  We propose a scheme for realizing subwavelength electromagnetic diode by
employing cascading nonlinear meta-atoms. One-way response is demonstrated on a
microwave transmission line comprising of three metallic ring resonators acting
as meta-atoms and a varactor as the nonlinear medium inclusion. Experiments
show that our implementation can operate simultaneously as forward diode and
backward diode at different frequencies. A transmission contrast of up to
14.7dB was achieved between forward and backward transmission. Subwavelength
size of our diode should be useful for miniaturization of integrated optical
nanocircuits.
