Anti-Stokes Light Scattering Mediated by Electron Transfer Across a Biased Plasmonic Nanojunction

Shuyi Liu,Adnan Hammud,Takashi Kumagai,Martin Wolf

doi:10.1021/acsphotonics.1c00402

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.

Highlights

Suppression in Imaging Gold Nanorods through Detection of Anti-Stokes Emission
An Ag tip−vacuum− Ag(111) junction at 10 K is irradiated by a focused narrowband cw laser, which leads to strong field enhancement when the incident wavelength matches the localized surface plasmon resonance (LSPR) of the junction
This may be explained by the fact that in the inelastic light scattering (ILS) measurement the LSPR excitation could occur in the tip base in addition to the very apex because the beam spot of ∼3 μm is much larger than the apex, whereas the LSPR excitation in scanning tunneling luminescence (STL) is strongly confined in the scanning tunneling microscope (STM) junction

Summary

■ RESULTS AND DISCUSSIONS

It should be noted that the intensity and the spectral shape of STL significantly deviates from the ILS spectra through the PL process in some cases (Supporting Information, Figure S4) This may be explained by the fact that in the ILS measurement the LSPR excitation could occur in the tip base in addition to the very apex because the beam spot of ∼3 μm is much larger than the apex (tens of nm), whereas the LSPR excitation in STL is strongly confined in the STM junction. The incident photons could interact with any excitation in the STM junction that occurs after Vbias-induced electron tunneling, for instance, hot-carrier generation in the tip or surface In this case, the intensity of the anti-Stokes scattering should be proportional to the tunneling current (jt), provided that the excitation is a oneelectron process. We observed a saturating behavior for the ILS intensity at very large jt, that is, very small tip−surface distances (Supporting Information, Figure S6), which may result from attenuation of the plasmonic field through quantum effects, for example, nonlocal screening and electron tunneling.[74]

■ CONCLUSIONS

■ REFERENCES

Background