Browsing by Author "Roberts, Jacob, advisor"

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Open Access
Dynamics of low-density ultracold plasmas in externally applied electric and magnetic fields
(Colorado State University. Libraries, 2013) Wilson, Truman M., author; Roberts, Jacob, advisor; Krueger, David, committee member; Lundeen, Stephen, committee member; Yalin, Azer, committee member
The experiments described in this thesis were focused on the influence of external electric and magnetic fields and electron evaporation on the evolution of ultracold plasmas (UCPs). The UCPs were created from the photoionization of 85Rb which was first captured in a magneto-optical trap (MOT) and then magnetically trapped and transferred by a set of magnetic coils attached to a motorized translation stage to a region of the vacuum chamber with a set of electrodes. The first experiment studied the response of the UCP to sharp electric field pulses, which included 2 cycles of a sine wave pulse. These experiments showed a resonant response to the 2 cycles of rf that was density dependent, but was not a collision based mechanism. Instead, the response was caused by a rapid energy transfer to individual electrons through the collective motion of the electron cloud in the UCP. This density-dependent response allowed us to develop a technique for measuring the expansion rate of the UCPs in our system. It was also observed in second set of experiments that electron evaporation from the UCP had a significant effect on the amount of energy that was transferred to the ions to drive the UCP expansion. Model calculations show that we should expect electron evaporation to have a more significant influence on the UCP expansion rate at the relatively low densities of the UCPs that we create compared to other experiments. By modeling electron evaporation during expansion, our data are consistent with evaporation reducing the electron temperature significantly, which lowers the overall UCP expansion rate. In addition to these studies, we also performed an experiment in which it was observed that in the presence of a magnetic field there was a significant increase in the initial UCP expansion rate coupled with a deceleration of the ion expansion at later times in the UCP evolution. Our observations to date are consistent with the magnetic field influencing electron screening and UCP formation. By restricting the electrons motion in the direction transverse to the magnetic field lines to circular orbits around the magnetic field lines, the electrons cannot move appropriately to screen the internal radial electric fields produced by the excess of ions. Studies of this effect are currently under way. Future studies include direct measurements of the electron temperature and collision rates between the components of the UCP as we move towards trapping the UCP in a Penning trap.
Open Access
Experimental realization of two-isotope collision-assisted Zeeman cooling
(Colorado State University. Libraries, 2013) Hamilton, Mathew, author; Roberts, Jacob, advisor; Lundeen, Stephen, committee member; Gelfand, Martin, committee member; Bartels, Randy, committee member
The work presented in this thesis focuses on the demonstration and initial evaluation of a novel non-evaporative cooling method called collision-assisted Zeeman cooling. For this realization, an ultracold gas consisting of a mixture of 87Rb and 8Rb was used. Cooling was accomplished through interisotope inelastic spin-exchange collisions that converted kinetic energy into magnetic energy. Continual optical pumping spin polarized the 85Rb which ensured that only kinetic energy reducing collisions occurred and the scattered pump photons carried entropy out of the system. Thus, cooling of the ultracold gas can be achieved without requiring the loss of any atoms in order to do so. This represents a theoretical advantage over forced evaporative cooling, which is the current state-of-the-art cooling technique in most experiments. This thesis discusses the details of collision-assisted Zeeman cooling, as well as how the theory of the technique has been extended from cooling a single species to cooling with two species. There are many predicted advantages from using two rather than one species of atom in this type of cooling: greater flexibility in finding favorable spin-exchange collision rates, easier requirements on the magnetic fields that must be used, and an additional means to mitigate reabsorption (the primary limitation in many if not most non-evaporative cooling techniques). The experimental considerations needed to prepare a system that simultaneously trapped two isotopes to be able to perform collision-assisted Zeeman cooling are discussed. Because this cooling scheme is highly reliant on the initial conditions of the system, a focused experiment examining the loading of the optical trap with both isotopes of Rb was conducted and the results of that experiment are described here. The first experimental observations of spin-exchange collisions in an ultracold gas mixture of Rb are described as a part of this work. The experiments where collision-assisted Zeeman cooling were demonstrated are then described and evaluated. In this first implementation of the cooling technique the initial densities were too low and optical-pump-induced heating and loss too high for achieving the full predicted performance of the cooling technique. Through additional modeling, these limitations were understood and the necessary improvements for the next iteration of CAZ cooling experiments are laid out at the end of this work.
