Free Space Measurements of Negative Refraction with Varying Angles of Incidence

We report free space, microwave measurements with varying incidence angle on a material that exhibits a negative index of refraction. The experiments measure the index of refraction directly, for normal incidence, on prisms with three different apex angles. Snell's Law and the measured index are then used to verify refraction for off-normal incidence. The metamaterials used to construct the prisms are comprised of parallel metallic layers of posts and either Greek key' spiral or split ring resonators. Despite this highly anisotropic composition, the measured retractive index was found to be nearly constant tor exit angles up to 52 degrees off the normal to the prism hypotenuse. In addition to being the first free space experimental verification of Snell's Law off the optical axis for this type of metamaterial, the measurements prove that the common parallel plate structures of negative index metamaterials are not acting as guided wave media. Pub. in IEEE Microwave and Wireless Components Letters, v15 n9 pp567-569, Sep 2005