We present a study of a cold strontium Rydberg gas. The narrowband laser excitation of Rydberg states in the range n = 20–80 from a 6 mK cloud of strontium atoms is detected using the spontaneous ionization of the Rydberg atoms. Using a high-resolution step-scanning technique, we perform detailed measurements of the Stark maps of selected Rydberg states. We find excellent agreement between the measured Stark maps and a numerical calculation based on an independent electron model. Finally we show that excitation of the second valence electron can be used to probe the dynamics of the Rydberg gas with nanosecond temporal resolution via autoionization.
We present a study of a cold strontium Rydberg gas. The narrowband laser excitation of Rydberg states in the range n = 20–80 from a 6 mK cloud of strontium atoms is detected using the spontaneous ionization of the Rydberg atoms. Using a high-resolution step-scanning technique, we perform detailed measurements of the Stark maps of selected Rydberg states. We find excellent agreement between the measured Stark maps and a numerical calculation based on an independent electron model. Finally we show that excitation of the second valence electron can be used to probe the dynamics of the Rydberg gas with nanosecond temporal resolution via autoionization.
Cold gases of Rydberg atoms are an ideal system in which to study the novel effects of strong interatomic interactions. This thesis describes the design and construction of the world's first experiment to study Rydberg states in a cold gas of an alkaline earth metal, in this particular case strontium. We have studied a wide range of Rydberg states, and have developed a sensitive ``step-scan" spectroscopic technique that detects the spontaneous ionization of the Rydberg gas. The step-scan method is used to acquire Stark maps, and these measurements verify a single-electron model for calculating dipole matrix-elements. From the matrix-elements, interaction strengths between strontium Rydberg atoms have been calculated for the first time. The presence of two valence electrons in an alkaline earth metal, such as strontium, offers a new angle on the study of Rydberg atoms. We create doubly excited ``autoionizing" states, the first such study in a cold gas. Autoionization is used as a high yield probe of Rydberg states, and enables a study of excitation dynamics with nanosecond time-resolution. We show that autoionization can quantitatively identify and elucidate state mixing in the Rydberg gas, and probe population transfer at the very onset of ultra-cold plasma formation.
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.
This thesis details the first measurements of Rydberg dipole blockade in a cold ensemble of divalent atoms. 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. Owing to the divalent nature of strontium, one electron can be excited to the Rydberg state, whilst the other lower-lying electron is available to undergo resonant optical excitation to autoionising states, which ionise in sub-nanosecond timescales. The remaining ions that are recorded on a micro-channel plate are proportional to the number of Rydberg atoms. The development of a narrow linewidth laser system necessary for an additional stage of cooling is explained and characterised. Two frequency stabilisation schemes are discussed: one to address the short-term laser frequency instabilities based on the Pound-Drever-Hall technique; the other to address the long-term laser frequency instabilities based on Lamb-dip spectroscopy in an atomic beam. The cooling dynamics on the narrow cooling transition is studied experimentally and modelled via theoretical simulations.
Dipole-dipole interactions, the strongest, longest-range interactions possible between two neutral atoms, cannot be better manifested anywhere else than in a Rydberg atomic system. Rydberg atoms, having high principal quantum numbers n>>1 and dipole moments that scale as n^2, provide a powerful tool to examine dipole-dipole interactions. Therefore, we have studied the production and production rates of strontium Rydberg atoms created using two-photon excitation and have explored their properties in two distinct experiments. In the first experiment, very-high-n (n~300) Rydberg atoms are produced in a tightly collimated atomic beam allowing spectroscopic studies of their energy levels and their Stark effects. Simulations using a two-active-electron model, developed by our theoretical collaborators, allow detailed analysis of the results and are in remarkable agreement with the experimental results. The high density of Rydberg atoms achieved ~ 5*10^5 cm^(-3), in this experiment will allow studies of strongly interacting Rydberg-Rydberg systems. The second experiment, in which a cold strontium Rydberg gas is excited in a magneto-optic trap, features an imaging technique offering both spatial and temporal resolution. We use this technique to observe and study the evolution of an ultra-cold strontium Rydberg gas which reveals the importance of Rydberg-Rydberg interactions in the early stages of this evolution. Strongly interacting Rydberg gas provides an opportunity to realize a very strongly-correlated ultra-cold plasma.
Clouds of ultracold strontium 5s48s1S0 or 5s47d1D2 Rydberg atoms are created by two-photon excitation of laser-cooled 5s21S0 atoms. The spontaneous evolution of the cloud of low orbital angular momentum (low-$\ensuremath{\ell}$) Rydberg states towards an ultracold neutral plasma is observed by imaging resonant light scattered from core ions, a technique that provides both spatial and temporal resolution. Evolution is observed to be faster for the S states, which display isotropic attractive interactions, than for the D states, which exhibit anisotropic, principally repulsive interactions. Immersion of the atoms in a dilute ultracold neutral plasma speeds up the evolution and allows the number of Rydberg atoms initially created to be determined.
This thesis describes an investigation of Rydberg excitation within a high-density sample of cold strontium atoms. This sample is prepared using a two-stage magneto-optical trap, cooling first on a broad singlet transition and then a narrow triplet transition. Rydberg atoms are then created using a two-photon, three-level ladder type scheme and detected using a novel autoionisation technique. It is shown that, in the regime of high optical thickness on the probing transition, a significant Rydberg population can be created by photons that are multiply scattered before leaving the cloud. The multiply scattered field is density-dependent and has strikingly different spectral properties from the incident laser light. This spectrum is convolved with the spectrum of the Rydberg atoms created via direct laser excitation and the two spectra can be isolated in post analysis. This technique provides a probe of the spectral distribution of the re-scattered light within the cloud, which may be qualitatively different from that of the transmitted light, and previously has not been measured directly. Additionally, this Rydberg population arising from the multiply scattered field can be seen in the spatial distribution of the Rydberg excitations within the atom sample. Finally, a careful analysis of the time dynamics of the Rydberg system reveals that multiple scattering co-exists with signatures of the Rydberg blockade in this strongly dissipative regime.