Ultra-strong coupling effects with quantum metamaterials

We study a semiconductor-based quantum metamaterial which has the optical characteristics of a metal in two directions, but behaves like a collection of artificial atoms, whose properties can be designed-in using quantum theory, in the third. We find that it supports a type of guided collective plasma resonance (CPR) mode which exhibits efficient optical coupling and long propagation distances. Furthermore, the coupling of the CPR mode with the 'artificial atom' transition leads to a case of Ultra-Strong-Coupling, demonstrated by a record vacuum Rabi splitting of 65 meV, a sizable fraction (42%), of the bare intersubband energy. (C) 2008 Elsevier Ltd. All rights reserved.