Nonlinear optical response of the Si(111)-(2×1) surface exciton: Influence of biexciton many-body correlations

A microscopic theory is presented which is capable of describing the influence of biexciton many-body correlations on the nonlinear optical response of the Si(111)-(2×1) surface exciton. The band structure and the quasiparticle wave functions of this surface are obtained by ab initio many-body perturbation theory within the GW method. These results are used as a basis for setting up semiconductor Bloch equations. Analyzing the optical response including many-body correlations in the coherent χ(3) limit, it is shown that for resonant and near-resonant excitation of the surface exciton, the correlations contribute significantly to the optical response. The binding energy of the surface biexciton is predicted to be about 18meV.