Abstract
The angular dependencies of the local field enhancement and the Raman emission enhancement are investigated, numerically and experimentally, for a plasmonic crystal double resonance SERS substrate consisting of a periodic array of gold disks above a gold film. We find that the local field enhancement is very sensitive to the incident angle. The Raman emission enhancement has a strong angular dependence on the detection direction, with the substrate "beaming" the Raman emission so that different Raman lines have different far-field patterns. We demonstrate that a stronger SERS signal results when the plasmonic substrate is illuminated with a collimated, rather than focused, laser beam.
Highlights
Metallic nanostructures can sustain surface plasmon modes which couple strongly with electromagnetic radiation [1,2,3]
The EM mechanism consists of three steps [4]. 1) The excitation field E0 with frequency ωL excites localized surface plasmons (LSPs) on the metallic nanostructures and the intensity of the electric field ELoc on the nanostructure surface is enhanced by a factor MLoc(ωL) = | ELoc |2/| E0 |2
For an isotropic Raman tensor, each molecule may be modeled by a dipole with d=αELoc that radiates at the Raman frequency ωR. 3) The fields generated by the Raman dipole excite surface plasmons in the metallic nanostructure, resulting in an enhancement by a factor MRaman(ωR) of the radiated power, compared to the hypothetical case of the dipole d being situated in free space
Summary
Metallic nanostructures can sustain surface plasmon modes which couple strongly with electromagnetic radiation [1,2,3]. The substrate is termed “double resonance” as it exhibits two hybridized resonances These result from the coupling between localized surface plasmons (LSPs) on the particles and surface plasmon polaritons (SPPs) on the film, and enable a large local field enhancement factor MLoc(ωL) and a large Raman emission enhancement factor MRaman(ωR) to be simultaneously achieved. We demonstrated that this structure can provide average SERS enhancement factors of nearly 109 [8]. Like the substrates we study in this paper, these have the advantage that the main beam is emitted surface-normal, rather than being directed into the substrate as in the case of Refs 21 and 22
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