Reaching the Limits of Enhancement in (Sub)Nanometer Metal Structures

Plasmonic enhancement has had remarkable success in optical coupling to the nanometer scale, enabling feats such as Raman spectroscopy with single molecule sensitivity. Here it is argued that much greater enhancements are possible in the near future by combining the gains of plasmonic resonances, directivity, subnanometer gaps, and permittivity near-zero materials. The pursuit of such extraordinary enhancements promises to bring new physics such as peering into the world of quantum optomechanics. It also promises new applications such as quantitative single molecule Raman spectroscopy and low photon number nonlinear optical switching. In addition, by pushing the limits of plasmonic enhancement, it is expected that the community will gain a greater appreciation of how physical phenomena such as nonlocality, surface scattering, and quantum tunneling each play a role in determining the ultimate performance.