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New Journal of Physics

Kramers–Henneberger atoms in flattened Gaussian beams

Ioksang Leung, Mingyu Zhu, P. X. Wang, Qi Wei

Published Feb 1, 2025• 1 citation
Publisher

Abstract

Abstract The dichotomy structure is one of the prominent features of Kramers–Henneberger (KH) atoms. Within a focused laser field, the wave function of KH atoms becomes polarized due to ponderomotive (PM) forces induced by light intensity gradients, leading to the disruption of the dichotomy structure. In a Gaussian-focused laser field, the PM forces are negligible only along the optical axis. To maintain the dichotomy structure of KH atoms over a broader area, we compared the result of flattened Gaussian laser beams with Gaussian-focused laser beams. By calculating the wave functions of ground KH state hydrogen atoms at various radial and angular locations in the laser focusing plane, it was demonstrated that under suitable laser parameters, the dichotomy structure can be fully or partially preserved in flattened Gaussian laser beams. Through the dichotomy structure of KH atoms, we can verify the existence of KH atoms using interference fringes of multi-photon ionization signals.

DOI
10.1088/1367-2630/adb2ba

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Abstract The dichotomy structure is one of the prominent features of Kramers–Henneberger (KH) atoms. Within a focused laser field, the wave function of KH atoms becomes polarized due to ponderomotive (PM) forces induced by light intensity gradients, leading to the disruption of the dichotomy structure. In a Gaussian-focused laser field, the PM forces are negligible only along the optical axis. To maintain the dichotomy structure of KH atoms over a broader area, we compared the result of flattened Gaussian laser beams with Gaussian-focused laser beams. By calculating the wave functions of ground KH state hydrogen atoms at various radial and angular locations in the laser focusing plane, it was demonstrated that under suitable laser parameters, the dichotomy structure can be fully or partially preserved in flattened Gaussian laser beams. Through the dichotomy structure of KH atoms, we can verify the existence of KH atoms using interference fringes of multi-photon ionization signals.
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.
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.
The principal element in any sort of optics is a lens. It is essential to create laser field configurations capable of focusing neutral atomic beams. There are two possibilities for focusing an atomic beam by laser light: by using the gradient force or by using the light pressure force. The force effecting the focusing of an atom in a beam should be proportional to the atomic displacement. This criterion must be satisfied for the force to produce the true “image”: if this criteria is not met, the image is blurred, i.e. there are aberrations. An approach to treat the focusing of atomic beam by laser light is to use the path integral technique to calculate the evolution of atomic wave function as it propagates in laser field. A particle-optical approach also can be used for treatment of focusing of atoms by laser light. The quadratic radial dependence of the potential is a necessary condition for focusing of atoms.
Abstract Multi-beam optical tweezers can be very effective and suitable for creating optical lattices and dexterous manipulation of the microscopic world. In an earlier work, a symmetrical multi-beam optical tweezers setup was proposed and theoretically investigated. In that study, however, we assumed that the incident beams were plane waves, and a plane wave cannot exist in real situations. The main aim of this paper is to investigate multi-beam optical tweezers under the realistic condition of using the Gaussian beams. The effects of changing the polarization state and the number of incident beams on potential landscapes are investigated. A comparison between the Gaussian beam and plane wave lattices is also performed. The results show that in the case of p-polarization, a very small spot (a sub- λ spot) can be achieved by increasing the number of incident beams. However, for s-polarized incident beams instead of a bright center, a dark center intensity distribution is obtained. The lattices constructed by the interference of the Gaussian beams have deeper traps in the central region compared to the plane wave lattices.
We report a study on light force on a beam of neutral two-level atoms superimposed upon a few-cycle-pulse Gaussian laser field under both resonant and off-resonant conditions. The phenomena of focusing, defocusing, and steering of the neutral atoms in the laser field are analyzed by solving the optical Bloch equation beyond the rotating-wave approximation and the force equation self-consistently. We find that two-level atoms in an atomic beam can be focused and defocused for large, positively and negatively detuned interactions even in the regime of extreme nonlinear optics. The so-called optical potential may be used for stable trapping of the neutral two-level atoms for large positively detuned interaction. This work successfully reproduces some of the features reported in recent experimental and theoretical works.