Tunneling and multiphoton ionization of atoms in two-color linear and circular laser fields and possibility of coherent polarization control of terahertz waves

Abstract

The tunneling and multiphoton ionization of atoms in an intense two-color linearly and circularly polarized laser fields are discussed in the Keldysh theory framework. We use the “imaginary-time” method, where tunneling of the photoelectron is described by the classical equations of motion but with purely imaginary “time.” Together with using of the saddle-point method, this allows to obtain the dependence of the total ionization rate and the net photoelectron current, generated due to the interaction of an intense two-color laser field with an atom, on the ratio of the second and fundamental harmonic amplitudes, their relative phase and an angle between harmonics. Application of the “imaginary-time” method also allows us to specify the parameters maximizing the net photocurrent and to determine the Coulomb correction to the ionization rate. We investigate the properties of polarization and spectral intensity of terahertz (THz) radiation and also the possibility of the coherent control of THz waves polarization in two-color scheme, through the relative phase between harmonics. We theoretically demonstrate that the amplification of THz radiation in the case of parallel co-rotating circular laser pulses is greater than for a combination of circularly and linearly polarized harmonics and provides the most promising conditions for increasing the efficiency of THz emission and coherent control of the THz beam polarization.