Phase sensitivity of harmonic emission with ultrashort laser pulses

We consider harmonic generation by atoms exposed to an intense laser pulse of a few femtoseconds. Our results, obtained by solving numerically the corresponding 3-dimensional time-dependent Schrodinger equation, demonstrate that the harmonic spectra are extremely sensitive to the phase of the laser field. Depending on this phase, the harmonics in the cutoff are resolved or not resolved. The position of the cutoff itself varies with the phase and the so-called "plateau" region exhibits two well-distinct parts: a series of well-defined harmonics followed in the high-frequency region by a series of broad peaks which are not separated any more by twice the laser field frequency. These results are explained in terms of both quantum and classical dynamics. We also show that this phase sensitivity may be exploited in order to probe the phase of the electric field of an ultrashort laser pulse in a single shot experiment. Our discussion about this new method of diagnosis takes into account propagation effects.