Materials with this property are sometimes called "left-handed", and materials with this property have permittivity and permeability less than zero.
Valentine and Xiang developed a three-dimensional structure with this property based on a cascaded fishnet structure of silver and magnesium fluoride, and Donhee Ham's group achieved this property by taking a two-dimensional electron gas and cooling it so it undergoes kinetic inductance. One paper posited that a slab with this property would be able to focus all radiative and non-radiative Fourier components of a 2D image, creating a (*) superlens. This property was originally postulated in a paper by Veselago, and was realized using a bunch of square copper split ring resonators by Shelby, Smith, and Schultz. Materials with this property are sometimes called "left-handed", and materials with this property have permittivity and permeability less than zero. Pendry's "invisibility cloak" and many metamaterials notably possess, for 10 points, what property, in which a dimensionless quantity appearing in Snell's Law is less than zero?
Valentine and Xiang developed a three-dimensional structure with this property based on a cascaded fishnet structure of silver and magnesium fluoride, and Donhee Ham's group achieved this property by taking a two-dimensional electron gas and cooling it so it undergoes kinetic inductance.
negative index of refraction [prompt on “metamaterials”, accept left-handed before mention]