Excitons in superlattices: Absorption asymmetry, dimensionality transition, and exciton localization

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Abstract
We calculate the field-dependent excitonic absorption spectra of semiconductor superlattices by diagonalizing the full Hamiltonian of the problem. Our model therefore includes both bound and continuum states of the excitons. The theoretical results are compared with experiments. We first show that the Coulomb interaction does not destroy the field-induced transition from the Franz-Keldysh to the Wannier-Stark regime, but causes an absorption asymmetry in the spectra, i.e., a dominance of Wannier-Stark transitions with negative indices. This effect, which has been observed before, is explained by a Coulomb-induced shift of oscillator strength from higher to lower energies in a simple model involving an effective Coulomb potential. We further address the problem of the transformation of the quasi-two-dimensional excitons of the Wannier-Stark levels to the three-dimensional miniband excitons obtained for zero field. For finite electric fields the quasibound miniband exciton states are explained as resonant states of the various Wannier-Stark excitons. We show that in contradiction to previous assumptions the Wannier-Stark ladder transitions are never suppressed by exciton localization. The observability of the transitions only depends on the relation between the zero-field miniband width and the linewidth of the transitions, but not on the ratio between miniband width and exciton binding energy.