INTERACTION OF ATOMS WITH SUPERSHORT LASER-PULSES AND THE GENERATION OF THE SUPERCONTINUUM

A theoretical model of emission from an atom excited by a strong laser field is proposed. The main ideas incorporated in the model are: (i) time-dependent AC Stark shift of Rydberg levels; (ii) field-induced level crossing providing efficient resonance coherent population of Rydberg levels; (iii) field-induced interference suppression of photoionisation from Rydberg levels in a strong field; (iv) two-step transition from the zone of coherently excited Rydberg levels to the ground level with spontaneous emission of a photon at the first step and stimulated emission of some laser photons at the second step. The model is shown to describe qualitatively the main features of the experimentally observed supercontinuum, i.e. a substantially broadened spectrum of emitted light.