  Electric dipole transitions between different cavity polariton branches or
between dressed atomic states with the same excitation number are strictly
forbidden in centro-symmetric systems. For doped quantum wells in 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 normal-mode energy splitting is tunable and can be in the
terahertz region. Here, we show that, by using asymmetric quantum wells, it is
possible to have allowed dipolar transitions between different polaritonic
branches, leading to the emission of terahertz photons. We present a quantum
field theory for such a system and predict that high-efficiency, widely tunable
terahertz lasing can be obtained.
