Optical Trapping of Atoms

Several different proposals have been published on how optically-cooled atoms with small velocities may be trapped in three-dimensional light traps . Letokhov proposed [14.33,34] to use the potential minima of a three-dimensional standing optical field composed by the superposition of three perpendicular standing waves for spatial trapping of cooled atoms, whereas Ashkin and Gordon calculated [14.35] that the dispersion forces in focussed Gaussian beams (Sect. 14.16) could be employed for trapping atoms. 

The example demonstrates that the negative potential energy in the potential minima (the nodes of the standing wave for Aw 0) is very small. The atoms must be cooled to temperatures below 1 K before they can be trapped. 

Note that these optical trapping methods reduce the velocity components (v, Vy,v ) to a small interval around v = 0. However, they do not compress 

When an atom with the polarizability a is brought into an inhomogeneous electric field E, a dipole moment p = aE is induced and the force 

The polarizability O ( ) depends on the frequency co of the optical field. It is related to the refractive index n co) of a gas with the atomic density N (Vol. 1, Sect. 2.6.3) 

For Aco Ys the polarizability a(co) increases nearly linearly with the detuning Aco. From (9.24) and (9.26) it follows that in an intense laser beam (S 1) with the intensity / = eocE the force Fd on an induced atomic dipole is 

This reveals that in a homogeneous field (for example, an extended plane wave) V/ = 0 and the dipole force becomes zero. For a Gaussian beam with the beam waist w propagating in the z-direction, the intensity I (r) in the x-y-plane is, according to Vol. 1, (5.32) 

The intensity gradient VI = -(4rlw )I ir)f points into the radial direction and the dipole force Fq is then directed toward the axis r = 0 for Aco 0 and radially outwards for Aco 0. 

For Am 0 the z-axis of an intense Gaussian laser beam with 7(r = 0) = To represents a minimum of the potential energy 

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Miller J D, Cline R A and Heinzen D J 1993 Far-off-resonance optical trapping of atoms Phys.Rev. A 47 R4567-70... 

Walker T, Sesko D and Wieman C 1990 Collective behavior of optically trapped neutral atoms Phys.Rev.Lett. 64 408-11... 

In this section I hope to show how the sensitivity of laser spectroscopy is exploited to obtain data on very low concentrations of atoms. In particular I will start off by considering a few laser atomic beam studies aimed at measuring optical isotope shifts and show how short-lived nuclei can be studied in this way. I shall also mention how it is possible to beat the natural linewidth and obtain supernatural spectra . The discussion of laser studies at low atomic concentrations then leads me onto consider experiments on laser cooling and trapping of atoms and ions. In this context I will also mention some experiments using the shelved electron idea to detect very weak transitions. Finally, I will say something about Rydberg atoms and the effects of atoms near metallic surfaces. 

Optical Cooling and Trapping of Atoms Fig. 9.25 Potential energy of... 

A whole chapter is devoted to time-resolved spectroscopy including the generation and detection of ultrashort light pulses. The principles of coherent spectroscopy, which have found widespread applications, are covered in a separate chapter. The combination of laser spectroscopy and collision physics, which has given new impetus to the study and control of chemical reactions, has deserved an extra chapter. In addition, more space has been given to optical cooling and trapping of atoms and ions. 

After a brief review of our work on optical molasses, we will present schemes for laser trapping of atoms that we are trying. We ako present several schemes for laser cooling atoms to temperatures of 10 % and possibly to 10 Finally, the realization of ultra-cold atoms has opened up the potential for new experiments. We will briefly mention a few areas of research that can be addressed with these atoms. 

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