  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.
