Many-body theory of quantum interference effects in semiconductor quantum well lasers

Summary form only given. In view of the progress of intersubband lasers, the proposal of lasers without population inversion (LWI) in intersubband transitions was made. It is based on the absorption cancellation via Fano interference of tunneling processes. For the demonstration of LWI in semiconductors, it is crucial to reveal the role of collective excitations and inhomogeneous-broadening. We consider a scheme with two closely spaced upper subbands a and a´ and a lower subband b within the conduction band of a multiple quantum well. The electrons are injected into and removed out of the subbands due to tunneling between the wells and quasi-continuum regions surrounding them. The tunneling involving the upper subbands experience Fano interference of two incoherent processes, which induces coherence between them and enables absorption cancellation. We calculate gain via the solution of semiconductor Bloch equations, which allows us to account for the Coulomb interaction between electrons. The effective masses are different in the subbands, and this results in a subband dispersion: transitions between different momenta of electrons have different resonant energies, or, in other words, the transition is inhomogeneously broadened. We find that the Coulomb interaction causes formation of collective modes (repellons) associated with the Fermi-edge singularity of the repelling pairs of an electron in the a subband and a hole in the b subband. The broadening of repellons is homogeneous and is much smaller than the inhomogeneous broadening due to single-particle modes. Large inhomogeneous broadening is known to destroy LWI.