  We demonstrate theoretically and experimentally a two-scale architecture able
to achieve giant field enhancement by simultaneously exploiting both the
Rayleigh anomaly and localized surface plasmon resonance. Metallic oligomers
composed of colloidal nanospheres are well-known for the ability to strongly
enhance the near-field at their plasmonic resonance. However, due to intrinsic
nonlocality of the dielectric response of the metals along with their inherent
loss, the achievable field enhancement has an ultimate constraint. In this
paper we demonstrate that combining plasmonic resonance enhancements from
oligomers, with feature size of tens of nanometers, with a Rayleigh anomaly
caused by a 1-D set of periodic nanorods, having a period on the order of the
excitation wavelength, provides a mean to produce enhancement beyond that
constrained by losses in near field resonances. Metallic oligomers are
chemically assembled in between the periodic set of nanorods that are
fabricated using lithographic methods. The nanorod periodicity is investigated
to induce the Rayleigh anomaly at the oligomers plasmonic resonance wavelength
to further enhance the field in the oligomers hot spots. A thorough study of
this structure is carried out by using an effective analytical-numerical model
which is also compared to full-wave simulation results. Experimental results
comparing enhancements in surface enhanced Raman scattering measurements with
and without nanorods demonstrate the effectiveness of a Rayleigh anomaly in
boosting the field enhancement. The proposed structure is expected to be
beneficial for many applications ranging from medical diagnostics to sensors
and solar cells.
