Nonperturbative phenomena of atoms and molecules irradiated by ultra-strong femtosecond laser pulses

We briefly review some remarkable breakthroughs in the field of intense-laser-irradiated atoms (or molecules)during the recent decades, including the multi-photon ionization, above threshold ionization, tunneling ionization, over-the-barrier ionization, nonsequential double ionization, ionization stabilization, and high-order harmonic generation.Motivated by the difficulties of pure quantum theories and classical methods in accounting for the strong-field phenom-ena, we proposed a semi-classical model, with which we have successfully reproduced and explained many experimental observations. We also give a perspective on possible directions in this field.