When a small metallic nanoparticle is irradiated by light,the oscillating electric field causes the conduction electrons to oscillate coherently.This collective oscillation of the electrons is called the dipole plasmonic resonances.When the size of a metal nanoparticle increases,higher-order plasmonic resonance modes such as electric quadrupole can be excited.The electric quadrupole where half of the electron cloud moves parallel to the applied field and half moves antiparallel is a subradiative plasmonic resonance mode with a small damping factor and a much narrower line-width,which weakly couples to the incident light.