Abstract

High harmonic generation in atoms and molecules exposed to intense IR laser fields is well described by the semi-classical three step model [1,2]: first an electron tunnel ionizes, then it is accelerated and driven back to its parent ion by the IR field, and finally the electron recombines with the parent ion, emitting a photon with an energy of integer multiples of the driving IR field. This picture is valid as long as the energy of the emitted photon is higher than the ionization potential of the atom or molecule (above-threshold harmonics, ATH), and has been verified in many experiments, see e.g. [3]. For ATH, and when the so-called short-trajectories dominate the spectra, it is possible to map the pump-probe delay into the harmonic number. This time-frequency mapping is the key principle of High Harmonic Spectroscopy, as different harmonics carry information about the dynamics that took place between the instant of ionization and recombination.

© 2015 IEEE