  The strong-coupling regime between an electronic transition and the photonic
mode of a optical resonator manifests itself in the lifting of the degeneracy
between the two modes and the creation of two polariton states with mixed
optical and electronic character. This phenomenon has been studied in atoms,
excitons in semiconductors and quantum electrodynamics circuits based on
Josephson junctions. Recently, there is also strong interest to study similar
effects using intersubband transitions in quantum wells in the terahertz, where
the ultra strong coupling regime can be reached and new physical effects have
been predicted. An other interesting feature of this system is that, in
contrast to systems based on superconductors, the ultra strong coupling regime
can be maintained up to room temperature. In this work, we demonstrate that
parabolic quantum wells coupled to LC circuit resonators in the ultra strong
coupling regime can achieve terahertz emission up to room temperature.
