Table 1. FOM of circular-hole double-fishnet negative-index photonic metamaterials obtained by embedding the structure in an effective homogeneous medium with refractive index $n=\sqrt{1.1}$ (i.e., no glass substrate). The FOMvalues in brackets refer the corresponding calculations for structures in air (n=1) on a glass substrate with refractive index n=1.5. The thickness of the spacer layer increases from s=30nm in the top to s=100nm in the bottom row. The metal thickness t=45nm is fixed. To keep a fixed vacuum wavelength λ=1.4µm, the lattice constant a, and the hole radius r have to be adjusted. The column labeled λ/a refers to the ratio of vacuum wavelength and lattice constant. The column to the right refers to the corresponding ratio for the material wavelength λ/n) in the embedding medium with refractive index $n=\sqrt{1.1}$ (glass substrate with refractive index n=1.5).