Abstract
Comprehensive reflectivity mapping of the angular dispersion of nanostructured arrays comprising of inverted pyramidal pits is demonstrated. By comparing equivalently structured dielectric and metallic arrays, diffraction and plasmonic features are readily distinguished. While the diffraction features match expected theory, localised plasmons are also observed with severely flattened energy dispersions. Using pit arrays with identical pitch, but graded pit dimensions, energy scaling of the localized plasmon is observed. These localised plasmons are found to match a simple model which confines surface plasmons onto the pit sidewalls thus allowing an intuitive picture of the plasmons to be developed. This model agrees well with a 2D finite-difference time-domain simulation which shows the same dependence on pit dimensions. We believe these tuneable plasmons are responsible for the surface-enhancement of the Raman scattering (SERS) of an attached layer of benzenethiol molecules. Such SERS substrates have a wide range of applications both in security, chemical identification, environmental monitoring and healthcare.
Highlights
The burgeoning interest in nanostructuring of dielectrics to make optical devices with novel properties has resulted in photonic crystals with extreme properties such as super-refraction [1], slow pulse propagation [2], negative refraction, and ultrasmall optical cavities [3]
This fabrication technique is well suited to the production of plasmonic nanostructures for Surface Enhanced Raman Scattering (SERS) applications because the etched surfaces are extremely reproducible, with the pyramid faces oriented at an inclination α=35.3° to the normal, and atomically smooth [9]
We present an additional simple model based on the confinement of propagating surface plasmon polaritons on the sides of a 2D V-groove
Summary
The burgeoning interest in nanostructuring of dielectrics to make optical devices with novel properties has resulted in photonic crystals with extreme properties such as super-refraction [1], slow pulse propagation [2], negative refraction, and ultrasmall optical cavities [3]. Understanding how Surface Plasmon Polaritons (SPPs) propagate on nanostructured metal surfaces is crucial in developing efficient SPR and SERS substrates. Specific localised plasmons are well understood for isolated particles of different shape, and for various grating structures, a general understanding of how plasmons localize on deformed metal surfaces is lacking. This is partly due to the current difficulty of solving Maxwell’s equations within the full three dimensional geometries including metallic media. This is evident in the controversial discussions about the role of plasmons in how light squeezes through sub-wavelength scale holes in metal films [3]. Nanostructured substrates for SERS using benzenethiol and aminothiophenol molecules as monolayer chemical markers [6]
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