  We demonstrate gain in a three-dimensional metal/semiconductor metamaterial
by the integration of optically active semiconductor quantum structures. The
rolling-up of a metallic structure on top of strained semiconductor layers
containing a quantum well allows us to achieve a three-dimensional superlattice
consisting of alternating layers of lossy metallic and amplifying gain
material. We show that the transmission through the superlattice can be
enhanced by exciting the quantum well optically under both pulsed or continuous
wave excitation. This points out that our structures can be used as a starting
point for arbitrary three-dimensional metamaterials including gain.
