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arXiv:1702.05039 · Journal of Physics B Atomic Molecular and Optical Physics

Probing interactions of thermal Sr Rydberg atoms using simultaneous optical and ion detection

R Hanley, Alistair D. Bounds, Paul H. Huillery, Niamh C. Keegan, Riccardo Faoro, Elizabeth M. Bridge, Kevin J. Weatherill, M. P. A. Jones

Published Apr 21, 2017• 8 citations
arXivHTMLPDF

Abstract

Abstract We demonstrate a method for probing interaction effects in a thermal beam of strontium atoms using simultaneous measurements of Rydberg EIT and spontaneously created ions or electrons. We present a Doppler-averaged optical Bloch equation model that reproduces the optical signals and allows us to connect the optical coherences and the populations. We use this to determine that the spontaneous ionization process in our system occurs due to collisions between Rydberg and ground state atoms in the EIT regime. We measure the cross section of this process to be 0.6 ± 0.2 σ geo , where σ geo is the geometrical cross section of the Rydberg atom. This result adds complementary insight to a range of recent studies of interacting thermal Rydberg ensembles.

DOI
10.1088/1361-6455/aa6e79

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Abstract We demonstrate a method for probing interaction effects in a thermal beam of strontium atoms using simultaneous measurements of Rydberg EIT and spontaneously created ions or electrons. We present a Doppler-averaged optical Bloch equation model that reproduces the optical signals and allows us to connect the optical coherences and the populations. We use this to determine that the spontaneous ionization process in our system occurs due to collisions between Rydberg and ground state atoms in the EIT regime. We measure the cross section of this process to be 0.6 ± 0.2 σ geo , where σ geo is the geometrical cross section of the Rydberg atom. This result adds complementary insight to a range of recent studies of interacting thermal Rydberg ensembles.
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We report the sudden and spontaneous evolution of an initially correlated gas of repulsively interacting Rydberg atoms to an ultracold plasma. Under continuous laser coupling we create a Rydberg ensemble in the strong blockade regime, which at longer times undergoes an ionization avalanche. By combining optical imaging and ion detection, we access the full information on the dynamical evolution of the system, including the rapid increase in the number of ions and a sudden depletion of the Rydberg and ground state densities. Rydberg-Rydberg interactions are observed to strongly affect the dynamics of plasma formation. Using a coupled rate-equation model to describe our data, we extract the average energy of electrons trapped in the plasma, and an effective cross section for ionizing collisions between Rydberg atoms and atoms in low-lying states. Our results suggest that the initial correlations of the Rydberg ensemble should persist through the avalanche. This would provide the means to overcome disorder-induced heating, and offer a route to enter new strongly coupled regimes.
We investigate the phenomenon of bistability in a thermal gas of cesium atoms excited to Rydberg states. We present both measurements and a numerical model of the phenomena based on collisions. By directly measuring the plasma frequency, we show that the origin of the bistable behavior lies in the creation of a plasma formed by ionized Rydberg atoms. Recombination of ions and electrons manifests as fluorescence which allows us to characterize the plasma properties and study the transient dynamics of the hysteresis that occurs. We determine scaling parameters for the point of plasma formation, and verify our numerical model by comparing measured and simulated spectra. These measurements yield a detailed microscopic picture of ionization and avalanche processes occurring in thermal Rydberg gases. From this set of measurements, we conclude that plasma formation is a fundamental ingredient in the optical bistability taking place in thermal Rydberg gases and imposes a limit on usable Rydberg densities for many applications.
Highly excited (Rydberg) atoms are produced and can exist for significant periods of time in various plasma environments. It is necessary to know cross sections for the various collision processes involving these atoms if a complete understanding of the physics of these environments is to be attained.\nThe experimental investigation of collisional phenomena between high Rydberg sodium atoms and electrons is described in this thesis. A fraction of the atoms in a ground state sodium beam is optically excited (using lasers) to well-defined, high Rydberg states (principal quantum numbers chosen around 40). Subsequent to their production the Rydberg atoms are allowed to interact with 25 eV electrons for a specified time interval following which the collision products are detected. The electrons are produced in a beam which is crossed at right angles with the sodium beam. Important collision processes are (1) electron impact ionization of the Rydberg atom, and (2) electron-Rydberg atom interactions in which the quantum state of the atom is changed (state changing collisions).\nResults are presented for an observed state changing process which is identified as angular momentum state changing only (no change in the value of the principal quantum number). Cross sections for this process are measured to be 10('-10) to 10('-9) cm('2). Problems encountered during the work which prevented the measurement of electron impact ionization cross sections are discussed. In the course of the work two other topics required study: (1) the process of electric field ionization of Rydberg atoms (used to detect the atoms), and (2) the effects of background (300 K) radiation (from the chamber walls, for example) on Rydberg atoms. Results of these studies are also presented.
This thesis describes the development of a new technique for measuring the spatial distribution of Rydberg atoms in a cold strontium gas. Strontium atoms are cooled and trapped in a magneto-optical trap and coherently excited to Rydberg states in a two-photon, three-level ladder scheme. Several methods of stabilizing the frequency to the cooling transition are discussed and characterized. A frequency stabilization scheme based on electromagnetically-induced transparency for the second laser required for Rydberg excitation is also explained. The Rydberg population dynamics are studied experimentally and modeled using an optical Bloch equation simulation. The divalent nature of strontium allows doubly excited “autoionizing” states to be accessed using resonant optical excitation. These states ionize in subnanosecond timescales, with the ions recorded on a micro-channel plate being proportional to the amount of Rydberg atoms. Translation of an autoionizing laser focused to a waist of 10 μm gives a spatially resolved Rydberg signal. A two-dimensional map of the Rydberg spatial distribution has been made using this autoionizing microscopy technique.
Abstract We present a first-principles density-matrix simulation of blackbody-radiation (BBR)-induced decoherence in cesium Rydberg electromagnetically induced transparency (EIT). The model incorporates spontaneous emission, blackbody-stimulated transitions, collisional broadening, transit-time-limited decoherence, and Doppler averaging across a truncated but converged Rydberg manifold within a Lindblad masterequation framework. From the steady-state solution, we extract both the temperaturedependent Rydberg coherence decay rate and the corresponding probe absorption coefficient.We show that the experimentally observed modification of probe transmission reported by Kaur et al. (2026) arises predominantly from BBR-induced decoherence of the Rydberg coherence, rather than from population redistribution among Rydberg levels. The model reproduces the observed temperature-dependent trends in probe absorption with quantitative accuracy. By employing a fully m j -resolved transition network, we establish a direct connection between microscopic decoherence processes and macroscopic optical observables. The decoherence effect on the optical response is sufficiently well-defined to serve as a measurable signal, enabling potential applications in Rydberg-based thermometry and radiometry.These results provide a self-consistent microscopic description of BBR-limited performance in thermal-vapor Rydberg EIT and establish a quantitative link between Rydberg-state decoherence and measurable changes in optical susceptibility, with direct relevance to RF-based sensing platforms.