  The nonlinear properties of a metamaterial sample composed of double-layer
metallic patterns and voltage controllable diodes are experimentally
investigated. Second harmonics and spectrum modulations are clearly observed in
a wide band of microwave frequencies, showing that this kind of metamaterial is
not only tunable by low DC bias voltage, but also behaves strong nonlinear
property under a small power incidence. These properties are difficult to be
found in normal, naturally occurring materials.
