Surface-Enhanced Raman Scattering Enabled by Metal-Coated Dielectric Microspheres

Dielectric microspheres have been demonstrated as an efficient tool for non-plasmonic surface-enhanced Raman scattering (SERS) due to the curvature-induced local electromagnetic enhancement. By combining a metal layer onto the microsphere surface, one can study SERS which is activated by both surface plasmons and microsphere-induced modulation of Raman emission. In this work, a new strategy of enhanced Raman scattering using silver-coated polystyrene microspheres is reported. The Raman signal up to 10(-6) M Rhodamine 6G molecules measured on the microsphere top surface is up to seven times larger than that measured from a flat silver film. An enhancement factor of approximate to 10(8) is estimated compared with the case of a non-plasmonic microsphere surface. The enhancement is attributed to localized surface plasmon resonances and also the modification of emission directionality which increases both Raman scattering and the collection efficiency of the detecting system. Besides, whispering gallery mode resonance potentially leads to additional cavity-field enhancement effect of Raman signals. Both experimental statistics and numerical calculations reveal that efficient enhancement occurs without purposely spectral matching optical resonance with Raman emission wavelengths.