Abstract
Commensurate gratings of deep-metallic grooves have highly localized cavity resonances which do not exist for purely periodic gratings. In this paper we present the experimental dispersion diagram of the resonances of a commensurate grating with three sub-wavelength cavities per period. We observe selective light localization within the cavities, transition from a localized to a delocalized mode and modifications of the coupling of modes with the external plane-wave that may lead to the generation of black modes. This unexpected complexity is analyzed via a theoretical study in full agreement with the experiments. These results open a way to the control of wavelength-dependent hot spot predicted in more complex commensurate gratings.
Highlights
Within the vast framework of controlling light properties at a sub-wavelength scale one important issue is to understand and produce - in a reproducible manner - field localization and enhancements at the surface of metals, one reason being that those are linked to the Surface Enhanced Raman Scattering (SERS) effect[1]
In the dispersion diagram (Fig.1d) they appear as dark lines, the two darkest ones corresponding to the two narrow resonances
We have presented the first experimental dispersion diagram and the calculated one of the cavities modes appearing in a grating with more three grooves per period
Summary
Within the vast framework of controlling light properties at a sub-wavelength scale one important issue is to understand and produce - in a reproducible manner - field localization and enhancements at the surface of metals, one reason being that those are linked to the Surface Enhanced Raman Scattering (SERS) effect[1]. Electromagnetic field enhancements at the surface of metals are created by the coupling of incident light with the surface plasmon modes of the metal. One vanishes at normal incidence and is excited over the whole diagram This mode undergoes a transition from a localized to a partly delocalized state, due to its coupling with horizontal surface plasmons as the incident angle increases. The third cavity resonance behaves in a manner similar to that of a simple period grating We analyze these features in the framework of the modal expansion approach [4] which is in full agreement with the experiments.
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