Near-field diffraction in a two-dimensional V-groove and its role in SERS.

Optical field distribution in micro-nano geometries of miniaturized optical devices is often significantly different from that in identical but macroscopic geometries. Plasmon effects and near-field diffraction can enhance the local field intensity, leading to enhanced cross section for light absorption and scattering, which can be utilized in substrate-enhanced spectroscopies for the detection of trace amounts of adsorbed chemicals. A specific problem is an ingenious but only empirically described way to enhance signal intensity in Raman spectroscopy by the use of a substrate patterned with gold coated micron size pyramidal pits. While Raman enhancement on nanostructured substrates is generally attributed to surface plasmons, here the micron size, and thus the sub-wavelength to near-wavelength dimensions suggest that resonant enhancement emanating from optical near-field diffraction might also play a role. To answer this question, light diffraction in a projection of the pyramidal pit: a V-groove, was modelled with a modified Neerhoff-Mur formalism suitable to calculate electromagnetic field distribution in sub-wavelength structures. Under the boundary conditions a perfect conductor screen was assumed, which excludes plasmon effects. The calculations show that interference in the cavity causes a modest resonant increase in local intensity and that near-field diffraction strongly influences the field distribution, which is explained with the electrodynamic edge effect. The magnitude of the resonant electric field on its own cannot account for the experimentally observed Raman enhancement. However, a resonant enhancement of a similar magnitude is expected for the emitted Stokes frequencies. In this case the geometry implements the conditions for the classical electromagnetic Raman enhancement, ~E(4), in a good agreement with experimental results.