Abstract
We study the presence of dark and bright modes in a planar metamaterial with a double rod unit cell introducing geometric asymmetry in rod lengths. The dark mode displays a Fano-type resonance with a sharp asymmetric profile, rendering it far more sensitive than the bright mode to slight variations of the dielectric environment. This peculiar feature may envisage the possible application of the asymmetric dimer metamaterial as an optical sensor for chemical or biological analysis, provided that the effect of material losses on the dark mode quality factor is properly taken into account.
Highlights
The flexibility in the geometry of metamaterials has enabled the tailoring of interactions between resonances in such structures, leading to exciting research possibilities such as negative index response [1], enhanced transmission [2] and electromagnetic cloaking [3]
As far as double rod metamolecules are concerned, there are a number of different ways to excite a quadrupolar dark plasmon mode: vertically stacking rod pairs displaced along their axis [10], introducing an additional orthogonal wire displaced from the symmetry centre [11], using oblique illumination of the symmetric dimer metasurface [12], introducing difference between rod lengths [13]
The excitation of dark modes due to the effect of symmetry breaking in the plasmonic dimer nanostructures has been studied
Summary
The flexibility in the geometry of metamaterials has enabled the tailoring of interactions between resonances in such structures, leading to exciting research possibilities such as negative index response [1], enhanced transmission [2] and electromagnetic cloaking [3]. With the introduction of asymmetry in the metamolecule geometry, trapped or “dark” modes can be excited [4,5] These dark mode resonances weakly couple to the free space [6,7] and present high values of the quality factor Q. In [13] a periodical array of dimers with different rod lengths is proposed to be the most simplified structure among the abovementioned dimer based metamaterials This metamaterial exhibits electromagnetically induced transparency, on the base of the dark and bright modes interaction. The dark mode, which appears due to asymmetry in the length of the gold nanorods, shows a high quality factor and a sharp dependence of its Fano resonance frequency on the environment refractive index. The dependence of the dark mode on structural parameters and its sensitivity to dielectric environment change is discussed in view of the possible application of the metamaterial under study for optical sensing, taking into account material losses
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