Quantitative comparison of local field enhancement from tip-apex and plasmonic nanofocusing excitation via plasmon-assisted field emission resonances

Abstract

Plasmonic nanofocusing via off-site excitation offers a promising approach to minimize background light scattering and enhance excitation efficiency in tip-enhanced optical spectroscopy. However, a comprehensive understanding of its effectiveness compared to direct tip-apex excitation remains limited. Here we introduce plasmon-assisted field emission resonances as a practicable approach for quantitative evaluation of the local field enhancement in a scanning-tunneling-microscope junction via off-site excitation compared to direct tip-apex excitation. By using single-groove pyramidal tips suitable for multi-wavelength excitations, we find that the near-field intensity is approximately 3.8 and 1.7 times higher for off-site excitation than for direct apex excitation at excitation wavelengths of 780 nm and 633 nm, respectively. These results are further supported by numerical electromagnetic field simulations. Our findings demonstrate the effective implementation of plasmonic nanofocusing in low-temperature scanning tunneling microscopy, paving the way for more precise and background-free tip-enhanced optical spectroscopy at the atomic scale.