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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Laser cooling of atoms and the study of ultracold atomic collisions

Vanderlei Salvador Bagnato, Luís Gustavo Marcassa

Published Jul 6, 1999• 0 citations
Publisher

Abstract

We present an overview of recent experiments we have done concerning the observation of photoassociative ionization in a cold sample of trapped sodium atoms. In a second stage, the process is used as a prototype to demonstrate the possibility of optical manipulation of interatomic collisions. This effect could be explored to manipulate the thermodynamical properties of a sample of degenerated quantum gas.

DOI
10.1117/12.358421

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We present an overview of recent experiments we have done concerning the observation of photoassociative ionization in a cold sample of trapped sodium atoms. In a second stage, the process is used as a prototype to demonstrate the possibility of optical manipulation of interatomic collisions. This effect could be explored to manipulate the thermodynamical properties of a sample of degenerated quantum gas.
Recent advances in laser cooling techniques have made possible a new type of spectroscopy. In the first experiments of this kind alkali atoms are laser cooled to temperatures around 1 mK and trapped, using laser fields, at densities around 1011 cm−3. Atoms colliding under such ultracold conditions can be resonantly photoassociated to bound states of the dimer molecules. These molecules can in turn be photoionized, providing a sensitive detection of the photoassociation spectrum and revealing in extremely high resolution the structure of the dimer molecules. The particular advantages of this technique include high resolution and signal‐to‐noise and the ability to reach extremely long range states of the molecule, accessible only through such slow collisions.
The cold-molecule field concerns the physics and the applications of molecular systems with translational temperature well below the 1 К range. The possibility of controlling all the motion as well as the internal quantum state of a sample of molecules is a long-term goal that opens the possibility for many new experiments and measurements ranging from fundamental constants to quantum chemistry and quantum computation. Although, direct laser cooling of the translational degree of freedom of molecules is still waiting for further technological or theoretical breakthroughs and ideas, many different techniques have proven to be successful in producing different types of cold as well as ultracold (i.e. T < 1 mK) molecules. In this chapter, we will concentrate on the description of one of these techniques: the photoassociation of laser cooled atoms. We will report on the status of the art of this technique for the case of cesium atoms, describing all the main experimental findings. In particular, we will illustrate the different photoassociation schemes for molecule formation, the detection schemes through photoionization, the molecule trapping in a magnetic or dipolar trap, the vibrational cooling into a single vibrational state and finally the present prospects for rotational cooling.
In this paper we first focus on the methods developed to control the position and the velocity of atoms, taking advantage of the radiative forces exerted on atoms placed in a laser beam. Temperatures in the range of μK can be reached for dilute atomic clouds trapped under vacuum in a very small region of space. The application to fountain clocks based on cold cesium atoms is presented. We then describe the characterization and the main features of Bose‐Einstein condensates, a new state of matter of purely quantum origin, which can be obtained by subsequent evaporative cooling. The methods in use for cooling molecules are considered, in particular the collision processes or the photoassociation of cold atoms. The possibility of changing interactions between ultracold particles is also explained and photoassociation is illustrated by the recent experiments of our group dealing with metastable helium atoms.
Specific molecular mechanisms are proposed for associative ionization collisions of ultracold sodium atoms in a hybrid optical trap. When an intense, strongly detuned optical trap laser is on, the ionization rate is modulated by molecular bound-state resonances which are strongly affected by field dressing. When the weak, slightly detuned optical molasses lasers are on to provide cooling, an excitation mechanism which produces the two excited atoms at large internuclear separation in different hyperfine states accounts for the lower observed ionization rate.