Enhancement of resolution and propagation length by sources with temporal decay in plasmonic devices

Abstract

Highly lossy nature of metals has severely limited the scope of practical applications of plasmonics. The conventional approach to circumvent this limitation has been to search for new materials with more favorable dielectric properties (e.g., reduced loss), or to incorporate gain media to overcome the inherent loss. In this study, however, we turn our attention to the source and show that the wealth of new SPP modes with simultaneous complex frequencies and complex wave vectors that are otherwise unreachable can be excited by imposing temporal decay on the excitation. Therefore, to understand the possible implications of these new modes and how to be able to tune them for specific applications, we propose a framework of pseudo-monochromatic modes that are generated by introducing exponential decays into otherwise monochromatic sources. Within this framework, the dispersion relation of complex SPPs is re-evaluated and cast to be a surface rather than a curve, depicting all possible omega-kpairs (both complex in general) that are supported by the given geometry. To demonstrate the potentials of the complex modes and the use of the framework to study them selectively, we have chosen two important, and somewhat limiting, features of SPPs to investigate; resolution in plasmonic lenses and propagation length in SPP waveguides. While the former is mainly used to validate the proposed method and the framework on the recent improvement of resolution in plasmonic superlenses, the latter provides a novel approach to extend the propagation length of the SPP modes in planar waveguides significantly. Since the improvement in propagation length due to the introduction of temporal decay to the excitation is rather counter-intuitive, the dispersion-based theoretical predictions (the proposed approach) have been validated via the FDTD simulations of Maxwell's equations in the same geometry without any a priori assumptions on the frequency or the wave vector.