Tunable plasmon–polarizmon resonance and hotspots in metal–silicon core–shell nanostructures

Metal nanostructures create near-field super hotspots under light irradiation with a range limited to a few nanometers. The intense field in the spot affords enhanced nonlinear optical processes, such as Raman spectroscopy. The intense field, however, can cause heavy distortion and thermal damage to the molecular specimen as well as heavy convolution with the metal electronic structure. In this work, we simulate concentric layered silicon–metal core–shell (and its inverse) nanostructures that may alleviate the disadvantages of the pure metal environment. Our results using Mie and finite-difference time-domain scattering studies show that, in addition to the super hotspot at the gold–silicon interface, there emerges a super hotspot at the silicon–vacuum interface, whose intensities anti-correlate and are tuned by tuning the silicon thickness. Moreover, the plasmonic resonance red shifts with the thickness of the silicon shell, reaching a terminal wavelength of ∼840 nm. These features are understood in terms of induced polarization charge at the silicon–metal and silicon–vacuum interfaces, which for high κ materials (13.32) can be significant. The metal–silicon system creates plasmon–polarizmon hotspots tunable in strength and wavelength content that can be designed to alleviate high field damage, useful for Raman scattering and photovoltaic applications. The integrated metal–silicon system also promises field enhancement of visible luminescence of silicon nanoparticles, useful for imaging and tracking applications.