Terahertz emission from ac Stark-split asymmetric intersubband transitions

Transitions between the two states of an ac Stark-split doublet are forbidden in centrosymmetric 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 wave-function symmetry can be broken by using asymmetric confining potentials. Here we develop the many-body theory describing the intradoublet emission of a resonantly pumped intersubband transition in a doped asymmetric quantum well, showing that in such a system the intradoublet 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 monolithic and tunable terahertz emitters.