Rydberg electron focused by Coulomb potential to enhance high-order harmonic generation via the chirped laser

Abstract

We theoretically study the high-order harmonic generation (HHG) from Rydberg hydrogen atom driven by a mid-infrared linearly polarized chirped laser pulse. It is shown that the high-order harmonic spectrum demonstrates an excessive high-yield broad-band supercontinuum. Based on the analyses of quantum-mechanical and classical models, it is found that the enhancement is caused by the motion of the Rydberg electron perpendicular to the polarization direction of the chirped laser pulse, which is induced by the attraction of the nucleus and initial orbital velocity of the Rydberg electron. This mechanism is different from the previous study (Chen et al 2015 Phys. Rev. A 91 043403), in which the motion of the Rydberg electron perpendicular to the polarization direction has little contribution to HHG.

Classical theory of laser-assisted spontaneous bremsstrahlung

“…In contrast, we are mostly concerned with the effect of the Coulomb interaction in the continuum, particularly the Coulomb focusing effect, which can be described classically, as follows, for example, from Refs. [18][19][20][21][22][23][24][25]. It would be unphysical to apply our approach to a short-range potential, since quantum effects play a crucial role in scattering of low-energy electrons by a short-range potential.…”

“…Here we investigate another feature of the process relevant to low-energy electron scattering when the Coulomb interaction cannot be treated perturbatively. It is known that the laser-induced atomic ionization processes can be enhanced by the Coulomb focusing [18][19][20][21][22][23][24][25]. The action of the Coulomb potential, in combination with multiple electron returns due to the laser field, focuses parts of the electron wave function, increasing the efficiency of such processes as multiphoton ionization.…”