ABSTRACT
Dong Sheng Guo1*, Jingtao Zhang2, Wei Yu2, Zhizhan Xu2, Xiaofeng Li3, Panming Fu3, and R.R. Freeman4 ]Huazhong University of Science & Technology, College of Optoelectronics & Information laboratory for High Intensity Optics, Shanghai Institute of Optics and Fine Mechanics, CAS, P. O. Box 800-211, Shanghai 201800 laboratory of Optical Physics, Institute of Physics, CAS, Beijing 100080, China department of Applied Science, University of California-Davis, Davis,California 95616 *Department of Physics, Southern University and A&M College, Baton Rouge, Louisiana 70813
Abstract. We report an unknown mechanism for high harmonic generation (HHG) in contrast to the well-known recombination mechanism. In this mechanism, the harmonics are generated when the photoelectrons exit the laser field. Our calculations show high harmonics in both odd and even orders also exhibit plateau and cut-off features similar to those with only odd orders. Comparison is made with spectra of harmonics produced from plasma and shows a good agreement. We reach a unified picture for above-threshold ionization (ATI) companied by HHG: there are two coexisting mechanisms to produce high harmonics. In the recombination mechanism, photoelectrons fall back to the original atomic bound state with an emission of odd-only harmonics. In the escape mechanism, photoelectrons escape from the laser field with an emission of both even and odd harmonics.
