Before going into localized surface plasmons (LSP’s), it is worthwhile discussing surface plasmons (SP’s). A surface plasmon polariton (SPP) is a coupled charge-density electromagnetic wave that is bound to the interface between a material with a negative dielectric constant (for instance, silver or gold) and a material with a positive dielectric constant (for instance, air or glass).
The SPP’s have attracted significant interest because it opens new ways to manipulate light on nanometer scales. Considerable efforts have been made towards integrated optical devices and circuits and future all-optical photonic chips. Even thought the SPP propagation along metal surfaces can only reach a few tens of micrometers , plasmon based components such as mirrors , waveguides  and interferometers  have been successfully fabricated. The SPP’s can also be used to observe molecular binding events with a detection limit of approximately 0.003 nm in adsorbate thickness . A great number of techniques have been used to launch SP’s. In Figure 1, a theoretical example using a 60 nm slit in 20 nm thick Au film is shown.