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Chinese Physics Letters

Temperature of the Remaining Cold Atoms after Two-Step Photoionization in an 87 Rb Vapor Cell Magneto-Optical Trap

Ya-Ping Ruan, Feng-Dong Jia, Long-Wei Liu, Zhen Sun, Wei Huang, Ping Xue, Xiang-Yuan Xu, Xing-Can Dai, Zhi-Ping Zhong

Published Jul 1, 2014• 1 citation
Publisher

Abstract

The temperature of the remaining cold 87 Rb atoms confined in a vapor cell magneto-optical trap after two-step photoionization has been measured. In the two-step photoionization process, the first excitation laser is served by the cooling laser and the second excitation laser is served by a continuous semiconductor laser with a wavelength of 450 nm. The results show that the temperature of the remaining cold atoms decreases as the intensity of the second excitation laser increases. Moreover, the relationship between the temperature T and number N of the remaining cold atoms generally follows a power law, while it deviates from the well-known T ∝ N 1/3 and the power factor is smaller than 1/3. We propose that ion-atom collisions occurring during a photoionization process strongly influence the temperature scaling law in an optically dense magneto-optical trap in the presence of an ionization laser. In addition, the forced evaporative cooling due to the combined effect of the detuning of the first excitation laser and the two-step photoionization plays a role in cooling the remaining cold atoms and results in the dependence of the power factor on the detuning of the first excitation laser.

DOI
10.1088/0256-307x/31/7/073401

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The temperature of the remaining cold 87 Rb atoms confined in a vapor cell magneto-optical trap after two-step photoionization has been measured. In the two-step photoionization process, the first excitation laser is served by the cooling laser and the second excitation laser is served by a continuous semiconductor laser with a wavelength of 450 nm. The results show that the temperature of the remaining cold atoms decreases as the intensity of the second excitation laser increases. Moreover, the relationship between the temperature T and number N of the remaining cold atoms generally follows a power law, while it deviates from the well-known T ∝ N 1/3 and the power factor is smaller than 1/3. We propose that ion-atom collisions occurring during a photoionization process strongly influence the temperature scaling law in an optically dense magneto-optical trap in the presence of an ionization laser. In addition, the forced evaporative cooling due to the combined effect of the detuning of the first excitation laser and the two-step photoionization plays a role in cooling the remaining cold atoms and results in the dependence of the power factor on the detuning of the first excitation laser.
The temperature of the remaining trapped 87 Rb atoms gradually decreases as the intensity of the ionizing lasers increases. And the relationship between the temperture and atom number of the remaining 87 Rb atoms follows an exponential law. This is mainly due to that the photoionized atoms in the state of 5 2 P 3/2 have higher velocity caused by the detuning of cooling laser, and they escape from the trap after becoming ions. In addition, ion-atom interaction may play a role at a higher ion density.
Evolution of the number and temperature of the remaining cold atoms in CW-laser photoionization of laser-cooled 87^{87}87Rb atoms
Based on the Rb+^++-Rb hybrid trap, we investigate the effect of ion-atom elastic collisions on the number and temperature of the remaining atoms. We measured the remaining atomic number and temperature as a function of the wavelength and intensity of the ionization laser, and whether the ion trap was turned on. Fittings with a single exponential decay function plus an offset to the number and radius of the remaining atoms are found to be in good agreement. We found a difference in the exponential factor of different wavelengths of ionization laser with the ion trap on or off. We suppose that the presence of electrons affects ion-atom collisions through disorder-induced heating. Our research contributes to a better understanding of how ultracold neutral plasma evolves, particularly the subsequent kinetics of atomic processes, which also serves as a useful reference for high-energy-density plasma.
Photoionization of a cold atomic sample offers intriguing possibilities for observing collective effects at extremely low temperatures. Irradiation of a rubidium condensate and of cold rubidium atoms within a magneto-optical trap (MOT) with laser pulses ionizing through one-photon and two-photon absorption processes was performed. Losses and modifications in the density profile of the remaining trapped cold cloud or the remaining condensate sample were examined as functions of the ionizing laser parameters. Ionization cross sections were measured for atoms in a MOT, while in magnetic traps losses larger than those expected for ionization process were measured.
We report the measurement of the absolute photoionization cross section for the 5P 3/2 state of 87 Rb at wavelength of 473 nm, which results in the photoelectron energies of 33 meV above the ionization threshold, using cold atoms confined in a vapor-loaded magneto-optical trap. The 87 Rb 5P 3/2 photoionization cross section at 473 nm is determined to be σ PI = 10.5 ± 2.2 Mb. Considering the spatial distribution of the trapped atoms, the average intensity of the ionization laser seen by an atom in the MOT instead of ionizing laser intensity I PI is used in our calculations for the photoionization cross sections. The excited state fraction is also accurately estimated using the latest experimental result.
This paper presents an experimental demonstration of light-induced evaporative cooling in a magneto-optical trap. An additional laser is used to interact with atoms at the edge of the atomic cloud in the trap. These atoms get an additional force and evaporated away from the trap by both the magnetic field and laser fields. The remaining atoms have lower kinetic energy and thus are cooled. It reports the measurements on the temperature and atomic number after the evaporative cooling with different parameters including the distance between the laser and the centre of the atomic cloud, the detuning, the intensity. The results show that the light-induced evaporative cooling is a way to generate an ultra-cold atom source.