Recent progress and frontiers in the electromagnetic mechanism of surface-enhanced Raman scattering

• We overviewed electromagnetic (EM) enhancement in surface-enhanced Raman scattering.
• Two-fold EM enhancement by plasmons is most successful theory.
• We also figured out problems and limitations in EM mechanism.
• Approximation in Fermi's golden rule may break down at hotspots.

The electromagnetic (EM) enhancement of the optical responses of molecules close to plasmonic metal nanostructures has been applied for ultra-sensitive detection in vibrational spectroscopy. Recently, the research fields of EM enhancement entered a new regime wherein the enhancement effects are connected to photochemical and photobiological phenomena. For this regime, the conventional theorem used to understand the EM enhancement effect should be re-examined because the experimental conditions are beyond the assumptions in the theorem. Thus, in this review, we firstly overview and summarize the EM mechanism in surface-enhanced Raman scattering (SERS), which is the most general optical response using an EM enhancement, and determine the limitations and problems of the EM mechanism in SERS. Secondly, we discuss the necessity of re-examining the EM mechanism with respect to three breakdowns of the approximations in Fermi's golden rule: the breakdown of the molecular electronic dynamics by the ultra-fast plasmonic de-excitation, the breakdown of the weak coupling between the plasmon and molecular exciton by strong EM enhancement, and the breakdown of the selection rule of SERS by the field-gradient effect. These breakdowns allow the observation and control of molecular functions that remain hidden by previous spectroscopic methods.

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