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Laser Physics Letters

Discrete exited states influence on the harmonics spectrum generated in two-colour laser fields

Sergey Yu Stremoukhov, A V Andreev

Published Dec 1, 2019• 4 citations
Publisher

Abstract

Abstract The contribution of excited discrete atomic levels to the generation of high- and low-order harmonics in the interaction of a single atom with a two-color laser field has been studied for the first time. The studies were carried out for two types of atoms with different symmetry of the ground state wave functions (Ag, Ne) and for a two-color laser field consisting of the linearly polarized fundamental and second harmonics of Ti:Sa laser ( ω + 2 ω ). The angle between the polarization of the ω + 2 ω laser field components is varied. A significant effect of the symmetry of populated excited levels on the polarization characteristics of the generated harmonics is shown. A new method of control over the polarization state of the generated radiation is proposed.

DOI
10.1088/1612-202x/ab5ae8

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Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)•Jan 20, 2003•0 citations
Abstract The contribution of excited discrete atomic levels to the generation of high- and low-order harmonics in the interaction of a single atom with a two-color laser field has been studied for the first time. The studies were carried out for two types of atoms with different symmetry of the ground state wave functions (Ag, Ne) and for a two-color laser field consisting of the linearly polarized fundamental and second harmonics of Ti:Sa laser ( ω + 2 ω ). The angle between the polarization of the ω + 2 ω laser field components is varied. A significant effect of the symmetry of populated excited levels on the polarization characteristics of the generated harmonics is shown. A new method of control over the polarization state of the generated radiation is proposed.
Abstract Here we discuss the new effects in the light-atom interaction of an atom and two-colour ( ω , 2 ω ) elliptically polarized femtosecond pump beams. We show that the spectrum of atomic response does not include the signal at summary frequency ( ω + 2 ω ) and its harmonics when the frequency components of the pumping field are counter-rotating and the atom is in s -ground state. In the case when the ground state of the atom is p -state ( l = 1), these components vanish in the response spectrum of the longitudinal atomic current. In both cases, the suppression of the summary frequency is accompanied by the suppression of signal at THz frequency.
Abstract We investigate features of electron emission from atoms in the field of the fundamental and second harmonic of a laser in conditions relevant to those realized in free-electron lasers. General expressions for the angular anisotropy and photoelectron spin polarization are obtained. These expressions are used for describing of the ionization of the valence shell of inert gas atoms and illustrated by calculations for neon. The possibility of control over the electron emission characteristics by varying the relative phase between harmonic (i.e., realization of one of the forms of the coherent control) is considered.
We present the most prominent features in two-colour coherent control of high-order harmonic generation using a strongly driven two-level atom model, which are known to generate high harmonics as well as hyper-Raman lines with a plateau and a cut-off. Furthermore, for one laser field case, we find, when the hyper-Raman lines are located at the position of the high harmonics, that the high harmonics are strongly enhanced due to the interference effect between the high harmonics and the hyper-Raman lines. Within these conditions, by combining the laser field with another much weaker laser field, we show plentiful and strong high harmonics in the light spectrum of the driven atomic system.
A theory of the nonlinear optical response of an atom interacting with a superposition of arbitrarily polarized fields is developed. The theory is based on the analytical solution of the boundary-value problem for an electron moving in a spherically symmetric intraatomic field and in the field of an external electromagnetic field. By means of the example of an argon atom interacting with a bichromatic field formed by the first and second harmonics of a Ti:sapphire laser, it is shown that, when an atom interacts with the field of two polarized pulses the polarization directions of which are not collinear, the response spectrum significantly depends on the laser radiation parameters—the duration and intensity of pulses, the time of delay between them, and the angle between the directions of polarization vectors. Generation of THz radiation is shown to be possible in the ionization-free regime due to intraatomic nonlinearity.
We have studied the excitation spectrum arising from the interaction of a two-level atom with two strong electromagnetic fields whose initially populated modes ω a and ω b are near resonance with the atomic transition frequency In the limit of high photon densities the Green's function of the system describing the electronic transition in question has been derived in a closed form and includes processes arising from the interaction of seven photon modes of both laser fields. The spectral function of the system is calculated when the frequency ω a of the one laser field is in resonance with the atomic transition frequency while the frequency ω b of the other field is varied. The excitation spectrum is found to consist, apart from the central peak at the excitation frequency ω a , of several lorentzian lines which are peaked in the neighbourhood of the frequencies ω a ± Δ, ω a ± 2Δ, ω a ± 3Δ and ω a ± 4Δ, where Δ = ω a - ω b and describe the type of optical mixing of light modes which is of the third, fifth, seventh and ninth order, respectively. Additional peaks appear in the spectra, which result from the interaction between the frequency modes in question. The probability amplitudes describing the relative intensities of the peaks in question are expressed as functions of the parameters η a = Ω a /Δ and η b = Ω b /Δ, where Ω a and Ω b are the Rabi Frequencies of the two laser fields, respectively. It is found that when η a ⩾ 1 and η b ⩾ 1 the expression for the relative intensities of the bands in question are remarkably enhanced and take extremely large positive or negative values. Results of numerical calculations for a wide range of values of η a and η b when η a = η b ⩾ 1 are presented graphically and the complete excitation spectrum is discussed in detail. The method may be useful to investigate the behaviour of excited states as well as those close to the limit of ionization.
The coherent control of ionization using two-color field has received considerable attention in recent years. In the case of two color laser field, because of additional degrees of freedom such as relative phase, intensities and relative polarizations, etc., the ionization of the system can be influenced significantly. In most papers, they investigated the ionisation behavior of system by using the combinations of the fundamental laser field (/spl omega/) and its second (2/spl omega/) or third (3/spl omega/) harmonic field. To our knowledge, a comparison between /spl omega/-2/spl omega/ and /spl omega/-3/spl omega/ is seldom dealt with. In this paper, we choose the combination of the fundamental laser field (/spl omega/) with its second (2/spl omega/) or third (3/spl omega/) harmonic field, and investigate the ionization of the system for the two kinds of combinations.