Infrared emission from tunneling electrons: The end of the rainbow in scanning tunneling microscopy

Electromagnetic radiation originating with localized surface plasmons in the metal-tip/metal-sample nanocavity of a scanning tunneling microscope is demonstrated to extend to a wavelength λ of at least 1.7 μm. Progressive spectral extension beyond λ∼1.0 μm occurs for increasing tip radius above ∼15 nm, reaching λ∼1.7 μm for tip radius ∼100 nm; these observations are corroborated by use of a simple physical model that relates the discrete plasmon mode frequencies to the tip radius. This spectral extension opens up a new regime for scanning tunneling microscope-based optical spectroscopy.