High-order harmonic generation from Rydberg atoms by using three-color laser pulse

The high-order harmonic generation is theoretically studied when Rydberg state hydrogen atoms are interated with a linearly polarized three-color laser field along the z-axis. It is found that high-order harmonics show a super-continuum structure as well as extensive intensity enhancement. By time-frequency analyses, it is revealed that a large group of electronic density focused from the area outside the z-axis to the nucleus. This extraordinary behavior of Rydberg electron which is quite different from the previous study gives rise to the remarkable enhancement of the super-continuum. Further theoretical research based on semi-classical and quantum-mechanical technique displays the electronic focus effect caused by the combined action of the initial Rydberg orbital velocity and nuclear attraction of the core. This mechanism can apparently enhance the high-order harmonics, leading to remarkable attosecond pulse.