The Electromagnetic Enhancement of SERS and the Modified Partition of Optical States in the Strong Matter-Coupling Regime.

Surface enhanced Raman Spectroscopy (SERS) is a powerful optical sensing technique that is based on enhanced Raman signals from molecules in proximity of rough metal surfaces. Experiments¹ have shown unexpected large enhancements in SERS, even up to 10¹⁴. Conventional electromagnetic theory accounts for enhancements only up to 10⁶, and the anomalous enhancements have even been attributed to an unknown chemical origin^2,3. We show that when one includes dynamics in fluctuations of an emitter strongly coupled to absorbing matter, such high gains are predicted due to tunneling out of photons from the strongly absorbing metal surface. This modification to the conventional partition of optical states into its radiative and non-radiative parts, is also imperative for emitters proximal to limiting small metal nanoparticles (< 10 nm in dimensions) which are fully absorbing. Some recent experiments have shown such anomalous gains in emission due to these extremely small nanoparticles. This effect can be exploited further in light generation, optical sensing and radiative heat transfer.
¹Zeisel, D., et al., Chemical Physics Letters, 283(5-6), 381-385 (1998)
²D.Fromm, et al., J.Chem.Phys.124,61101 (2006)
³K. Kneipp, J. Phys. Chem. C 120, 21076 (2016)