Focused Surface Plasmons for Enhanced Raman Scattering*

Surface plasmon polaritons launched at concentric arcs can be focused into a sub-wavelength wide focal spot of high near-field light intensity. When increasing the number of arcs from one to eight the focused intensity increases by a factor of 30. The focused plasmons give raise to enhanced Raman scattering from R6G molecules placed in the focal area. By exploiting the polarization dependence of the focusing we establish an enhancement of the Raman signal by a factor of three. Our results show that focusing of propagating surface plasmons on flat metal surfaces may be an alternative to localized plasmons on metal nanostructures for achieving enhanced Raman scattering. In particular, a flat metal substrate enables better control over the local electric fields and the placement of analyte molecules, and, therefore, ultimately better fidelity of Raman spectra.