Temporal profile and time control of harmonic generation
We calculate the harmonic spectrum and the time profile of the emission of harmonics emitted by a hydrogen atom exposed to a low-frequency and moderate-intensity laser pulse. This calculation has been performed by solving numerically the corresponding three-dimensional time-dependent Schrodinger equation. The results are compared to those obtained with the strong-field approximation model. We show that the semiclassical interpretation based on the existence of one or several return times is valid for harmonics at the end of the plateau or beyond the cutoff. We also demonstrate that it is possible to control in time the emission of one given harmonic by using an external field whose polarization depends on time.