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Abstract:
Light scattering from plasmonic nanojunctions is routinely used to assess their optical properties. However, the microscopic mechanism remains imperfectly understood, and an accurate description requires the experiment in a well-defined environment with a highly precise control of the nanojunction. Here we report on inelastic light scattering (ILS) from plasmonic scanning tunneling microscope (STM) junctions under ultrahigh vacuum and cryogenic conditions. We particularly focus on anti-Stokes continuum generation in the ILS spectra with a narrowband continuous-wave laser excitation, which appears when an electrical bias is applied between the tip and the surface. This anti-Stokes continuum is commonly observed for various STM junctions at ∼10 K, corroborating that it is a universal phenomenon in electrically biased plasmonic nanojunctions. We propose that the microscopic mechanism underlying the anti-Stokes continuum generation is explained by ILS accompanied by electron transfer across the STM junction, whereby the excess energy is provided by the applied bias voltage. This process occurs through either photoluminescence (PL) or electronic Raman scattering (ERS). By recording the ILS spectra in parallel with STM-induced luminescence, we show that ERS becomes dominant when the excitation wavelength matches the plasmonic resonance of the STM junction, whereas PL mainly contributes to the off-resonance excitation. Our results provide an in-depth understanding of ILS by plasmonic nanojunctions and demonstrate that the anti-Stokes continuum can arise from a nonthermal mechanism.