Open Access
Measurements of electron-ion collision rates and Rydberg atom populations in ultracold plasmas by using short electric field pulses
(Colorado State University. Libraries, 2017) Chen, Wei-Ting, author; Roberts, Jacob, advisor; Robinson, R. Steve, committee member; Harton, John, committee member; Krapf, Diego, committee member
Ultracold plasmas are good tools for studying fundamental plasma physics. In particular, these plasmas are well-suited to study so-called strong coupling physics the physics of plasmas where nearest-neighbor Coulomb interactions become large enough to cause spatial correlations and break assumptions. An ultracold plasma makes such a good tool because it is it is free of interactions with neutral atoms, and has a well controlled and tunable initial conditions. The UCPs in this work were created from the photoionization of cold 85Rb atoms. The experiments described in this thesis are focused on the measurements of damping of electron center-of-mass oscillations. We developed a method that uses two short electric field pulses to map the temporal profile of the oscillation amplitude. We found that the damping of such oscillations can result from dephasing which is a collisionless mechanism or from electron-ion collisions or a combination of both. Thus, we separate the study of two pulse measurements into two parts. The first part of the two short electric field pulse measurement is about the measurements and modeling of in the collisionless damping regime. The second part will focus on the regime where the damping is dominated by electron-ion collisions where we not only observed strong coupling influence on electron-ion collision rates, but also saw break down of one or more standard assumptions used in plasma physics calculations. Rydberg atoms can be formed in ultracold plasmas through three-body recombination process. Our setup was capable of measuring Rydberg atoms in a energy range above the bottleneck energy. We measured the Rydberg populations at different temperatures, and our preliminary results agree well with a parameter-free calculation. However, there are some unexplained parts of our measurements on early time Rydberg populations. This means more studies are needed in the future in order to interpret our results and make use of them. Future work includes measurements of the strong coupling influence on electron-ion collision rates in a magnetized ultracold plasma, measurement of Rydberg population below the bottleneck energy, a detailed study of evaporations in ultracold plasmas.
Open Access
Near-resonant and resonant light in ultracold gases
(Colorado State University. Libraries, 2020) Gilbert, Jonathan, author; Roberts, Jacob, advisor; Yost, Dylan, committee member; Bradley, Mark, committee member; Marconi, Mario, committee member
This dissertation describes experiments and calculations involving light manipulation of atoms and light propagation in ultracold gases. There are three major sections to this dissertation. Each section presents a research topic connected to the main subject of near-resonant and resonant light in ultracold gases. First, this dissertation details the theoretical description and experimental implementation of a novel cooling technique for ultracold atoms trapped in a confining potential. Manipulating the internal states of atoms by applying near-resonant laser pulses at specified times leads to high energy atoms being preferentially selected and then slowed to achieve cooling. We call the technique "spatially truncated optical pumping (STOP) cooling." Advantages of the technique include its straightforward adaptability into experiments already using a magneto-optical trap; its applicability to any species that can be laser cooled and trapped in a confining potential; it does not depend on highly specific transitions for cooling; it does not depend on number loss for cooling. We present experimental results from applying the technique to an ultracold gas of 87Rb. We also present theoretical predictions of expected cooling rates, along with possible improvements to our apparatus that could lead to further cooling. Next, this dissertation details numerical calculations of near-resonant light propagation through a highly absorptive elongated ultracold gas. The confined gas modeled by these calculations are representative of gases commonly found in ultracold atom experiments. The spatial density distribution and spatial extent of these gases leads to a substantial gradient in the index of refraction. In addition, these gases can have a smaller spatial extent than that of the cross section of a laser beam that illuminates them. We present calculations that show the index variation in these systems can lead to frequency-dependent focusing or defocusing of incident near-resonant light. In some cases, focusing results in light intensities inside of the gas that are over an order of magnitude higher than the incident value. Additionally, we show that refraction and diffraction of the light results in non-intuitive patterns forming in the directions perpendicular to the light propagation. Lastly, this dissertation details the theoretical treatment and experimental measurements of the time-dependent absorption and phase response of an ultracold gas that is suddenly illuminated by near-resonant light. These studies focus on dynamics occurring over timescales on the order of an atomic excited state lifetime. Because the atoms cannot respond instantaneously to the applied light, both the absorption response and phase response require time to develop, with the phase response being slower than the absorption response. Related polarization effects such as Faraday rotation are due to phase shifts imparted by the gas, and therefore these effects also require time to develop. We detail our experimental measurements of the time-dependent development of Faraday rotation in an ultracold gas of 85Rb and compare the results to predictions using a theoretical approach based on solving optical Bloch equations. We identify how parameters such as the applied magnetic field strength and optical thickness of the gas influence the response timescales of the gas.
