Cooling and Pumping

Juha Javanainen

Rebekah Ferrier, Roberts, Jacob, Krueger, David +2 more

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.

Juha Javanainen

M. Hemmerling, Gordon R. M. Robb

We investigate the possibility of cooling an atomic gas enclosed in an optical cavity using blue-detuned laser light of sufficient intensity that excitation of the atoms cannot be neglected. We consider an ensemble of two-level atoms confined inside a simple Fabry–Perot cavity in two different geometric configurations: in one (‘cavity-pump’ configuration) the pump field is directed along the cavity axis and in the other (‘atom-pump’ configuration) the pump field is directed perpendicular to the cavity axis. Numerical simulations of the semi-classical models for each configuration are compared. Both configurations demonstrate cooling using a blue-detuned pump field. It is shown that in the cavity-pump configuration there is no collective enhancement of the cooling rate over that of free space blue-cooling. In contrast, the atom-pump configuration demonstrates collective enhancement of the cooling rate and intracavity field intensity.

Andrei V. Ivanov, Yu. Rozhdestvensky

Methods of coherent pumping through dipole-allowed 5d levels of RE ion are proposed for laser cooling. The coherent and complete population transfer between the ground and the first excited levels of 4f multiplet is achieved by using the different Raman techniques, namely two-photon scattering, adiabatic passage method, and π-pulse pumping. It is shown that the multiplication of the number of electrons that participate in cooling cycle leads to increasing of the cooling power and to acceleration of the cooling process. The increasing of cooling efficiency of 0.5% compared to the direct pumping between 4f levels is attained through the use of dipole-allowed optical transitions. Performed estimates show that the sample temperature can achieve 94 K for current purity materials. The calculations are obtained for Yb3+:CaF2 system.

Andreas H. Trabesinger