R-3 Repository :: Browsing by Author "Killian, T.C."

Browsing by Author "Killian, T.C."

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A high-throughput three-dimensional cell migration assay for toxicity screening with mobile device-based macroscopic image analysis
(Nature Publishing Group, 2013) Timm, David M.; Chen, Jianbo; Sing, David; Gage, Jacob A.; Haisler, William L.; Neeley, Shane K.; Raphael, Robert M.; Dehghani, Mehdi; Rosenblatt, Kevin P.; Killian, T.C.; Tseng, Hubert; Souza, Glauco R.
There is a growing demand for in vitro assays for toxicity screening in three-dimensional (3D) environments. In this study, 3D cell culture using magnetic levitation was used to create an assay in which cells were patterned into 3D rings that close over time. The rate of closure was determined from time-lapse images taken with a mobile device and related to drug concentration. Rings of human embryonic kidney cells (HEK293) and tracheal smooth muscle cells (SMCs) were tested with ibuprofen and sodium dodecyl sulfate (SDS). Ring closure correlated with the viability and migration of cells in two dimensions (2D). Images taken using a mobile device were similar in analysis to images taken with a microscope. Ring closure may serve as a promising label-free and quantitative assay for high-throughput in vivo toxicity in 3D cultures.
Absorption Imaging of Ultracold Neutral Plasmas
(2005-04) Simien, C.E.; Chen, Y.C.; Gupta, P.; Laha, S.; Martinez, Y.N.; Mickelson, P.G.; Nagel, S.B.; Killian, T.C.
Bose-Einstein Condensation of 84Sr
(American Physical Society, 2009) Martinez de Escobar, Y.N.; Mickelson, P.G.; Yan, M.; DeSalvo, B.J.; Nagel, S.B.; Killian, T.C.
We report Bose-Einstein condensation of Sr84 in an optical dipole trap. Efficient laser cooling on the narrow intercombination line and an ideal s-wave scattering length allow the creation of large condensates (N0∼3×105) even though the natural abundance of this isotope is only 0.6%. Condensation is heralded by the emergence of a low-velocity component in time-of-flight images.
Bose-Einstein condensation of 88Sr through sympathetic cooling with 87Sr
(2010-05-05) Mickelson, P.G.; Martinez de Escobar, Y.N.; Yan, M.; DeSalvo, B.J.; Killian, T.C.
We report Bose-Einstein condensation of Sr88, which has a small, negative s-wave scattering length (a88=−2a0). We overcome the poor evaporative cooling characteristics of this isotope by sympathetic cooling with Sr87 atoms. Sr87 is effective in this role despite the fact that it is a fermion because of the large ground-state degeneracy arising from a nuclear spin of I=9/2, which reduces the impact of Pauli blocking of collisions. We observe a limited number of atoms in the condensate (Nmax≈104) that is consistent with the value of a88 and the optical dipole trap parameters.
Controlling Condensate Collapse and Expansion with an Optical Feshbach Resonance
(American Physical Society, 2013) Yan, Mi; DeSalvo, B.J.; Ramachandhran, B.; Pu, H.; Killian, T.C.
We demonstrate control of the collapse and expansion of an Sr88 Bose-Einstein condensate using an optical Feshbach resonance near the S01−P13 intercombination transition at 689 nm. Significant changes in dynamics are caused by modifications of scattering length by up to ±10abg, where the background scattering length of Sr88 is abg=−2a0 (1a0=0.053 nm). Changes in scattering length are monitored through changes in the size of the condensate after a time-of-flight measurement. Because the background scattering length is close to zero, blue detuning of the optical Feshbach resonance laser with respect to a photoassociative resonance leads to increased interaction energy and a faster condensate expansion, whereas red detuning triggers a collapse of the condensate. The results are modeled with the time-dependent nonlinear Gross-Pitaevskii equation.
