Resonance Behaviors of Localized Surface Plasmon on an Ag/GaN Nano-Grating Interface for Light-Emitting Diode Application

Although a metal grating structure is usually fabricated for momentum-matching a surface plasmon polariton (SPP) with photon, for surface plasmon (SP) coupling application in a light-emitting diode (LED), localized surface plasmon (LSP) on such a structure also plays an important role. We numerically study the LSP resonance behaviors, including the localized resonance behavior of counter-propagating SPP interference, of an Ag grating on GaN. It is found that the resonance behaviors of LSP are controlled not only by the geometry of a grating ridge, but also by grating period, particularly when the grating period is small. In a grating with sharp ridge, large ridge height or width, LSP features of dense charge distributions around the boundaries between ridges and connecting valleys exist. The spectral positions of such LSP features are weakly dependent on the ridge width. Among such features, those with their mode field oscillations across a ridge and hence distributions in an extended space around the ridge can more strongly couple with the quantum wells of an LED. Because of the short coverage range of SPP evanescent field, the localized resonance feature caused by counter-propagating SPP interference has a shorter coupling range. For LED application, an LSP mode with field oscillation across a ridge is preferred.