  It is shown here that rendering the width of nano-grooves into a tunable
parameter presents a new means of light trapping in subwavelength gratings.
Particularly, in gratings with groove-widths below 150 nm, the plasmon coupling
between the perimeter walls of the groove becomes dominant and thus turns
groove width into the principle geometric parameter in determining light
trapping. Using this parameter, we investigate the prospect of tunable optical
functionalities. An analytical formula is derived by treating each nano-groove
as a plasmonic waveguide resonator. The resulting change in the cavity
effective mode index is examined; these results are in close agreement with
numerical simulations. It is shown that the tunable nano-groove resonator
presented here accurately defines waveguiding, slow-light, and light-trapping
regimes.