Creating and studying ion acoustic waves in ultracold neutral plasmas
(2012) Killian, T.C.; McQuillen, P.; O'Neil, T.M.; Castro, J.; Rice Quantum Institute
Creating Non-Maxwellian Velocity Distributions in Ultracold Plasmas
(arXiv, 2011) Castro, J.; Bannasch, G.; McQuillen, P.; Pohl, T.; Killian, T.C.; Rice Quantum Institute
Creation of Rydberg Polarons in a Bose Gas
(American Physical Society, 2018) Camargo, F.; Schmidt, R.; Whalen, J.D.; Ding, R.; Woehl, G. Jr.; Yoshida, S.; Burgdörfer, J.; Dunning, F.B.; Sadeghpour, H.R.; Demler, E.; Killian, T.C.
We report spectroscopic observation of Rydberg polarons in an atomic Bose gas. Polarons are created by excitation of Rydberg atoms as impurities in a strontium Bose-Einstein condensate. They are distinguished from previously studied polarons by macroscopic occupation of bound molecular states that arise from scattering of the weakly bound Rydberg electron from ground-state atoms. The absence of a p-wave resonance in the low-energy electron-atom scattering in Sr introduces a universal behavior in the Rydberg spectral line shape and in scaling of the spectral width (narrowing) with the Rydberg principal quantum number, n. Spectral features are described with a functional determinant approach (FDA) that solves an extended Fröhlich Hamiltonian for a mobile impurity in a Bose gas. Excited states of polyatomic Rydberg molecules (trimers, tetrameters, and pentamers) are experimentally resolved and accurately reproduced with a FDA.
Degenerate Fermi Gas of 87Sr
(2010-07-13) DeSalvo, B.J.; Yan, M.; Mickelson, P.G.; Martinez de Escobar, Y.N.; Killian, T.C.
We report quantum degeneracy in a gas of ultracold fermionic Sr87 atoms. By evaporatively cooling a mixture of spin states in an optical dipole trap for 10.5 s, we obtain samples well into the degenerate regime with T/TF=0.26+0.05−0.06. The main signature of degeneracy is a change in the momentum distribution as measured by time-of-flight imaging, and we also observe a decrease in evaporation efficiency below T/TF∼0.5.
Demonstrating universal scaling for dynamics of Yukawa one-component plasmas after an interaction quench
(American Physical Society, 2016) Langin, T.K.; Strickler, T.; Maksimovic, N.; McQuillen, P.; Pohl, T.; Vrinceanu, D.; Killian, T.C.
The Yukawa one-component plasma (OCP) model is a paradigm for describing plasmas that contain one component of interest and one or more other components that can be treated as a neutralizing, screening background. In appropriately scaled units, interactions are characterized entirely by a screening parameter, κ. As a result, systems of similar κ show the same dynamics, regardless of the underlying parameters (e.g., density and temperature). We demonstrate this behavior using ultracold neutral plasmas (UNPs) created by photoionizing a cold (T≤10 mK) gas. The ions in UNP systems are well described by the Yukawa model, with the electrons providing the screening. Creation of the plasma through photoionization can be thought of as a rapid quench of the interaction potential from κ=∞ to a final κ value set by the electron density and temperature. We demonstrate experimentally that the postquench dynamics are universal in κ over a factor of 30 in density and an order of magnitude in temperature. Results are compared with molecular-dynamics simulations. We also demonstrate that features of the postquench kinetic energy evolution, such as disorder-induced heating and kinetic-energy oscillations, can be used to determine the plasma density and the electron temperature.
Electron Screening and Kinetic-Energy Oscillations in a Strongly Coupled Plasma
(2004-12-20) Chen, Y.C.; Simien, C.E.; Laha, S.; Gupta, P.; Martinez, Y.N.; Mickelson, P.G.; Nagel, S.B.; Killian, T.C.; Rice Quantum Institute
Electron-Temperature Evolution in Expanding Ultracold Neutral Plasmas
(2007-08-17) Gupta, P.; Laha, S.; Simien, C.E.; Gao, H.; Castro, J.; Killian, T.C.; Pohl, T.
Emergence of kinetic behavior in streaming ultracold neutral plasmas
(AIP Publishing LLC, 2015) McQuillen, P.; Castro, J.; Bradshaw, S.J.; Killian, T.C.
