  Semiconductor superlattices are strongly nonlinear media offering several
technological challenges associated with the generation of high-frequency
Gigahertz radiation and very effective frequency multiplication up to several
Terahertz. However, charge accumulation, traps and interface defects lead to
pronounced asymmetries in the nonlinear current flow, from which high harmonic
generation stems. This problem requires a full non-perturbative solution of
asymmetric current flow under irradiation, which we deliver in this paper
within the Boltzmann-Bloch approach. We investigate the nonlinear output on
both frequency and time domains and demonstrate a significant enhancement of
even harmonics by tuning the interface quality. Moreover, we find that
increasing arbitrarily the input power is not a solution for high nonlinear
output, in contrast with materials described by conventional susceptibilities.
There is a complex combination of asymmetry and power values leading to maximum
high harmonic generation.
