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Efficient focusing with a concave lens based on a photonic crystal with an unusual effective index of refraction

This work reports measurements with a concave lens based on a photonic crystal (PC) structure, which was designed to have an effective index of refraction neff < 1 or even < 0, and which is intended as a model system for future down‐scaled optical elements based on PCs with an unusual effective index. The dimensions of the model PC were optimized for experiments in the microwave regime around 10 GHz. Calculations for a material with no losses allowed to select the wavelengths for which the lens could be expected to behave as a homogeneous meta‐material with an unusual effective refractive index. The field distribution behind the lens was measured, and good focusing efficiencies for neff < 1 were found for perfect and strongly disturbed PC's in reasonable accordance with the predictions. The (down‐scaled) model system investigated thus can serve as a reference for testing PC‐based optical elements made from materials which so far elude reliable predictions, e.g. doped semiconductors, nanorod assemblies, or meta‐materials with anisotropic behavior. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)