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International Conference Laser Optics (ICLO)

High optical harmonics polarization state due to incident field spatial inhomogeneity

A. V. Andreev, Sergey Yu Stremoukhov, Olga A. Shoutova

Published Jun 1, 2016• 0 citations
Publisher

Abstract

High optical harmonics generation process in atomic gases interacting with bichromatic laser field in non-collinear scheme is studied with the aim of polarization state of atomic response investigation in dependence on incident field spatial properties. It is shown that the non-dipole transitions (the impact of which increases with laser field intensity) play the crucial role in discussed process.

DOI
10.1109/lo.2016.7549790

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Journal of Physics B Atomic Molecular and Optical Physics•Apr 28, 1995•5 citations
High optical harmonics generation process in atomic gases interacting with bichromatic laser field in non-collinear scheme is studied with the aim of polarization state of atomic response investigation in dependence on incident field spatial properties. It is shown that the non-dipole transitions (the impact of which increases with laser field intensity) play the crucial role in discussed process.
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.
The theory of the interaction of a centrosymmetric atom with a superstrong spatially inhomogeneous laser field is developed. This theory employs the two-level approximation to describe the dynamics of spatially nonlocal interactions related to variations in the populations of the levels. We propose a model that includes spatially nonlocal interactions, such as magnetic-dipole and quadrupole interactions and interactions due to the gradient of the ponderomotive potential of the field. We consider the interaction of a homogeneous medium of centrosymmetric atoms with a superstrong laser field, which is represented as a superposition of two plane-wave ultrashort pulses propagating at some angle with respect to each other. Perturbation theory, which is valid for the fields of moderate intensity, is developed for the atomic response. The results of numerical simulations are compared with the predictions of this perturbation theory. The specific features of the nonlinear-optical response of an atom in a superstrong field are investigated. The angular distribution of the second- and third-harmonic emission is calculated in the constant-field approximation for different polarisations of the incident field.
The dynamics of high-intense laser fields at coherent Raman self-scattering are investigated by the use of numerical simulation. The processes under investigation take place at the Raman scattering on electronic states in atoms with transition to be forbidden in electro-dipole approximation, on vibrational or rotational states in molecules, on states connected with the spin flip and other. It is found that the threshold intensities exist provided that the pulse duration is smaller than dephasing time of the Raman transition in the medium. Under this, the following processes occur: an effective generation of the high-order harmonics comes up; the plateau forms in the harmonic spectrum. The maximum which arises on this plateau shifts to the higher-harmonic numbers in case of the higher threshold intensities of the pulse. It is shown how the transformation of the initial pulse intensity into high-order harmonics intensity can be considerably increased with the different initial parameters of the pulse and the medium.
We study the generation of higher harmonics using intense light field in an interacting bosonic gas loaded in a one-dimensional optical lattice. We find that the strong light pulse can generate reasonably high harmonics in the insulating regime, while the superfluid regime exhibits only a few harmonics. In the insulating regime, the number of harmonics increases with the variation in the strength of the light field. This originates primarily due to the field-driven resonant and non-resonant excitations in the neutral Mott state and their recombination with the ground state. If the repulsive interaction between two atoms (UUU) is close to the strength of the light field (A0A_0A0​), the resonant quasiparticle-quasihole pairs on nearest-neighbor sites, namely dipole states are found to play a dominant role in the generating higher harmonics. However, in the strong-field limit A0≫UA_0\gg UA0​≫U, the nonresonant states where quasiparticle-quasihole pairs are not on nearest-neighbor sites give rise to higher harmonics. We conclude with a possible experimental outlook of the obtained results.
Recently we have demonstrated the quantum nature of light in strongly laser driven atoms, and we have shown how the process of high harmonic generation can be used for the creation of highly non-classical light states, in particular superpositions of two coherent states, i.e., optical Schr\{o}dinger states [M. Lewenstein et al., Generation of optical Schr\odinger cat states in intense laser-matter interactions, Nat. Phys., 17 1104-1108 (2021)]. Here, we investigate the quantum optical description of the interaction, incorporating many atoms and the back-action of the high harmonic generation or above threshold ionization processes on the coherent state of the driving laser field. We show how the conditioning on high harmonic generation and above threshold ionization can lead to non-classical light states, and we discuss the key parameters that can be used for controlling and characterizing the non-classical states obtained after high harmonic generation. The theoretical results have been experimentally confirmed by demonstrating how the obtained coherent state superposition changes from an optical to kitten state by changing the number of atoms participating in the high harmonic generation process. Additionally, we experimentally verify the applicability of the approach for generating high photon number non-classical light states. This is shown by demonstrating the generation of a 9--photon optical state which, to our knowledge, is the highest photon number optical state experimentally reported. Our findings anticipate the development of new methods that naturally lead to the creation of high photon number controllable coherent state superpositions, advancing investigations in quantum technology.
An interaction of an atomic/molecular medium with two strong electric fields E L (t) and E M (t) is investigated. The frequency omega L of the first (laser) field E L (t) is close to a transition frequency omega 21 between an upper (2) and a lower (1) atomic levels while the second field E M (t) is of a much lower frequency omega M << omega 21 . The medium is a dipole gas characterized by a nonzero averaged value of electric dipole moment in the states 1 and/or 2. As analytical solution of equations for the density matrix of a dipole atom/molecule interacting with the fields E L (t) and E M (t) is found using the second approximation of the averaging method by Krylov-Bogoliubov-Mitropolskii. The solution obtained is valid in a much broader range of field strengths than an analogous solution that could have been found by the usual time-dependent perturbation theory. Using the solution obtained for the density matrix, polarization of a dipole gas interacting with the fields E L (t) and E M (t) is analytically calculated. This analytical expression for polarization of such media can be utilized for study of various nonlinear optical phenomena. One of the phenomena is a generation of waves at frequencies omega L +or-p omega M and (p+1) omega M (where p=1,2,3,...) in a dipole gas. Another phenomenon is an amplification of the incident wave of the lower frequency omega M due to a stimulated scattering in a dipole gas.