Optical spectroscopy is a powerful tool for chemical analysis, providing a wealth of information on structure, dynamics, and physical properties of matter. However, the diffraction limit of light hampers to directly resolve nanoscale structures. This physical limitation can be overcome by near-field optics. In particular, localized surface plasmon resonance of metal nanostructures gives rise to strong confinement and enhancement of electromagnetic fields, enabling ultrasensitive optical spectroscopy. I will discuss our recent development on atomic-scale optical spectroscopy at surfaces using a combination of low-temperature scanning tunneling microscopy and quantum plasmonics.
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