We delineate the four principal surface plasmon polariton coupling and interaction mechanisms in subwavelength gratings, and demonstrate their significant roles in shaping the optical response of plasmonic gratings. Within the framework of width-graded metal-insulator-metal nano-gratings, mode confinement and wave guiding result in multiwavelength light localization provided conditions of adiabatic mode transformation are satisfied. The field is enhanced further through fine tuning of the groove-width (w), groove-depth (L) and groove-to-groove-separation (d). By juxtaposing the resonance modes of width-graded and non-graded gratings and defining the adiabaticity condition, we demonstrate the criticality of w and d in achieving adiabatic mode transformation among the grooves. We observe that the resonant wavelength of a graded grating corresponds to the properties of a single groove when the grooves are adiabatically coupled. We show that L plays an important function in defining the span of localized wavelengths. We show that multiwavelength resonant modes with intensity enhancement exceeding 3 orders of magnitude are possible with w < 30nm and 300nm < d < 900nm for a range of fixed values of L. This study presents a novel paradigm of deep-subwavelength adiabatically-coupled width-graded gratings - illustrating its versatility in design, hence its viability for applications ranging from surface enhanced Raman spectroscopy to multispectral imaging.
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