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Highly enhanced Raman scattering of graphene on a plasmonic nano-structure platform is demonstrated. The plasmonic platform consists of silver nano-structures in a periodic array on top of a gold mirror. The gold mirror is used to move the hot spot to the top surface of the silver nano-structures, where the graphene is located. Two different nano-structures, ring and crescent, are studied. The actual Raman intensity is enhanced by a factor of 890 for the G-peak of graphene on crescents as compared to graphene on a silicon dioxide surface. The highest enhancement is observed for the G-peak as compared to the 2D-peak. The results are quantitatively well-matched with a theoretical model using an overlap integral of incident electric field intensities with the corresponding intensities of Raman signals at the G- and 2D-peaks. The interaction of light with nano-structures is simulated using finite element method (FEM).

A two-dimensional array of gold nano-patches on a highly reflective mirror is proposed for refractive index sensing based on changes in the reflected colors. The grating on the mirror creates localized surface plasmon resonances resulting in a minimum in the visible reflectance spectra. The wavelength of the resonance can be tuned by changing the width of the nano-patches and is also dependent on the refractive index of the surrounding medium. The color variation due to change in the refractive index is measured and used to realize a simple low-cost sensor with a refractive index resolution better than 10⁻⁵ just using image processing. The efficacy of the proposed sensor is also demonstrated for surface sensing by depositing thin layers of silicon dioxide. The color difference due to the addition of a 3 nm thick layer of silicon dioxide is detectable by the naked eye and deposition thickness of 2 Å can be resolved using image processing.

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