  Transitions between the two states of an AC Stark-split doublet are forbidden
in centro-symmetric systems, and thus almost impossible to observe in
experiments performed with atomic clouds. However, electrons trapped in
nanoscopic heterostructures can behave as artificial atoms, with the advantage
that the wavefunction symmetry can be broken by using asymmetric confining
potentials. Here we develop the many-body theory describing the intra-doublet
emission of a resonantly pumped intersubband transition in a doped asymmetric
quantum well, showing that in such a system the intra-doublet emission can be
orders of magnitude higher than in previously studied systems. This emission
channel, which lies in the terahertz range, and whose frequency depends upon
the pump power, opens the way to the realization of a new class of monolithic
and tunable terahertz emitters.
