Principle underlying the localized surface plasmon resonance (LSPR) phenomenon and the enhancement in the electric field generated by an LSPR. a Electromagnetic field interactions with a metal nanoparticle. b Simulation of the electric field distribution near a spherical metal nanoparticle. c Optical models of rough metal surfaces prepared with different metal gap sizes (d corresponds to the distance between two cylinders, and R is the radius of the metal cylinder). d Calculated electric field enhancements obtained from different spacings, using the geometry in (c). e Divergence of the electric field at the sample for 2R = d, from an incident beam at 2.7 eV.