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Journal of Experimental and Theoretical Physics Letters

Nonlinear polarization response of an atomic gas medium in the field of a high-intensity femtosecond laser pulse

Ekaterina A. Volkova, A. M. Popov, Olga V. Tikhonova

Published Dec 1, 2011• 34 citations
Publisher

Abstract

Polarization response that appears in silver vapors in the field of a high-intensity femtosecond Ti:sapphire laser has been studied by the direct numerical integration of the time-dependent Schrödinger equation. The regions of applicability have been determined for perturbation theory and the power series expansion of the polarization in the field. The contribution of free electrons to the response at the frequency of the interacting field has been calculated, which is due to the photoionization process and limits the Kerr effect. An important contribution of laser-excited atomic states to nonlinear atomic responses of neutral atoms has been demonstrated.

DOI
10.1134/s0021364011190180

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Polarization response that appears in silver vapors in the field of a high-intensity femtosecond Ti:sapphire laser has been studied by the direct numerical integration of the time-dependent Schrödinger equation. The regions of applicability have been determined for perturbation theory and the power series expansion of the polarization in the field. The contribution of free electrons to the response at the frequency of the interacting field has been calculated, which is due to the photoionization process and limits the Kerr effect. An important contribution of laser-excited atomic states to nonlinear atomic responses of neutral atoms has been demonstrated.
The nonlinear polarization response of a quantum system modeling a silver atom in the field of high-intensity radiation in the IR and UV spectral ranges has been studied by direct numerical integration of a nonstationary Schrödinger equation. The domains of applicability of perturbation theory and polarization expansion in powers of the field intensity are determined. The contribution of excited atoms and electrons in a continuum to the atomic polarization response at the field frequency, which arises due to the radiation-induced excitation and photoionization processes, is analyzed. Features of the nonlinear response to an external field under conditions of atom stabilization are considered.
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 investigated the nonlinear susceptibility of atoms induced by high intensity, ultrashort laser pulses using a numerical solution of the time-dependent Schrödinger equation. We found that the instantaneous nonlinear susceptibility becomes saturated at high intensity. We also found that the saturation is closely linked to depletion of the ground state. Based on the numerical results, a simple model that generalizes the nonlinear susceptibility of atoms for high intensity, ultrashort laser pulses is proposed. We also investigated the ionization-induced dipole moment and found that the amplitude of the dipole moment induced by an ionized electron is, in general, smaller than that induced by a free-electron, and is attributable to a residual interaction between the ionized-electron and its parent ion.
It is commonly assumed that for low-intensity short optical pulses far from resonance, the third-order optical nonlinear response is instantaneous. We solve the three-dimensional time-dependent Schrödinger equation for the hydrogen atom and show that this is not the case: the polarization is not simply proportional to the cube of the electric field even at low intensities. We analyze the fundamental-frequency and third-harmonic nonlinear susceptibilities of hydrogen, investigate their dependence on intensity, and find that the delays in the Kerr response rapidly approach the femtosecond time-scale at higher intensities, while the delays in the third harmonic generation remain much lower. We also propose an experimental scheme to detect and characterize the above effects.
The dynamics and quantum correlations are studied in a system of two interacting atoms in a resonance femtosecond laser field. The polarization response of a medium consisting of such pairs of interacting atoms is analyzed. Regimes are found in which the polarization response of the medium under study contains the intense component at a tunable low frequency. It is shown that the dipole response of the medium can be suppressed using the entanglement effect and symmetry properties of collective atomic states.
The nonlinear optical response of a dilute atomic gas to ultrashort high-intensity midinfrared pulse excitation is calculated fully microscopically. The optically induced polarization dynamics is evaluated for the interacting many-electron system in a gas of hydrogen atoms. It is shown that the many-body effects during the excitation distinctly influence not only the atomic ionization dynamics, but also the nonlinear polarization response in the midinfrared regime. The delicate balance between the Kerr focusing and the ionization-induced defocusing is dramatically modified and a significant decrease of the nonlinear refractive index is predicted for increasing wavelength of the exciting pulse.