Intensity figures of merit of a metal-insulator-metal thin-film stack in the spatial and frequency domains

Abstract

Quality factors Q and figures of merit (FoM) are widely used to characterize the enhancement of a property of an observable mode or in optical sensing using plasmonic samples. We study a simple 1D metal-insulator-metal (MIM) planar thin-film stack sustaining surface plasmon polariton (SPP) and planar waveguide (PWG) modes. One evaluates the quality factors Q and the intensity FoM (IFoM) of the transmitted flux of modes of this MIM thin-film stack in the spatial and frequency domains. In the spatial domain, the observables are studied as a function of the in-plane wave vector kρ/k0 at a given frequency ω whereas in the frequency domain as a function of the frequency ω at a given in-plane wave vector kρ/k0. The quality factors Q and the IFoM of the MIM thin film stack in the spatial domain are at least six times larger than in the frequency domain (see Table 1). Compared with a conventional insulator-metal-insulator (IMI) thin film stack, the quality factor Q and the IFoM of a MIM stack are about 15 and 150 times larger in the spatial domain and only five times larger in both Q and IFoM in the frequency domain. As a general trend, the SPP mode is essential for the intensity enhancement in both MIM and IMI thin film stacks, but the SPP-PWG avoided crossing in MIM exalts this enhancement. In summary, this paper shows that it is most convenient to work in the spatial domain where the intensity is higher and leakage weaker giving a larger quality factor Q and IFoM. It also demonstrates that by optimizing the independent variables, namely the in-plane wave vector kρ/k0, and frequency ω, one can obtain Q and IFoM higher than in some 2D/3D nanoscale samples.