Optimization of arrays of gold nanodisks for plasmon-mediated Brillouin light scattering

Distributions of electric fields in two-dimensional arrays of gold nanodisks on a Si₃N₄ membrane, with light incident through the membrane, are modeled with the aim of determining array geometries for effective plasmon-mediated Brillouin light scattering ("surface-enhanced Brillouin scattering") from phonons or magnons in a specimen placed in contact with such an array. Particular attention is devoted to average intensities and higher-wavevector components of the fields in a plane 2 nm from the circular nanodisk/vacuum interface, which is anticipated to be in the vicinity of the surface of a specimen. For nanodisks with diameters of 50 nm, the average intensity near the circular nanodisk/vacuum interface increases as the angle of the incident light approaches the normal of the Si ₃N₄ surface. At low angles of incidence relative to the Si₃N₄ normal, average intensities also increase with decreasing array spacing, primarily because of the corresponding changes in fractional coverage area of the gold. The highest average intensities (with near-normal incidence and 70 nm array periodicity) are found to be ̃3 times that of the incident light. More significantly, higher-wavevector components of the fields are found to have intensities comparable to the incident light. This finding provides evidence for the feasibility of using surface plasmons in nanodisk or nanoline arrays to mediate Brillouin scattering from phonons or magnons with wavelengths of a few tens of nanometers, which would extend the wavevector range of Brillouin-scattering metrology by an order of magnitude.