Ab initio theory of Fano resonances in plasmonic systems
© 2014 EPFL
The transition from localized to delocalized plasmons (i.e. the transition from a situation where the decay length of a travelling surface plasma wave is greater than its propagation distance to a situation where it is smaller) and hence the onset of plasmon delocalization is studied in a single 2D silver nanoparticle of increasing length. A fourier analysis in the near-field of the nanoparticle is used as the main tool for analysis. This method, along with far-field scattering spectra simulations and the near-field profile directly above and along the length of the nanoparticle are used to investigate and clearly show the transition from localized to delocalized modes. In particular, it is found that for a finite sized rectangular nanoparticle, both the emerging odd and even delocalized modes are nothing but a superposition of many standing wave plasmon modes. As a consequence, even very short metal films can support delocalized plasmons that bounce back and forth along the film.
The original formalism developed by Fano to describe asymetric lineshapes observed in atom spectroscopy has proven very useful to desribe many recent experiments in plasmonic nanostructures. However, this formalism fails to describe many important experimentl situations, since it does not account for losses in the metal used to build the nanostructure. Recently, we have developed an ab initio theory for Fano resonances in plasmonic nanostructures and metamaterials using the Feshbach formalism. This theory reveals the role played by the electromagnetic modes and material losses in the system, and enables the engineering of Fano resonances in arbitrary geometries. A general formula for the asymmetric resonance in a nonconservative system is also derived. The influence of the electromagnetic interactions on the resonance line shape is discussed and it is shown that intrinsic losses drive the resonance contrast, while its width is determined mostly by the coupling strength between the nonradiative mode and the continuum.
The following figure shows the structural decomposition of a Fano resonance in a dolmen-type plasmonic structure in air: beam 1 supports a radiative dipolar mode, whereas beams 2 and 3 support a nonradiative quadrupolar mode. (a) Local density of states (LDOS) of a dipole emitter placed 50 nm from the end of one beam of the doublebeam structure, (b) reflectance of a single beam, (c) reflectance of the composite system.
This ab initio theory for Fano resonances in plasmonic nanostructures and metamaterials provides deep insights into the mechanisms that lead to Fano resonances in plasmonic systems. The influence of the electromagnetic interactions on the resonance line shapes can be analyzed and this novel formalism introduces a critical screening parameter driven by the intrinsic losses in the material. The resonance width is determined mostly by the coupling strength between the nonradiative mode and the continuum. The formula can be derived from the classical model of two damped coupled oscillators and is in perfect agreement with numerical simulations of complex plasmonic systems.
Check the corresponding publication: PDF External link: doi: 10.1103/PhysRevB83.235427