HIGH-ORDER HARMONIC-GENERATION PROCESSES IN CLASSICAL AND QUANTUM ANHARMONIC-OSCILLATORS

We present a theoretical study of high-order harmonic generation by a slowly driven ''Duffing'' anharmonic oscillator. The power spectra are shown to display a plateau of high harmonics, which ends up with a sh arp cutoff. The classical dynamics is analyzed with the adiabatic inva riance theorem, which yields a simple interpretation to this character istic behavior. We compare with the quantum case by solving numericall y the time-dependent Schrodinger equation, and outline the similarity between classical damping and quantum-mechanical ionization processes. This allows us, in particular, to interpret the existence of intrinsi c phases between high harmonics and the driving field. We further disc uss the implication of these relaxation processes on the coherence of high harmonics, as well as the existence of interference processes yie lding quasiresonant structures in intensity dependences.