Session J1: Ultracold Plasmas and Strongly Interacting Rydberg Systems

Observation of Blueshifted Ultralong-Range Cs$_2$ Rydberg Molecules

We observe ultralong-range blue-shifted Cs$_{2}$ molecular states near $ns_{1/2}$ Rydberg states in an optical dipole trap, where $31\leq n\leq34$. The accidental near degeneracy of $(n-4)l$ and $ns$ Rydberg states for $l>2$ in Cs, due to the small fractional $ns$ quantum defect, leads to non-adiabatic coupling among these states, producing potential wells above the $ns$ thresholds. Two important consequences of admixing high angular momentum states with $ns$ states are the formation of large permanent dipole moments, $\sim 15-100\,$Debye, and accessibility of these states via two-photon association. The observed states are in excellent agreement with theory.   

The pursuit of more strongly coupled plasmas using the Rydberg blockade

Ultracold neutral plasmas (UNP), created by photoionizing laser-cooled atoms near (above or below) the ionization threshold, offer a superb system in which to study the properties of strongly coupled plasmas. In strongly coupled Coulomb systems, the Coulomb coupling parameter $\Gamma $ is defined as the ratio of Coulomb interaction energy to kinetic energy and exceeds unity. When UNPs are created by exciting atoms just above the ionization threshold, the potential energy inherited from the spatial disorder of the atoms is rapidly redistributed between potential and kinetic degrees of freedom, limiting $\Gamma $ to about 2 or less. One approach for reducing this disorder induced heating is to ionize a spatially ordered gas, such as a fully blockaded Rydberg gas which exhibits spatial correlations due to localized suppression of subsequent Rydberg excitations. Towards this effort we have recently demonstrated the use of a diffuse seed UNP's electrons to collisionally drive Rydberg atoms toward a plasma state. We will describe our efforts to apply this technique to a Rydberg-blockaded sample and present high resolution measurements of the resultant ion coupling parameter.   

Velocity Relaxation in Strongly and Weakly Coupled Ultracold Plasmas

In this talk, we present direct measurements of thermalization rates in strongly coupled ultracold neutral plasmas, which are created by photoionizing strontium atoms in a magneto-optical trap. Because strong coupling can be achieved in ultracold plasmas at low densities, it is possible to probe collisional processes with time-resolved optical diagnostics. Spin-selective excitation of the $^{\mathrm{2}}$S$_{\mathrm{1/2}}-^{\mathrm{2}}$P$_{\mathrm{1/2}}$ transition at 422 nm in Sr$+$ ions allows us to perturb and probe the ion velocity distribution and directly measure collisional thermalization rates. We have measured relaxation rates with time resolution on the order of 100 ns. Currently, efforts are underway to achieve better time resolutions on the order of 10 ns, which may give us better insight into deviations from Landau-Spitzer collision theory. Also, we discuss efforts to use laser heating to produce plasmas with lower coupling parameters to connect our results to theory for the weakly coupled regime.   

Quadrupling the strong coupling in ultracold neutral plasmas

Ultracold neutral plasmas provide fertile ground for investigating the properties of strongly coupled neutral systems. To this end, a priority in the field is to generate ultracold plasmas with higher values of the strong coupling parameter $\Gamma$, which is given by the ratio of the nearest neighbor Coulomb potential energy to the average kinetic energy of the ions. Simulations predict that exciting the plasma ions to higher ionization states can increase the strongly coupling in an ultracold plasma by as much as a factor of 4. We describe an experiment in laser-cooled calcium in which laser pulses are used to singly ionize neutral atoms in a magneto-optical trap. A second set of laser pulses are used to produce Ca$^{2+}$. The maximum value of $\Gamma$ depends on the time at which the second ionization pulse arrives. This talk will describe the experiment and recent results.   

Particle distribution in a supersonic beam ultracold plasma

MOT plasmas have well defined initial phase space distributions, owing to the technique of laser cooling. To determine the distribution function of the nitric oxide plasma formed in a seeded supersonic expansion, we perform molecular beam dynamic calculations, incorporating sudden freeze and point-source approximations. Convolution of the resulting NO phase space distribution with the pulsed laser field illumination geometry yields the initial phase space distribution of our beam plasma. We are working to image the spatial part of this distribution and measure its temporal evolution.   

Influence of Electron Evaporative Cooling on Ultracold Plasma Expansion

The expansion of ultracold neutral plasmas (UCP) is driven by the thermal pressure of the electron component and is therefore sensitive to the electron temperature. At lower densities (less than 10$^8$ /cm$^3$), evaporative cooling has a substantial impact on the UCP expansion rate. We studied these effects of electron evaporation in this density range. Owing to the low density, the effects of three-body recombination were negligible. We observed a significant decrease in the ultracold plasma expansion rate when the electron evaporation rate was deliberately increased. We modeled the expansion by taking into account the change in electron temperature owing to evaporation as well as adiabatic expansion and found good agreement with our expansion data. We also developed a simple model for initial evaporation over a range of ultracold plasma densities, sizes, and electron temperatures to determine over what parameter range electron evaporation has a significant effect.   

Early Ultracold Plasma Expansion and Formation in a Weak Magnetic Field

The effects of a weak magnetic field on the formation and initial expansion rate of ultracold plasmas are reported. We observed an increase in the amount of electrons that escape during the initial formation when the Larmor radius of the electrons in the ultracold plasma approaches the spatial size of the plasma. This also corresponded to a radical increase in the initial expansion rate of the plasma. Observations of the late time expansion showed a deceleration of the expansion velocity under certain plasma conditions.