In the hotspot, a molecule will be subjected to a much stronger electric field than that of the incident field. 3,4 The focusing stems from resonance excitation/emission by oscillations of “free” electrons in the conduction band of the nanometal. The enhanced scattering/sensitivity arises from the ability of metal nanostructures to focus the incident light of certain wavelengths (in the visible for gold) to a small spatial nanoregion (hotspot) within a few nanometers from the metal nanostructures.
The process affords enhanced sensitivity of vibration fingerprint recognition by the otherwise highly weak Raman spectroscopy. Surface enhanced Raman scattering (SERS) 1,2 provides increases in scattering cross sections of an external light (laser light, for example) by sample molecules of several orders of magnitude when the molecules are placed within atomic distances from sharp nanoscale metal structures. The integrated metal–silicon system also promises field enhancement of visible luminescence of silicon nanoparticles, useful for imaging and tracking applications. 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. 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.
Moreover, the plasmonic resonance red shifts with the thickness of the silicon shell, reaching a terminal wavelength of ∼840 nm. 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. 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. 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. The intense field in the spot affords enhanced nonlinear optical processes, such as Raman spectroscopy. Metal nanostructures create near-field super hotspots under light irradiation with a range limited to a few nanometers.
0 kommentar(er)
