Origin of unusual even-order harmonic generation by a vortex laser.

ABSTRACT

When a spatially-inhomogeneous few-cycle vortex laser interacting with quantum wells, besides the general odd-order harmonics occur, unusual "even-order" ones can be found, which are clarified to possess even orders, however their topological charge numbers are not consistent with those predicted from the vortex transformation criterion (i.e., topological charge number should be directly proportional to its harmonic order for any a harmonic). The origin is the broader spectral width of odd-order harmonics tails at the positions of even-order harmonics due to the short-duration pulse excitation, whose contribution overwhelms the spatial-inhomogeneity degree.