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

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.

Optical Properties of Quantum-Confined Semiconductor Structures Driven by Strong Terahertz Fields

“…Howevwer, the FKE was also expected to be obtained for low fields. The coexistence of both WSL and FKE in the same structure was demonstrated and a consistent explanation of both effects was given by Linder et al, (Linder, 1997) showing that the FKE basically consists of an intensity modulation of the WSL transitions. Figure 14(a) shows calculated excitonic absorption spectra over a wide range of the electric field for a narrow-miniband superlattice.…”

“…Excitonic absorption spectra over a wide range of the electric field for a narrow-miniband superlattice. (a) calculated with excitonic effect, (b) calculated without excitonic effect (Linder, 1997) Figure 15 shows excitonic absorption spectra over a wide range of the electric field for a wide-miniband superlattice. In this case, the superlattice is closer to a 3D system.…”

Excitonic influences on the anticrossing gap of Wannier–Stark levels in a wide well superlattice

“…Chuang [10] developed a general method, which allows us to calculate excitonic levels for many kinds of problems in the physics of semiconductor heterostructures [8].…”

“…By introducing relative coordinates r = (ρ, ϕ), and keeping in mind that only excitons with zero masscenter-motion are optical active [8], we find for the exciton Hamiltonian…”

Interacting dynamic Wannier-Stark ladder driven by a periodic pulse train

“…Incidentally, it was illustrated in the usual WSL that the absorption spectra of the excitonic WSL bear a strong resemblance to those of the nonexcitonic WSL differential in p , apart from that the peak positions in the former are somewhat shifted to the lower-energy side from the peak positions in the latter by the extent of the binding energies. 51 Now, let one briefly take account of the arrowed portions in the spectra of Fig. 7 for x = 0.9.…”