  We study the properties of a tunable nonlinear metamaterial operating at
microwave frequencies. We fabricate the nonlinear metamaterial composed of
double split-ring resonators and wires where a varactor diode is introduced
into each resonator so that the magnetic resonance can be tuned dynamically by
varying the input power. We show that at higher powers the transmission of the
metamaterial becomes power dependent, and we demonstrate experimentally
power-dependent transmission properties and selective generation of higher
harmonics.
