the tip apex. The image quality, such as the contrast and the resolution, can be further improved by invoking higher-order scattering effects in Raman scattering process because it provides better confinement of light field than the linear scattering due to the nonlinear effects, and better contrast due to the suppression of the luminescence background. We have shown this by utilizing the near-field effects in coherent anti-Stoke Raman scattering (CARS), and have obtained high quality image with a spatial resolution of 15nm [3].
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