We consider harmonic generation by atoms exposed to an intense laser pulse
of a few femtoseconds. Our results, obtained by solving numerically the cor
responding 3-dimensional time-dependent Schrodinger equation, demonstrate t
hat 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 o
f 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 fr
equency. These results are explained in terms of both quantum and classical
dynamics. We also show that this phase sensitivity may be exploited in ord
er 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 diagnosi
s takes into account propagation effects.