2-COLOR PHASE-CONTROL OF HIGH-ORDER HARMONIC-GENERATION IN INTENSE LASER FIELDS

We present a time-independent generalized Floquet approach for nonpert urbative treatment of high-order harmonic generation (HG) in intense o ne- and two-color laser fields. The procedure consists of the followin g elements: (i) determination of the complex quasienergy eigenvalue an d eigenfunction by means of the non-Hermitian Floquet formalism, where in the Floquet Hamiltonian is discretized by the complex-scaling gener alized pseudospectral technique [Wang, Chu, and Laughlin, Phys. Rev. A 50, 3208 (1994)], and (ii) calculation of the HG rates based on the a pproach that implies the classical treatment of the electromagnetic fi eld and quantal treatment of the atom. The method is applied to the no nperturbative study of HG by the hydrogen atom in strong laser fields with the fundamental frequencies 532 and 775 nm and their third harmon ics. The results show a strong dependence on the relative phase delta between the fundamental frequency field and its harmonic. For the inte nsities used in calculations (1x10(13) and 5x10(13) W/cm(2) for the fu ndamental frequency 532 nm and 1x10(13) and 3x10(13) W/cm(2) for the f undamental frequency 775 nm, the harmonic intensity being 10 and 100 t imes weaker), the total photon emission rate has its maximum at delta= 0 and minimum at delta=pi. However, this tendency, while valid for the first several HG peaks, is reversed for the higher HG peaks. The HG s pectrum for delta=pi is broader and the peak heights decrease more slo wly compared to the case of delta=0. These results have their analog i n the multiphoton above-threshold detachment study performed recently for H- ions [Telnov, Wang, and Chu, Phys. Rev. A 51, 4797 (1995)].