Open Access
Off-resonant RF heating of strongly magnetized electrons in ultracold neutral plasma
(Colorado State University. Libraries, 2021) Guthrie, John M., author; Roberts, Jacob, advisor; Fairbank, William, Jr., committee member; Gelfand, Martin, committee member; Wilson, Jesse, committee member
Magnetic fields are common in many plasma systems. Ultracold neutral plasmas (UCPs) are capable of not only accessing strong Coulomb coupling physics but also strong and extreme electron magnetization regimes, as well. These magnetization regimes, as defined by Baalrud and Daligault [S. Baalrud and J. Daligault, Phys. Rev. E, 96, 043202 (2017)], are predicted to modify screening or binary collision properties as the electron cyclotron radius approaches or subceeds the relevant plasma length scales. UCPs provide an advantageous testing ground for measuring magnetized electron-ion interactions, such as collisional heating induced by applied off-resonant RF fields. The experiments described in this thesis are focused on observations of RF heating in a UCP made from a photoionized cloud of ultracold 85Rb at three electron magnetization strengths that span the weakly-strongly magnetized boundary to the strongly-extremely magnetized boundary. Relative comparisons between heating rates at different magnetic fields were measured with ~20% precision, and an absolute determination of the heating rate near the weak-strong magnetization boundary is determined with ~40% precision. The results from these experiments were compared to theoretical predictions we developed that account for the finite-RF amplitude conditions used in the UCP measurements. This finite-amplitude heating rate theory is shown to be an extension of low-amplitude magnetized AC conductivity treatments as well as unmagnetized nonlinear collisional radiation absorption treatments. Mixed agreement was discovered between our observations and the theory for the three magnetic fields investigated: 10.6, 65, and 134 G. The measured absolute RF heating rate at 10.6 G and the relative rate between 134 and 10.6 G are in agreement with predictions within uncertainty; the relative rate between 65 and 10.6 G was observed to be a factor of ~3 lower than the predictions, with an absolute difference---in terms of the measurement uncertainty---on the order of 10σ. The implications of this disagreement are discussed, and future measurements that can be conducted with this technique are presented.
Open Access
Spatially-selective optical pumping cooling and two-isotope collision-assisted Zeeman cooling
(Colorado State University. Libraries, 2014) Wilson, Rebekah Ferrier, author; Roberts, Jacob, advisor; Krueger, David, committee member; Lundeen, Stephen, committee member; Marconi, Mario, committee member
In this thesis I describe two non-evaporative cooling schemes for cooling Rb atoms. The first is a Sisyphus-like ultracold gas cooling scheme called Spatially-selecTive Optical Pumping (STOP) cooling. In principle, STOP cooling has wide applicability to both atoms and molecules. STOP cooling works by exploiting the fact that atoms or molecules in a confining potential can be optically pumped out of an otherwise dark state in a spatially-selective way. Selecting atoms or molecules for optical pumping out of a dark state in a region of high potential energy and then waiting a fixed time after the optical pumping allows for the creation of a group of high kinetic energy atoms or molecules moving in a known direction. These can then be slowed using external fields (such as the scattering force from a resonant laser beam) and optically pumped back into the dark state, cooling the gas and closing the cooling cycle. I present theoretical modeling of the STOP cooling technique, including predictions of achievable cooling rates. I have conducted an experimental study of the cooling technique for a single cooling cycle, observing one dimensional cooling rates in excess of 100 micro-K per second in an ultracold gas of 87Rb atoms. I will also comment on the prospects for improving the cooling performance beyond that presented in this work. The second cooling scheme I investigated is called Two-Isotope Collision Assisted Zeeman (2-CAZ) cooling. Through a combination of spin-exchange collisions in a magnetic field and optical pumping, it is possible to cool a gas of atoms without requiring the loss of atoms from the gas. I investigated 2-CAZ cooling using 85Rb and 87Rb. I was able to experimentally confirm that the measured 2-CAZ cooling rate agreed with a cooling rate predicted though a simple analytic model. As part of the measured cooling rate, I quantitatively characterized the heating rates associated with our actual implementation of this cooling technique and found hyperfine-changing collisions to be a significant limitation for the 85/87Rb gas mixture. Possible improvements to this experiment will be discussed as well as the prospects for improved cooling performance using an atom without hyperfine structure as the optically pumped atom.