Nonmodal Plasmonics: Controlling the Forced Optical Response of Nanostructures

Abstract

The operation of numerous physical systems and devices relies on concentrating their state space around a few carefully engineered eigenstates governing the dynamics. In photonics, these discrete degrees of freedom are typically the resonant modes of a structure. However, whenever a light source drives the structure, a continuum of additional nonmodal states generates its forced response, undermining its underlying physics and hindering control through discretization. Dealing with this nonmodal continuum poses a challenge to the design of nanophotonic systems aiming to combine compact sources and nanostructures into unified functional platforms. Here, we present a route to control forced nanostructures by engineering a discrete set of nonmodal degrees of freedom, originating from joint nanostructure-source antiresonances. We experimentally demonstrate that the forced response of ultrathin gold films is shaped by pairs of resonant-antiresonant plasmons, exhibiting joint creation and annihilation in momentum-energy space. Tuning their excitation, we show that 10 nm films can appear "black": exhibiting strong spectroangular wideband absorption.