Inhomogeneous and homogeneous broadening of excitonic spectra due to disorder
Two different approaches for the calculation of optical properties of disordered semiconductors are first compared: the equation of motion approach formulated for the configurationally averaged linear optical polarization and the tight-binding real-space approach with subsequent configurational aver...
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|Zusammenfassung:||Two different approaches for the calculation of optical properties of disordered semiconductors are first compared: the equation of motion approach formulated for the configurationally averaged linear optical polarization and the tight-binding real-space approach with subsequent configurational averaging. The latter turned out to be more successful for the present study. In particular, based on the one-dimensional real-space tight-binding model the equation of motion for the linear and nonlinear optical polarization is solved numerically, including the many-particle Coulomb interaction within the dynamical-truncation limit. Disorder-induced features of linear and nonlinear optical spectra are investigated in the main part of this thesis. The influence of the length scale of disorder on shift and width of linear spectra of excitons is studied for the case of weak disorder. The results are interpreted in terms of the averaging of the disorder potential due to the relative motion of electrons and holes. This mechanism strongly depends on disorder length scale. We find that for length scale smaller than the Bohr radius there is a universal behavior. Nonlinear two-dimensional Fourier-transform spectra are calculated. It is found that in a disordered semiconductor heterostructure various couplings (in the sense of Fano-couplings) become possible which are absent in the ordered counterpart because of selection rules. They lead to disorder-induced dephasing and an interpretation of the excitonic line in terms of an energy dependent dephasing rate. It is also shown that two-exciton states obscure the spectra. It is therefore important to use such excitation conditions that do not yield large bi- or two-exciton contributions to the nonlinear response if disorder effects on the exciton are to be studied, i.e., co-circular polarized excitation laser pulses.|