Coherent control and giant enhancement of multiphoton ionization and high-order-harmonic generation driven by intense frequency-comb laser fields: Anab initiotheoretical investigation

Abstract

We present an ab initio theoretical investigation of the coherent control and significant enhancement of multiphoton ionization (MPI) and high-order-harmonic generation (HHG) of atoms and molecules by means of intense frequency-comb laser fields. We show that the nonperiodically or quasiperiodically time-dependent Schr\odinger equation for the frequency-comb laser excitation problem can be transformed exactly into a time-independent non-Hermitian generalized Floquet matrix eigenvalue problem by means of the many-mode Floquet theory and the complex-scaling transformation. The generalized Floquet Hamiltonian can be optimally discretized and the complex quasienergy eigenvalues and eigenfunctions can be solved accurately and efficiently by means of the generalized pseudospectral method. The procedure is applied to a case study of the resonance-enhanced MPI and HHG of atomic hydrogen driven by an intense frequency-comb laser field. Our study shows that both the MPI and HHG rates can be coherently controlled by tuning the laser parameters such as the pulse-to-pulse carrier-envelope-phase (CEP) shift. In particular, both the MPI and HHG rates exhibit dramatic enhancement by tuning the CEP shift, due to the phenomenon of multiphoton resonances.