Optimizing Optical Negative Index Materials: Feedback from Fabrication

This work is concerned with challenges in optimizing optical metamaterials with a negative effective permeability and a negative effective permittivity (double-negative metamaterials, DNM). We demonstrate the huge importance of linking the fabrication parameters and the results of optical and structural characterization of fabricated prototypes with a given optimal design environment. Specifically, the role of material deposition rate in the performance of optical plasmonic metamaterials is studied. Initially, simpler prototype samples with a negative effective permeability in the visible range have been optimized and subsequently fabricated using different material deposition rates in an electron beam evaporation system. A lower deposition rate resulted in a better quality metallic surface and consequently a lower loss. To simulate the effect of metallic surface roughness, an adjustment of the complex permittivity for silver is performed in each sample by fitting the transmission and reflection spectra. The adjustment is introduced by a wavelength-dependent ‘lossfactor’ for the imaginary part of bulk metal permittivity. An averaged constant loss-factor is chosen for the best available fabrication technique and is used in the optimal design of DNM prototypes. A fabricated DNM sample is in good agreement with the results of the pre-fabrication optimization. The sample is double-negative for wavelengths between 805 and 815 nm with a real part of refractive index of about -1.0 at 813 nm, while the ratio -Re(n)/Im(n) reaches a maximum value of close to 1.3 at that wavelength.