We create streaming ultracold neutral plasmas by tailoring the photoionizing laser beam that creates the plasma. By varying the electron temperature, we control the relative velocity of the streaming populations, and, in conjunction with variation of the plasma density, this controls the ion collisionality of the colliding streams. Laser-induced fluorescence is used to map the spatially resolved density and velocity distribution function for the ions. We identify the lack of local thermal equilibrium and distinct populations of interpenetrating, counter-streaming ions as signatures of kinetic behavior. Experimental data are compared with results from a one-dimensional, two-fluid numerical simulation.
Experimental Measurement of Self-Diffusion in a Strongly Coupled Plasma
(American Physical Society, 2016) Strickler, T.S.; Langin, T.K.; McQuillen, P.; Daligault, J.; Killian, T.C.
We present a study of the collisional relaxation of ion velocities in a strongly coupled, ultracold neutral plasma on short time scales compared to the inverse collision rate. The measured average velocity of a tagged population of ions is shown to be equivalent to the ion-velocity autocorrelation function. We thus gain access to fundamental aspects of the single-particle dynamics in strongly coupled plasmas and to the ion self-diffusion constant under conditions where experimental measurements have been lacking. Nonexponential decay towards equilibrium of the average velocity heralds non-Markovian dynamics that are not predicted by traditional descriptions of weakly coupled plasmas. This demonstrates the utility of ultracold neutral plasmas for studying the effects of strong coupling on collisional processes, which is of interest for dense laboratory and astrophysical plasmas.
Experimental Realization of an Exact Solution to the Vlasov Equations for an Expanding Plasma
(2007-10-08) Laha, S.; Gupta, P.; Simien, C.E.; Gao, H.; Castro, J.; Pohl, T.; Killian, T.C.
Imaging the evolution of an ultracold strontium Rydberg gas
(American Physical Society, 2013) McQuillen, P.; Zhang, X.; Strickler, T.; Dunning, F.B.; Killian, T.C.; Rice Quantum Institute
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-ℓ) 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.
Inelastic and elastic collision rates for triplet states of ultracold strontium
(2009-06-05) Traverso, A.; Chakraborty, R.; Martinez de Escobar, Y.N.; Mickelson, P.G.; Nagel, S.B.; Yan, M.; Killian, T.C.
Ion Acoustic Waves in Ultracold Neutral Plasmas
(American Physical Society, 2010) Castro, J.; McQuillen, P.; Killian, T.C.; Rice Quantum Institute
We photoionize laser-cooled atoms with a laser beam possessing spatially periodic intensity modulations to create ultracold neutral plasmas with controlled density perturbations. Laser-induced fluorescence imaging reveals that the density perturbations oscillate in space and time, and the dispersion relation of the oscillations matches that of ion acoustic waves, which are long-wavelength, electrostatic, density waves.
Ion holes in the hydrodynamic regime in ultracold neutral plasmas
(American Institute of Physics, 2013) McQuillen, P.; Castro, J.; Strickler, T.; Bradshaw, S.J.; Killian, T.C.
We describe the creation of localized density perturbations, or ion holes, in an ultracold neutral plasma in the hydrodynamic regime, and show that the holes propagate at the local ion acoustic wave speed. We also observe the process of hole splitting, which results from the formation of a density depletion initially at rest in the plasma. One-dimensional, two-fluid hydrodynamic simulations describe the results well. Measurements of the ion velocity distribution also show the effects of the ion hole and confirm the hydrodynamic conditions in the plasma.
Ion temperature evolution in an ultracold neutral plasma
(AIP Publishing, 2015) McQuillen, P.; Strickler, T.; Langin, T.; Killian, T.C.
We study the long-time evolution of the ion temperature in an expanding ultracold neutral plasma using spatially resolved, laser-induced-fluorescence spectroscopy. Adiabatic cooling reduces the ion temperature by an order of magnitude during the plasma expansion, to temperatures as low as 0.2 K. Cooling is limited by heat exchange between ions and the much hotter electrons. We also present evidence for an additional heating mechanism and discuss possible sources. Data are described by a model of the plasma evolution, including the effects of ion-electron heat exchange. We show that for appropriate initial conditions, the degree of Coulomb coupling of ions in the plasma increases during expansion.

Browsing by Author "Killian, T.C."

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