We have developed a sensitive spectroscopic technique for study of a dilute ultracold plasma (UCP) using a laser induced autoionization of Rydberg atoms. In our experiment the ultracold ⁴⁰Ca Rydberg atoms and ions are prepared in a magneto-optical trap by several cw lasers. The laser beam diameters are order of 2×10^-3 m. The technique allows to detect the plasma with ion and electron densities below 10⁹ m^-3. For observation of the autoionization effect we used the two-photon Rydberg transition 4s3d ¹D₂ – 90 ¹D₂ (with lasers 672 nm and 798 nm) and the ionization two-photon channel with lasers 423 nm and 390 nm. The autoionization resonance is observed as a variation of the resonance fluorescence of the ⁴⁰Ca ions at a wavelength of 397 nm. The dependence of the autoionization resonance magnitude on the ion density is recorded. The ability to create an UCP with well-controlled parameters allows us to calibrate of the autoionization resonances. The technique can be applied to detect small electric fields by means of ⁴⁰Ca Rydberg atoms. The developed technique can be useful for the measurements of the small fields in development of the ultra-precise atomic clock, as well as for experimental simulations of the ultracold low-density plasma in the Earth's ionosphere.

Dependence of the resonance amplitude at the 4s3d ¹D₂ – 90 ¹D₂ Rydberg transition on the power P₃₉₀ of the ionizing laser (λ = 390 nm) and the ion density in the UCP. The peak density of the neutral atoms is $n_a = 10^{15}$m^-3.

B.B. Zelener, E.V. Vilshanskaya, S.A. Saakyan, V.A. Sautenkov, B.V. Zelener, V.E. Fortov
JETP Letters 113, Issue 2 (2021).