Tunable enhancement of harmonic radiation in coupled quantum wells

A few-cycle ultrashort laser field propagating through strongly coupled quantum wells (CQW) is numerically investigated. The results show that the harmonic signal can be tuned by the structure-control of CQW or enhanced due to propagation effects. If the structure of the CQW is spatial inversion symmetric, a disguised harmonic at the second-order harmonic position is disclosed within the normal odd-order harmonic sequence. However, if the structure of the CQW is adjusted to break the inversion symmetry, the odd-order and even-order harmonics both occur, whose intensities are also influenced by propagation effects. Graphical abstract (a) Electric fields, (b) population differences, and (c) harmonic distributions for different well heights of barrier U 1 at propagation distance of z = 120 µm.