Free-electron density effects on the exciton dynamics in coupled quantum wells

We have studied the alterations produced by the free-electron density on the dipole moment generated by the exciton oscillations in an asymmetric coupled quantum-well system. This physical situation is possible for the case of carrier injection, when the electron concentration is greater than the hole concentration, and free electrons and excitons coexist. Excitons are directly created by hole-assisted electron resonant tunneling, where doped layers supply electrons and holes. Many-body interaction and elastic scattering strongly influence the coherent dynamics of excitons, leading to time-dependent modifications of the resonant energy levels and of the level-splitting energies. As a consequence, nonperiodic charge oscillations and nonlinear regimes appear for electron densities higher than ${10}^{10}{\mathrm{cm}}^{\mathrm{-}2}.$ Beyond electron densities of ${10}^{11}{\mathrm{cm}}^{\mathrm{-}2}$ the exciton generation can be inhibited, leading to tunneling collapse.