Quantum mechanical limit to plasmonic enhancement as observed by surface-enhanced Raman scattering

Abstract

Plasmonic nanostructures enable light to be concentrated into nanoscale 'hotspots', wherein the intensity of light can be enhanced by orders of magnitude. This plasmonic enhancement significantly boosts the efficiency of nanoscale light-matter interactions, enabling unique linear and nonlinear optical applications. Large enhancements are often observed within narrow gaps or at sharp tips, as predicted by the classical electromagnetic theory. Only recently has it become appreciated that quantum mechanical effects could emerge as the feature size approaches atomic length-scale. Here we experimentally demonstrate, through observations of surface-enhanced Raman scattering, that the emergence of electron tunnelling at optical frequencies limits the maximum achievable plasmonic enhancement. Such quantum mechanical effects are revealed for metallic nanostructures with gap-widths in the single-digit angstrom range by correlating each structure with its optical properties. This work furthers our understanding of quantum mechanical effects in plasmonic systems and could enable future applications of quantum plasmonics.

Quantum description of the optical response of charged monolayer–thick metallic patch nanoantennas

“…The conclusions derived from the JM calculations have been later confirmed by the experimental data 40,[66][67][68][69] , and ab-initio calculations at a full atomistic level [70][71][72] .…”

Highly sensitive, reproducible and uniform SERS substrates with a high density of three-dimensionally distributed hotspots: gyroid-structured Au periodic metallic materials

“…SPR enhancement has been reported to dramatically increase with the reduction of the gap width, especially at a gap width of less than 5 nm. 36,37 Although accurately evaluating the gap distribution in 3D gyroid networks with different VFs is difficult, simulated TEM projections along different plane directions can provide insight. Based on the TEM image of the cross-section of the biotemplate (Supplementary Figure S1), we selected several plane directions as examples and systematically investigated their gap distributions with an increasing VF via simulated TEM projections (Videos S1-S7).…”

Nonlocal optical response in metallic nanostructures

“…1. Schematic illustrations of (a) electron beam interacting with silver nanoparticle on thin substrate, [13][14][15][16] (b) electromagnetic interaction of two nearby metal nanoparticles, [17][18][19][20][21][22][23] (c) gold nanoparticles in close proximity of gold film, 24 and (d) optical measurements of two gold-coated atomic-force microscopy tips in close proximity. 25 (c) and (d) courtesy of C. Ciracì and J. Baumberg, respectively.…”

“…92,93,178 These unphysical attributes of the LRA are corrected in DFT 65,67,126,127,170 and hydrodynamic 88,92,93 approaches due to the inclusion of nonlocal response and electron spill-out (only DFT). Recent measurements on dimers with subnanometer gaps using both optical techniques 19,22,23,25 and EELS 18,179 show lack of agreement with the LRA, and, in the touching case, also display limits on the resonance energies of the bonding plasmon modes. However, due to the indirect nature of the measurements the explanation for the discrepancy between LRA and the observed measurements is not conclusive with possible interpretations being provided from both quantum tunneling 65,126 and nonlocal response 69 perspectives.…”