Basics and Frontiers of Electromagnetic Mechanism of SERS Hotspots

Electromagnetic (EM) enhancement of Raman signals by plasmon resonance has been considered as the main mechanism of surface-enhanced Raman scattering (SERS). This mechanism is referred to EM mechanism. Recently, development in nanotechnology and calculation methods have realized the experimental and theoretical conditions beyond the conventional assumptions in EM mechanism and thus have brought the necessity to update the mechanism. In particular, hotspots (HSs), where single molecule SERS detections become possible, show various exotic phenomena related to both SERS and cavity quantum electrodynamic (QED). Thus, the scope of this chapter is to report new directions of EM mechanism, mainly from the viewpoint of HSs by QED. We here discusses four main topics: (1) cavity QED description of EM mechanism; (2) strong coupling between plasmon and molecular exciton resonances; (3) contribution of subradiant plasmon resonance to EM mechanism; and (4) EM mechanism of one-dimensionally extended SERS HSs.