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Results in Physics

Self-deformation of Kramers–Henneberger atoms in focused laser beams of circular polarization

Jun-Dong Xiang, Pingxiao Wang, Jiaxiang Wang, Qi Wei

Published Jan 24, 2024• 0 citations

Abstract

In intense field laser dressing of atoms, electrons follow the laser oscillation and undergo quiver motion just like free electrons but still weakly bound to the atomic core, thus forming a set of specific dressed states named Kramers-Henneberger (KH) states. For decades, it was widely believed that ground KH state hydrogen atoms in a circularly polarized laser field exhibit the symmetric structure of a perfect ring. But our study shows this is not a common structure because of the fact that KH atoms can only exist in intense laser fields which are practically focused laser beams. In focused laser field, KH state electrons have to experience the ponderomotive force, which might polarize the atom and reshape its structure. Only in a laser beam with small quiver amplitude and high frequency, can the symmetric circular structure be fully or partially preserved.

DOI: 10.1016/j.rinp.2024.107391

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In intense field laser dressing of atoms, electrons follow the laser oscillation and undergo quiver motion just like free electrons but still weakly bound to the atomic core, thus forming a set of specific dressed states named Kramers-Henneberger (KH) states. For decades, it was widely believed that ground KH state hydrogen atoms in a circularly polarized laser field exhibit the symmetric structure of a perfect ring. But our study shows this is not a common structure because of the fact that KH atoms can only exist in intense laser fields which are practically focused laser beams. In focused laser field, KH state electrons have to experience the ponderomotive force, which might polarize the atom and reshape its structure. Only in a laser beam with small quiver amplitude and high frequency, can the symmetric circular structure be fully or partially preserved.

The Electric Stark effect of a Kramers-Henneberger (KH) state of hydrogen atoms in both linearly and circularly polarized laser fields is studied. For the ground KH state of H atoms with a small quiver amplitude, the quadratic Stark effect is observed. For a large quiver amplitude, the Stark effect is quadratic only in a weak electric field and quickly changes to linear as the electric field increases. The atomic structure of the KH state is very sensitive to the electric field and can be easily polarized. The huge polarizability and induced dipole moment are comparable to those of Rydberg atoms.

We present a generalized model for the intensity dependent response of atoms in strong IR laser fields, describing deviations in the nonlinear response at the frequency of the driving field from the standard model (linear Kerr effect + plasma defocusing) based on the formation of the stable KH states. According to our numerical simulations, shaping the driving laser pulse allows one to reveal a prominent resonance structure in the Kerr response of an individual atom, which can only be explained by population transfer into excited KH states.

We discuss the connection between strong-field ionization, saturation of the Kerr response and the formation of the Kramers–Henneberger (KH) atom and long-living excitations in intense infrared (IR) external fields. We present a generalized model for the intensity-dependent response of atoms in strong IR laser fields, describing deviations in the nonlinear response at the frequency of the driving field from the standard model. We show that shaping the driving laser pulse allows one to reveal signatures of the excited KH states in the Kerr response of an individual atom.

A theoretical study is made on the steady-state self-focusing and self-defocusing of a circularly-polarized laser beam, occurring in an atomic vapor mixed with a buffer gas at a relatively high pressure. The presence of the buffer gas removes the theoretical difficulties arising from hole-burning, the transit time, and hyperfine pumping. It also localizes the optically-pumped atoms in the vicinity of the laser beam, so that self-focusing occurs even at extremely weak light intensities. A preliminary experiment is also descirbed, the results of which show that optical-pumping self-focusing can be observed at laser intensities down to 30 µW.

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