Nonreciprocal active metamaterials

We show experimentally that active electromagnetic metamaterials are suitable for the implementation of a large class of nonreciprocal media characterized by magnetoelectric constitutive parameters that cannot be achieved in passive materials. We design, fabricate, and characterize an isolator made of such an active metamaterial slab and show that it is essentially transparent to microwaves propagating in one direction and opaque in the opposite direction.