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Scientific Reports

Manipulation of photoassociation of ultracold Cs atoms with tunable scattering length by external magnetic fields

Guosheng Feng, Yuqing Li, Xiaofeng Wang, Jizhou Wu, V. B. Sovkov, Jie Ma, Liantuan Xiao, Suotang Jia

Published Oct 16, 2017• 10 citations
Publisher

Abstract

We demonstrate that for ultracold, optically trapped Cs atoms the photoassociation (PA) can be manipulated by using external uniform magnetic fields due to the alteration of the scattering wavefunction in the region of the free-bound optical transition. We present PA-induced atom loss measurements with the same intensity for PA laser but different external magnetic fields, and analyze main contributions of the PA to the variation of the number of atoms in the trap. The PA rate exhibits a strong dependence on the changing uniform magnetic field. The experimental data are simulated within the model of a single-channel one-well rectangular potential, whose depth is adjusted so as to assure the predicted variation of the scattering length with the magnetic field. The computational and experimental results are in a reasonable agreement to each other. The same model is used to illustrate some general properties of the two-body quantum system in the near-threshold state.

DOI
10.1038/s41598-017-13534-6

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We demonstrate that for ultracold, optically trapped Cs atoms the photoassociation (PA) can be manipulated by using external uniform magnetic fields due to the alteration of the scattering wavefunction in the region of the free-bound optical transition. We present PA-induced atom loss measurements with the same intensity for PA laser but different external magnetic fields, and analyze main contributions of the PA to the variation of the number of atoms in the trap. The PA rate exhibits a strong dependence on the changing uniform magnetic field. The experimental data are simulated within the model of a single-channel one-well rectangular potential, whose depth is adjusted so as to assure the predicted variation of the scattering length with the magnetic field. The computational and experimental results are in a reasonable agreement to each other. The same model is used to illustrate some general properties of the two-body quantum system in the near-threshold state.
We study the effects of magnetic Feshbach resonance on the shifts in photoassociation (PA) spectra of ultracold Cs atoms. A series of atom loss spectra show a linear variation of the frequency shift with the PA laser intensity at different magnetic fields near the d-wave Feshbach resonance of optically trapped Cs atoms. The magnetic field-dependence of the slope of the shift on the PA laser intensity exhibits a dispersive change near the Feshbach resonance. The theoretical formula derived from a model based on Fano resonance fits well with the experimental data. Using a model rectangular potential with tunable well depth and applying the Franck-Condon principle, we obtain numerical results, which are found to be largely in disagreement with the experimental findings.
We investigate an effective control of the laser-induced frequency shift of ultracold cesium molecules formed by photoassociation (PA) in a magnetically levitated crossed dipole trap via an external magnetic field. A series of molecular PA spectra have been measured with increasing the PA laser intensity at different bias fields. We find that the slope of laser-induced frequency shift is strongly dependent on the external magnetic field: the slope can be increased up to a maximum of ∼6 times compared with a slope without an external magnetic field. A qualitatively theoretical explanation has been offered by considering the external magnetic field that can be used to manipulate the interaction between ultracold atoms.
It is crucial to control the collision between ultracold atoms by applying external fields. We developed a theoretical model for investigating the s-wave scattering of ultracold atoms controlled by the magnetic field and laser field. The calculation is performed by using the close-coupling method and mapped Fourier grid method. Due to the interference between the photoassociation and bound-to-bound transitions, the bound state in the continuum, which is a resonance with a vanishing width, occurs at the magnetic field position near the magnetic Feshbach resonance. The widths of resonances in the neighborhood of the bound state in the continuum are narrow. Changing the laser intensity can shift the magnetic field position where the bound state in the continuum occurs through modifying the ground molecular state to induce wide resonances at desired magnetic field positions. By increasing the resonance width, the tunability of the real part of the scattering length at resonances can be significantly improved. Changing the laser intensity can also adjust the coupling between the ground and excited molecular states. When the coupling between the ground and excited molecular states approaches zero, a resonance is induced, and the photoassociation and bound-to-bound transitions are both significantly suppressed at this resonance. Therefore, the atomic loss peak due to spontaneous emission does not appear at this resonance. The magnetic field position of this resonance is stable against the change in laser frequency.
We report a new method to detect photoassociation (PA) resonance signals through the collective dipole oscillations produced by a momentum transfer from the PA excitation lasers to an ultracold atomic cloud confined by a magnetic trap. Using the amplitudes of these oscillations for several molecular excited states, we derive a precise value for the s -wave scattering length between the ultracold atoms. This new and simple method provides results in excellent agreement with previous spectroscopic PA measurements based on atom losses or temperature rise. We present a simple model interpreting the momentum and energy transferred from the PA process to trap confined ultracold atoms.
We demonstrate nonlinear Fano effect in ultracold atom-molecule system composed of Photoassociation (PA) near a narrow d-wave Feshbach resonance for Cs atoms in the hyperfine state F = 3, mF = 3. A series of PA spectra of ultracold Cs atoms trapped in a crossed dipole trap are recorded near the Feshbach resonance. We measure PA rate as a function of magnetic field and clearly find Fano effect with characteristically asymmetric line shapes. Meanwhile, we investigate variations of spectral broadening and shifts as magnetic field around the Feshbach resonance. Our results also show Fano effect has a great effect on both spectral broadening and slope of spectral shift. We develop a nonlinear Fano theory based on magnato-optical quantum interferences in this ultracold atom-molecule system. The theory provides a remarkable agreement with our experimental results.
We have calculated the time-dependent dynamics of two ultracold Na atoms in an atom trap where a time-dependent magnetic field B(t)B(t)B(t) moves a Feshbach resonance state across the energy threshold for a binary collision. Our coupled-channel scattering calculations, which reproduce the observed properties of such resonances in sodium atom collisions, can be reduced to an effective two-channel configuration-interaction model for one bound state and one continuum. The model is adapted to describe the time-dependent dynamics induced by B(t)B(t)B(t) for two atoms trapped either in a strongly confining single well of an optical lattice or in an optical potential in the presence of a Bose-Einstein condensate. We show that a simple Landau-Zener curve crossing model gives quantitative agreement with exact calculations of field-induced transition rates. If B(t)B(t)B(t) sweeps the resonance across threshold from above, two atoms in the ground state of the trap potential can be efficiently converted to translationally cold dimer molecules. If the resonance is swept from below, the atoms can be removed from the ground state and placed in hot vibrational levels of the trap. Our calculations reproduce the rapid atom loss rates observed in a Na Bose-Einstein condensate due to sweeping a Feshbach resonance state through the binary collision threshold.