We consider harmonic generation by atoms exposed to an intense laser p
ulse of a few femtoseconds. Our results, obtained by solving numerical
ly the corresponding three-dimensional time-dependent Schrodinger equa
tion, demonstrate the strong sensitivity of the harmonic spectra to th
e phase of the laser. These results are explained in terms of both qua
ntum and classical dynamics. We show that this phase sensitivity may b
e exploited in order to probe the laser phase for ultrashort pulses. O
ur discussion about this new method of diagnosis takes into account pr
opagation effects.