Inter-branch terahertz lasing in asymmetric intersubband polariton systems

In doped quantum wells embedded into semiconductor microcavities, the strong coupling between an intersubband transition in the conduction band and a cavity mode produces two branches of intersubband cavity polaritons, whose energy splitting is tunable and can be in the terahertz region. Electric dipole transitions between different cavity polariton branches, like transitions between dressed atomic states with the same excitation number, are strictly forbidden in centro-symmetric systems. We showed that, breaking the symmetry using asymmetric quantum wells, it is possible to have allowed dipolar transitions between different polaritonic branches, leading to the emission of terahertz photons. We developed a quantum field theory for such a system, and predict that high-efficiency, widely tunable terahertz lasing can be obtained