Advances in nonlinear, active, and anisotropic artificial impedance surfaces

We discuss recent advances in nonlinear, active, and anisotropic artificial impedance surfaces. These structures are a form of planar metamaterial in which the impedance is defined by a periodic metallic structure which may include nonlinear or active components. In order to achieve high anisotropy in arbitrary patterns, nonperiodic strucutres are required. By adding nonlinear elements such as diodes, varactors, and other components, power-dependent and frequency self-tuning structures may be built. In addition, by incorporating active components such as non-Foster circuit elements, is is possible to make broadband fast-wave and slow-wave structures which provide a wide degree of control over the interaction between electromagnetic waves and the surface.