  We present an effective method to generate second harmonic (SH) waves using
nonlinear metamaterial composed of coupled split ring resonators (CSRRs) with
varactor (variable capacitance) diodes. The CSRR structure has two resonant
modes: a symmetric mode that resonates at the fundamental frequency and an
anti-symmetric mode that resonates at the SH frequency. Resonant fundamental
waves in the symmetric mode generate resonant SH waves in the anti-symmetric
mode. The double resonance contributes to effective SH radiation. In the
experiment, we observe 19.6 dB enhancement in the SH radiation in comparison
with the nonlinear metamaterial that resonates only for the fundamental waves.
