Localized surface plasmons (LSPs) are nonpropagating excitations

of the free-electron plasma in metal nanostructures excited by

electromagnetic radiation at optical frequencies [1]. The optical

excitation of LSPs results in the formation of hot spots featuring

high electric field enhancement over subwavelength spatial regions.

This effect opens up a wide range of device applications [2], such as

solar cells, plasmon-enhanced photodetectors, optical manipulators,

modulators, thermal emitters, surface-enhanced Raman scattering

substrates, apertureless near-field scanning optical microscopy, and

heat-assisted magnetic recording, and it is also ideal to enhance

nonlinear optical effects.