Multipole Radiations from Large Gold Nanospheres Excited by Evanescent Wave.

We proposed the use of the evanescent wave generated in a total internal reflection configuration to excite large gold nanospheres and investigated the radiations of the high-order plasmon modes supported in gold nanospheres. It was revealed that the evanescent wave excitation is equivalent to the excitation by using both the incident and reflected light, offering us the opportunity to control the orientation of the electric field used to excite nanoparticles. In addition, it was found that the scattering light intensity is greatly enhanced and the background noise is considerably suppressed, making it possible to detect the radiations from high-order plasmon modes. Moreover, the influence of the mirror images on the scattering induced by a metal substrate is eliminated as compared with the surface plasmon polariton excitation. By exciting a gold nanosphere with -polarized light and detecting the scattering light with a -polarized analyzer, we were able to reveal the radiation from the electric quadrupole mode of the gold nanosphere in both the spatial and the frequency domains. Our findings are important for characterizing the radiations from the high-order modes of large nanoparticles and useful for designing nanoscale photonic devices.