Enhancement of Resolution and Propagation Length by Sources with Temporal Decay in Plasmonic Devices

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, by imposing temporal decay on the excitation, SPP modes with simultaneous complex frequencies and complex wave vectors can be excited with enhanced resolution and propagation length. Therefore, to understand the underlying physics of these phenomena and, in turn, 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 $\omega-k$ pairs (both complex in general) that are supported by the given geometry. Since the improvement in resolution and propagation length due to the introduction of temporal decay to the excitation is rather counter-intuitive (i.e., adding temporal loss improves the propagation length), the dispersion-based theoretical predictions 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. Moreover, improvement in resolution with the temporal decay has been demonstrated in a plasmonic superlens structure to further validate the predictions.