Rainbow light trapping in ultrathin plasmonic nanogratings

Abstract

Light incident on nanoscale metal-insulator-metal (MIM) plasmonic gratings generates surface plasmon polaritons (SPPs) which resonate and propagate within the grating structure. The SPP resonant wavelength can be altered by introducing a gradient in the width of the MIM grooves. Specifically, a symmetrically graded grating with a narrow central groove leads to a gradient in the effective refractive index such that the index increases in the direction of the central groove. This index gradient guides non-localized SPP waves towards the grating center. This waveguiding of SPPs along with localized SPP modes within the narrow central grooves give rise to multi-wavelength electric field enhancement at the grating center. SPP coupling across the grooves leads to an increase in electromagnetic field strength with decreasing groove width. However, standard nanofabrication techniques limit the minimum width of the grooves to approximately 50nm, preventing maximum field enhancement. Herein we report on the development of a novel nanoplasmonic graded grating with a 10 nm central groove flanked by increasingly wider grooves on either side, which are fabricated using thin film RF magnetron sputter deposition technique. These structures are studied using COMSOL Multiphysics modelling in which we vary the groove width, groove separation and groove depth, and thus demonstrate localization of broadband incident light. Raman spectroscopy and fluorescence microscopy are used to demonstrate field enhancement at several visible wavelengths.