The optical properties of natural substances are based on the fact that only the electric part of the light is manipulated. However, photonic metamaterials offer the possibility to modify both the electric and the magnetic part of the light. This can lead to a negative refractive index in optics, for instance. Photonic metamaterials are artificial materials consisting of responsive elements which are smaller than the relevant wavelength of light. Thus, the complex interaction between light and these elements can be solely described by effective parameters, the permittivity and the permeability.
In this thesis, we investigate the optical properties of so-called fishnet metamaterials in both experiment and theory. In these fishnet metamaterials, for example, a negative phase and group velocity occur in the spectral range in which the refractive index is negative. We describe experiments performed on a low-loss negative-index sample operating at telecommunication wavelengths. Furthermore, we present a metamaterial which shows a negative refractive index in the visible spectrum for the first time. Additionally, we examine the complex interaction between the responsive elements of our metamaterials. So far, recent metamaterials are only based on a single layer. Therefore, we have fabricated samples with more than one layer and study the changes of the optical properties with the number of layers.