Intraband dynamics at the semiconductor band edge: Shortcomings of the Bloch equation method

The validity of the semiconductor Bloch equations (SEE) depends on the approximate decomposition of an intraband correlation function into a product of interband transition densities. We analyze the consequences of this approximation on the intraband dynamics of an optically excited semiconductor. As a special example where the SEE treatment becomes questionable we consider the THz emission of a narrow band superlattice in a static bias field. A comparison of the second order SEE solution with a rigorous second order treatment of this system helps one identify the weak points of the SEE approach and understand the physical background of its failure.