Excitonic Emission of THz Radiation: Experimental Evidence of the Shortcomings of the Bloch Equation Method

THz-emission experiments are presented which demonstrate a limitation of the semiconductor Bloch equations (SBE) for the description of the Coulomb interaction in semiconductors optically excited near the band gap. In a narrow-miniband superlattice, the field dependence of the THz emission frequency shows an anticrossing behavior characteristic for excitonic transitions. This proves the dominance of excitonic contributions, contradicting predictions based upon the SBE. The data support recent theoretical work which elucidated that the inherent Hartree-Fock approximation of the SBE can lead to an incorrect description of the Coulomb interaction.