Time-frequency analysis of high harmonic generation

We investigate the question of when each harmonic is emitted in high-order harmonic generation in atomic gases irradiated by high-power laser pulses. Our numerical computation of the Wigner and Husimi distributions of the dipole acceleration indicates that low-order harmonics are generated mainly via transitions between bound states and are emitted at times when the laser fields amplitude takes on its maximum or minimum value. The Wigner and Husimi distributions computed quantum mechanically can be explained semiclassically in terms of the three-step model. The phenomenon of high harmonic generation is thus an example that shows a strong degree of quantum-classical correspondence.