GENERATION OF ULTRASHORT PULSES OF HARMONICS

We study harmonic generation by a single atom exposed to two short per pendicularly polarized laser pulses. The two perpendicular electric fi elds oscillate at two different frequencies omega(1) and omega(2) Henc e, the resultant field has a polarization which depends on time. Since harmonics are emitted when the resultant oscillating field is linearl y polarized, it is expected that a short pulse of harmonics may be emi tted if the external field is linearly polarized during a short period of time. We show that, indeed, the atom may emit an ultrashort pulse of a given harmonic. This result has been obtained by time-frequency a nalyzing the acceleration of the induced dipole moment with a filter w hose frequency bandwidth is smaller than twice the frequency of the ex ternal field. Our calculation of the dipole acceleration is based on t he numerical solution of the time-dependent Schrodinger equation. We t hen address the question of how far it is possible to reduce the durat ion of the emitted pulse of one given harmonic by adjusting both omega (1) and omega(2) and keeping the amplitude of this pulse significant. In order to answer to this question, we used the quantum version of th e two-step model [M. Lewenstein et al., Phys. Rev. A 49, 2117 (1994)].