A surface plasmon resonance sensing and imaging platform based on plasmonic non-uniform nano-gratings with narrow groove (sub-10 nm) is presented. In these nanogratings, normally incident optical radiation is directly coupled to surface plasmons without the requirements of any other conventional surface plasmon coupling mechanisms such as prism-based or grating-based coupling. Theoretical analysis of practically realizable plasmonic non-uniform nano-gratings with rounded tops and slanted sidewalls is carried out to numerically to determine reflectance and differential reflectance signals when the localized refractive index of the medium around the gold layer present in these nano-gratings is changed. This change in the localized refractive index can occur due to the binding of biomolecules to the gold layer. Two kinds of plasmonic non-uniform nano-gratings are studied using finite difference time domain (FDTD) modelling: gold nano-gratings (GNGs) and gold-coated silicon nano-gratings (GSNGs). The plasmonic non-uniform nano-gratings being proposed, more specifically the GSNGs, can be easily fabricated with the presently existing nanofabrication and thin film deposition methods as opposed to uniform nano-gratings (with parallel sidewalls) that are very difficult to fabricate. The plasmonic non-uniform nano-gratings with narrow grooves eliminate the strict requirements on the angle of incidence for coupling of light into surface plasmons, which are needed in conventional prism-based coupling mechanisms. By employing FDTD calculations, we demonstrate that these plasmonic non-uniform nano-gratings provide very high differential reflectance amplitude values, which are indicative of high sensitivities of the SPR or SPRi sensors when the localized refractive index around the sensors is varied. Moreover, the sensors being proposed in this paper provide a maximum sensitivity of localized refractive index sensing (i.e. surface sensitivity or SS) of 70 nm/nm with a figure of merit of the localized sensor (FOMS) of 1.5 nm -1 . This sensitivity of localized refractive index sensing is the highest reported thus far in comparison with previously reported plasmonic sensors. Moreover, these plasmonic non-uniform nano-grating based sensors exhibit significantly better performance when compared with conventional SPR or SPRi sensors based on the Kretschmann configuration.

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Vanadium dioxide (VO 2 ) is a phase change material that undergoes a reversible semiconductor-to-metal transition that can be triggered thermally, electrically, or optically. VO 2 exhibits this sharp first-order phase transition at a temperature of %68 C. [1,2] This phase transformation from monoclinic phase to rutile metallic phase leads to a considerable change in the electrical and optical properties of VO 2 thin films and nanostructures, primarily in the infrared (IR) and near-infrared regime (NIR) spectral range. As this phase transition is reversible, occurs near room temperature, and can occur at ultrafast speeds, [3][4][5][6] several applications of VO 2 films and nanostructures have emerged, such as temperature-dependent sensors, [7] optoelectronics, [2] optical switches, [8,9] and smart windows. [10,11] In recent years, there have been several studies on the plasmonic behavior of VO 2 nanostructures. [9,12,13] The temperaturedependent variation of the refractive index of VO 2 enables thermally induced modulation of plasmon resonance wavelengths in the VO 2 nanostructures. [12] More recently, composite materials consisting of metallic thin films or nanostructures of plasmonically active metals (such as gold and silver) either embedded in or surrounded by VO 2 have gained importance, as greater tunability of the plasmon resonances can be achieved in these composite structures. [14][15][16] Tunable plasmon resonances have been studied in VO 2 -Au nanocomposites, with gold plasmon resonance wavelengths blueshifting with an increase in temperature. [17] Devices with nanoparticles of Au present on top of thin films of VO 2 have also been studied. [18] More recently, a VO 2 /Au/ VO 2 thermochromic structure was studied in which the thermochromic characteristics of the structure were dependent on the gold deposition thickness. [19] Metamaterials based on VO 2 /Au lamellar stacks have also been reported, [20] such that their optical dispersion phase changes upon semiconductor-to-metal transition of the VO 2 layers. A VO 2 /Au/VO 2 sandwich structure was proposed for smart windows such that the color of the windows could be attained by controlling the gold thickness. [21] More recently, several lithography-based techniques have been employed-such as electron beam lithography, [14,15] focused ion beam (FIB) milling, [22] and nanosphere lithography-for the fabrication of the composite materials consisting of plasmonic nanostructures and VO 2 . Modulation of optical transmission in the NIR spectral regime was reported with subwavelength nanohole arrays in a metal-VO 2 double-layer film. [22] Plasmonic switches based on a combination of electron beam lithography (EBL)-fabricated aluminum nanohole arrays and VO 2 have also been reported for use in the C, L, and U optical communication bands. [23] Although precise nanostructures could be fabricated using electron-beam lithography and FIB milling, large-area fabrication of plasmonic switching devices using these methods is time-consuming and expensive. Patterned VO 2 n...

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