Light scattering by a nanoparticle and a dipole placed near a dielectric surface covered by a thin metallic film.
On the basis of Maxwell's equations a light scattering system of axial symmetry is considered, which consists of a nanoparticle, a dipole and a metal film (covering a dielectric support). Nanoparticle (NP) and dipole are situated on an axis of symmetry and the dipole is oriented along the axis and placed between film and nanoparticle. The field enhancement factor F and dipole energy flux D are calculated by the Green's function method: the initial system of Maxwell's equations is reduced to a system of boundary integral equations, and solutions are obtained by the boundary element method. Illumination of the scattering system by a radially polarized Bessel light beam causes a field enhancement in the vicinity of the film surface. The metallic NP closely placed at the film surface acts as nano-antenna. Surface plasmons excited in the particle and film convert the incident propagating EM field into non-propagating evanescent near-field. Then the field is confined and strongly enhanced in a particle/film gap. The enhancement of Raman radiation depends on many factors: size and shape of NP, permittivities of all materials, light wavelength, film thickness, angle of light beam, and - very strongly - on the gap distance. The field enhancement in a gap ~1 nm can be 10(3) and more and the Raman radiation enhancement factor can reach huge values ~10(10)-10(12). Whereas for small nanoparticles the field enhancement factor F and the dipole energy flux D do not depend on the direction of the exciting beam and on the angle of emission, a strong influence is found for extended particles. This influence is plausibly explained by a larger overlap between the electric field of the exciting beam or the emitted radiation field with the near field distribution of the nanoparticle leading to higher F and D values, respectively.
Geshev
Pavel I
PI
Fischer
Ulrich C
UC
Fuchs
Harald
H