| Popis: |
Enhancement of local electromagnetic fields is instrumental for engineering of light absorption, emission, scattering, chemical reactions, and other processes. Nanostructured composites with plasmonic inclusions have been shown as promising candidates to concentrate electromagnetic waves in nanometer-sized “hot spots”. Unfortunately, majority of high-performance plasmonic structures are resonance-based, and therefore their performance is relatively narrow-band. Here we present a novel material system that has potential to realize broadband enhancement of local intensity and explain the origin of this behavior. The proposed material platform comprises an array of aligned plasmonic cones arranged in a periodic planar lattice. From the effective medium standpoint, such structure represents a uniaxial material whose effective permittivity varies along the cone. Importantly, there exists a relatively wide range of wavelengths where one component of the effective permittivity tensor crosses zero within the composite. According to previous research, strong enhancement of local field is expected in the vicinity of epsilon-near-zero point in homogeneous materials with spatially varying permittivity, often called transitional metamaterials. We show, however, that due to strong structural nonlocality electromagnetic response of nanocone media does not follow this recipe. In fact, we demonstrate that the incoming radiation is coupled into an additional electromagnetic wave that propagates towards the tip of the cone causing a strong enhancement to the local field. We present a comprehensive description of this phenomenon. |
