# Collisionally Induced Laser Cooling of Ultradense Gases

Apparatus for collisional laser cooling of ultradense atomic and molecular gases

Thermographic image of high pressure cell after 30 seconds of laser irradiation.

The blue region in the center is the cell window, cooled from the gas from behind.

We investigate the laser cooling of "macroscopic" atomic ensembles by collisional redistribution. For the experiments, we use a rubidium-argon gas mixture at 200 bar pressure. The achieved cooling power approaches 100 mW, which is more than a factor 10000 above the cooling power achieved in experiments on the Doppler cooling of dilute atomic gases. Frequent collisions of rubidium atoms with the argon buffer gas shift rubidium transitions into resonance with the exciting, far red detuned laser field, while the subsequent spontaneous decay occurs near the unperturbed transition frequency. During this process, kinetic energy is extracted from the sample, and the temperature reduces.

We are also investigating the collisionally induced laser cooling scheme for molecular gases. In other work, we have verified the Kennard-Stepanov relation, a Boltzmann-type thermodynamic frequency scaling of the ratio between absorption and emission, for both atomic and molecular gas samples.

Laser Cooling by Collisional Redistribution of Radiation

U. Vogl and M. Weitz

Nature

**461**, 70 (2009), arXiv:0906.2904, News & Views

P. Moroshkin, L. Weller, A. Saß, J. Klärs, and M. Weitz

Phys. Rev. Lett.

**113**, 063002 (2014), arXiv:1401.0731

D. Gelbwaser-Klimovsky, K. Szcygielski, U. Vogl, A. Saß, R. Alicki, G. Kurizki, and M. Weitz

Phys. Rev. A

**91**, 023431 (2015), arXiv: 1502.08019