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Journal of Experimental and Theoretical Physics Letters

Diagnostics of a Diluted Ultracold Plasma Using the Autoionization Effect of Rydberg States of 40Ca Atoms

Boris B. Zelener, E V Vilshanskaya, S. A. Saakyan, Vladimir A. Sautenkov, B. V. Zelener, Vladimir Evgenievich Fortov

Published Jan 1, 2021• 7 citations
Publisher

Abstract

A sensitive method to study a dilute ultracold plasma based on the autoionization resonances of Rydberg atoms is demonstrated. Ultracold 40Ca Rydberg atoms are prepared in a continuously operating magneto-optical trap. A method that allows the detection of the plasma with ion and electron densities below 109 m–3 is shown. Experimental studies of resonances at the two-photon $$4s3d{{\,}^{1}}{{D}_{2}}{-} 90{{\,}^{1}}{{D}_{2}}$$ Rydberg transition have been carried out using 672 and 798 nm resonant laser beams. The autoionization resonance is observed during fluorescence of single-charged 40Ca ions at a wavelength of 397 nm. The dependence of the amplitude of autoionization resonance on the density of ions in the continuously generated ultracold plasma is obtained.

DOI
10.1134/s0021364021020107

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A sensitive method to study a dilute ultracold plasma based on the autoionization resonances of Rydberg atoms is demonstrated. Ultracold 40Ca Rydberg atoms are prepared in a continuously operating magneto-optical trap. A method that allows the detection of the plasma with ion and electron densities below 109 m–3 is shown. Experimental studies of resonances at the two-photon 4s3d 1D2−90 1D24s3d{{\,}^{1}}{{D}_{2}}{-} 90{{\,}^{1}}{{D}_{2}}4s3d1D2​−901D2​ Rydberg transition have been carried out using 672 and 798 nm resonant laser beams. The autoionization resonance is observed during fluorescence of single-charged 40Ca ions at a wavelength of 397 nm. The dependence of the amplitude of autoionization resonance on the density of ions in the continuously generated ultracold plasma is obtained.
Signals of ultracold plasma are observed by two-photon ionization of laser-cooled caesium atoms in a magneto-optical trap. Recombination of ions and electrons into Rydberg atoms during the expansion of ultracold plasma is investigated by using state-selective field ionization spectroscopy. The dependences of recombination on initial electron temperature (1–70 K) and initial ion density (∼10 10 cm −3 ) are investigated. The measured dependence on initial ion density is N 1.547+0.004 at a delay time of 5 μs. The recombination rate rapidly declines as initial electron temperature increases when delay time is increased. The distributions of Rydberg atoms on different values of principal quantum number n, i.e. n = 30–60, at an initial electron temperature of 3.3 K are also investigated. The main experimental results are approximately explained by the three-body recombination theory.
In this work, we discuss two-photon excitation and diagnostic of ultracold Rydberg atoms in a magneto-optical trap. Lithium atoms were excited by using ultraviolet cw laser. For identification of Rydberg transitions, we recorded resonance fluorescence of ultracold atoms. Spectra of transitions 2P- n S, 2p- n D were measured. Our results are in good agreement with calculations and experimental data available in literature. Presented work is a part of our project focused on preparation and study of Rydberg matter and ultracold plasma.
Summary form only given. We report on recent experimental and theoretical investigations of highly excited atoms prepared from a dense atomic sample in a magneto-optical trap (MOT). The goals of the work are two-fold. First, by exciting atoms above the ionization potential (IP) we can study the properties of an ultracold plasma of electrons (T<1 K) and ions (T<1 mK). Second, tuning the excitation below the IP, we can populate high-lying Rydberg states whose atomic radii are comparable to the interparticle spacing, possibly spawning new collective phenomena. In our initial experiments we study Rydberg states. We collect 10/sup 7/ /sup 85/Rb atoms in the MOT at a density of 10/sup 10//cm/sup 3/. Atoms in the 5s ground state are excited into Rydberg states using single photon excitation.
In this Letter we discuss our approach that makes possible creation of the steady-state ultracold plasma having various densities and temperatures by means of continuous two-step optical excitation of calcium atoms in the magneto-optical trap. A strongly coupled ultracold plasma can be used as an excellent test platform for studying many-body interactions associated with various plasma phenomena. The parameters of the plasma are studied using laser-induced fluorescence of calcium ions. The experimental results are well described by a simple theoretical model involving equilibration of the continuous source of charged particles by the hydrodynamical ion outflux and three-body recombination. The ultracold plasma with the peak ion density of 2.7×10^{6} cm^{-3} and the minimum electron temperature near 2 K has been prepared. Our steady-state approach in combination with the strong magnetic confinement of the plasma will make it possible to reach extremely strong coupling in such system.
Photoionizing laser-cooled atoms produces ultracold neutral plasmas with initial temperatures of 1–1000 K and densities as high as 10 10 cm −3 . Applied radio frequency fields can excite plasma oscillations that are used to monitor the expansion of the unconfined plasma. Significant three-body recombination of electrons and ions into Rydberg atoms takes place during the plasma expansion. Previous experiments have been done with xenon, but a new experiment is planned with laser-cooled strontium. The strontium ion has an optically allowed transition at a convenient blue wavelength. This will allow direct imaging of the plasma through fluorescence or absorption, and may enable laser cooling and trapping of the plasma.
We present the first experiment to observe an ultracold neutral plasma. The plasma, which was created by photoionization of laser cooled atoms, has charge densities as high as 2×109 cm−3, and the temperatures of electrons and ions are as low as 100 mK and 10 μK, respectively. The plasma has a lifetime of about 100 μs, much longer than predicted by recombination rates. When the laser that excites the atoms is tuned below the ionization limit we create a sample of very highly excited cold Rydberg atoms. At our highest densities and during a time of a few microseconds, in which the Rydberg atoms are essentially stationary, the ensemble evolves towards an unbound plasma-like state.