Cold and optically dense gases of atomic rubidium are produced using buffer gas cooling in combination with rapid, high-flow vapor injection. The observed rubidium density is 3 × 10 12 cm −3 at a gas temperature of ∼20 K, leading to an optical density of the order of 200.
Spectroscopy of alkali-metal--buffer-gas mixtures at high pressures from single-digit to several hundred bars in the regime of substantial collisional broadening is relevant in a wide range of fields, ranging from collisional redistribution cooling to laboratory astrophysics. Here we report on spectroscopic measurements of dense rubidium--noble-gas mixtures recorded in a pressure cell equipped with soldered sapphire optical viewports, which allows for the controlled realization of extreme conditions of high temperature and high pressure in a table top laboratory experiment. In the gas cell, we have recorded absorption and emission spectra of rubidium subject to 188 bars of helium buffer-gas pressure at 548-K temperature. The spectra to good accuracy follow a Boltzmann-type Kennard-Stepanov frequency scaling of the ratio of absorption and emission spectral profiles. Further, the long optical path length in the cell allowed us to both record spectra of rubidium-argon mixtures at moderate temperatures and high pressures and observe redistributional laser cooling in this system.
We demonstrate experimentally the generation of one-dimensional cold gases of 87Rb atoms by diffuse laser cooling. A horizontal slender vacuum glass tube with a length of 105 cm and diameter of 2 cm is used in our experiment. The diffuse laser light inside the tube, which is generated by multireflection of injected lasers, cools the background vapor atoms. With 250 mW of cooling light and 50 mW of repumping light, an evenly distributed 1-m-long profile of cold atom cloud is obtained. We observe a factor 4 improvement in the atomic optical density for a typical cooling duration of 170 ms and a sub-Doppler atomic temperature of 25 $\ensuremath{\mu}\mathrm{K}$. The central number of detected cold atoms remains constant for a free-fall duration of 30 ms. Such samples are ideal for many quantum optical experiments involving electromagnetically induced transparency, electronically highly excited (Rydberg) atoms, and quantum precision measurements.
We describe experiments on the laser cooling of both helium-rubidium and argon-rubidium gas mixtures by collisional redistribution of radiation. Frequent alkali-noble gas collisions in the ultradense gas, with typically 200 bar of noble buffer gas pressure, shift a highly red detuned optical beam into resonance with a rubidium Dline transition, while spontaneous decay occurs close to the unshifted atomic resonance frequency. The technique allows for the laser cooling of macroscopic ensembles of gas atoms. The use of helium as a buffer gas leads to smaller temperature changes within the gas volume due to the high thermal conductivity of this buffer gas, as compared to the heavier argon noble gas, while the heat transfer within the cell is improved.
Atomic vapor of four different paramagnetic species: gold, silver, lithium, and rubidium, is produced and studied inside several buffer gases: helium, nitrogen, neon, and argon. The paramagnetic atoms are injected into the buffer gas using laser ablation. Wires with diameters 25, 50, and $100\text{ }\ensuremath{\mu}\text{m}$ are used as ablation targets for gold and silver; bulk targets are used for lithium and rubidium. The buffer gas cools and confines the ablated atoms, slowing down their transport to the cell walls. Buffer gas temperatures between 20 and 295 K and densities between ${10}^{16}\text{ }{\text{cm}}^{\ensuremath{-}3}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3}$ are explored. Peak paramagnetic atom densities of ${10}^{11}\text{ }{\text{cm}}^{\ensuremath{-}3}$ are routinely achieved. The longest observed paramagnetic vapor density decay times are 110 ms for silver at 20 K and 4 ms for lithium at 120 K. The candidates for the principal paramagnetic-atom loss mechanism are impurities in the buffer gas, dimer formation, and atom loss on sputtered clusters.
Abstract Optical spectroscopy of atoms and molecules is a field where one usually operates very far from thermal equilibrium conditions. A prominent example is spectroscopy of thin vapors, where the pump irradiation leads to a non-equilibrium distribution within the electronic structure that is well shielded from the environment. Here we describe experimental work investigating absorption and emission lines of rubidium vapor subject to a noble buffer gas environment with pressure 100–200 bar, a regime interpolating between usual gas phase and liquid/solid state conditions. Frequent elastic collisions in the dense buffer gas sample cause a large coupling to the environment. We give a detailed account of recent observations of the Kennard–Stepanov scaling, a Boltzmann-like thermodynamic frequency scaling between absorption and emission profiles, for both atomic and molecular rubidium species in the gaseous environment. Our observations are interpreted as due to the thermalization of alkali-noble gas submanifolds in both ground and electronically excited states respectively. Both pressure broadening and shift of the high pressure buffer gas D -lines system are determined. We also discuss some prospects, including possible advances in collisional laser cooling and optical thermometry.
Laser cooling via collisional redistribution of fluorescence utilizes dense mixtures of alkali metals with noble buffer gases. Typical pressure values of the buffer gas are of the order of a few hundred bar, ensuring a frequent number of collisions between the two atomic species. The energy levels of the alkali atoms are thus perturbed so that excitation using far red-detuned laser light becomes feasible. Energy is then extracted via spontaneous emission occuring close to the unperturbed atomic resonances. Optimization of the cooling effect strongly depends on excitation conditions, namely the choice of detuning of the cooling beam, which in turn is dependent on the used pressure. We here report on spectroscopy measurements of atomic rubidium under high pressure buffer gas conditions. While thermal deflection spectroscopy has been previously used to measure the temperature change of the laser-cooled gas, an alternative approach for temperature measurements utilizing the Kennard-Stepanov relation is also investigated. We here exhibit that the collisionally thermalized atomic resonances well follow this thermodynamic scaling, allowing for temperature extraction of the dense gas mixture.