Extraction efficiency of highly confined surface plasmon-polaritons to far-field radiation: an upper limit

Abstract

We propose a unique method determining an upper limit of extraction efficiency of the surface plasmon-polaritons (SPPs), E-SP, which are highly confined on a corrugated metal surface. The method is based on measurement of the spectral bandwidth of a grating-induced absorption spectrum as a function of metal dielectric constant. After finding the fact that E-SP exhibits an extremely linear relationship with the collision frequency Gamma of metal over a SPP band below the surface plasmon frequency, an upper limit of E-SP can be determined by an asymptotic estimation as Gamma -> 0 for total decay rates of the confined SPPs. Our method based on the bandwidth measurement is inherently free from the ambiguity and underestimation difficulties pertaining to the previous prism-coupling approaches for E-SP estimation. It will also be quite applicable for evaluating SPP-mediated light-emitting diodes (LEDs) of which total external efficiency is dominantly restricted by the upper limit of E-SP. Especially for the case when SPP excitation probability approaches unity, the proposed method would excellently figure out the maximum realizable external efficiency of SPP-mediated LEDs.