For applications in the near infrared or visible wave range, the fabrication of planar metamaterials envisages the use of sophisticated electron beam lithography techniques, therefore a reduction in the complexity of their planar geometry is highly desirable, since this would decrease production time and cost dramatically. 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. In spite of the various reports in the recent literature, only recently an analytical model��leading to a generalization of Fano formula to electromagnetic fields and lossy materials��has been proposed [21], enabling the study of these asymmetric resonances in metallic nanostructures and paving the way to their engineering.

However, influence of structural parameters and intrinsic material losses on sensing properties of Fano resonance based metamaterials has not been studied properly so far.In this work, we study numerically a planar metamaterial composed of gold nanorod dimers and report on the parametric analysis of modes excited in the IR spectrum by electromagnetic wave normally incident onto the metasurface. 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 Entinostat for optical sensing, taking also into account material losses.2.?Metamaterial Geometry and Numerical ModelThe metamaterial dimer structure consists of two metallic Au nanorods on an Indium Tin Oxide (ITO)-coated glass substrate, with the ITO acting as an adhesive layer for the gold. A schematic representation of the unit cell of the dimer structure is shown in Figure 1a, with gold rods of length L1 and L2 respectively separated by a gap g = 50 nm. Both rods have equal widths w = 70 nm and thickness 30 nm. The ITO layer has a thickness of 25 nm with permittivity of 3.8. The Au permittivity at frequency �� is described in terms of the Drude model:?=1?��p2��2+i�ئ�c(1)with a plasma frequency ��p = 1.37 �� 1016 s?1 and a collision frequency ��c = 1.2 �� 1014 s?1, to account for the scattering losses in the gold film [13,22].