Rainbow-trapping by adiabatic tuning of intragroove plasmon coupling

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.