External field-dependent enhancement of internal Coulomb interactions in time-resolved four-wave mixing

We show that the nonlinear Coulomb interactions of Wannier-Stark states in a biased GaAs/Alₓ{Ga}_{1{{-}}x}$As superlattice can be controlled by changing the external applied field. In time-resolved four-wave mixing this field control implies a change from a nonlinear polarization signal dominated by phase-space occupation to a Coulomb interaction-induced signal when the bias is increased at high excitation densities. We give a detailed theoretical explanation of the experimental observation based on a numerical solution of the semiconductor Bloch equations in a Wannier-Stark state basis.