Near-Field Raman Spectroscopy: Enhancing Spatial Resolution Using Metallic Tips
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Optical microscopy is a well-established technology of microanalysis for material science, biology, pathological diagnostics, industry, and so on. However, the smallest analyzing dimension of the sample resolvable with a conventional optical microscope is a half wavelength of light or several hundreds of nanometers in visible region because of the nature of the light wave. This is a so-called diffraction limit of light, which is not small enough to observe the structures of interest in the current advanced sciences and technologies, including protein molecules, quantum nanodevices, and self-assembled molecules. Near-field optical microscopy is a tool to overcome the barrier of wavelength by confining photons in nanoscale volume.
A combination of the state-of-the-art near-field technology with the conventional optical measurements enables us to explore the nanoworld with visible light. Especially, near-field Raman spectroscopy makes it possible to assign molecules, to analyze chemical behavior of molecules, and to observe molecular dynamics at nanometric or molecular scale. Lasers and detectors of visible region are available in Raman spectroscopy, and quenching phenomenon and photobleaching could be avoided; hence near-field Raman spectroscopy is suitable for molecular sensing with a nanometric spatial resolution. Because the cross section of Raman scattering is much smaller than fluorescence and infrared absorption cross section, it is necessary to enhance Raman cross section for its accurate measurement. This can be performed by using a metallic tip that also works as one of a near-field probe for detection. In this article, we present how the metallic tip works as a near-field probe at first and then show the current progress in near-field Raman spectroscopy using a metallic tip.