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Journal of Optics B Quantum and Semiclassical Optics

Quasiclassical laser cooling in an intense standing wave

Прудников Олег Николаевич, E. Arimondo

Published Jun 9, 2004• 12 citations
Publisher

Abstract

We study the atom focusing realized through the interaction with an intense standing light wave. We use a numerical integration scheme taking into account correctly the large spatial gradient of the force and of the diffusion coefficient in an intense laser field. We find that for a small laser detuning and close to a minimum of the optical potential the Doppler force repels the atom from the minimum, leading to an increase of the spatial width of the localized atomic fraction.

DOI
10.1088/1464-4266/6/7/011

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We study the atom focusing realized through the interaction with an intense standing light wave. We use a numerical integration scheme taking into account correctly the large spatial gradient of the force and of the diffusion coefficient in an intense laser field. We find that for a small laser detuning and close to a minimum of the optical potential the Doppler force repels the atom from the minimum, leading to an increase of the spatial width of the localized atomic fraction.
We have investigated the localization of atoms in two configurations of laser fields: monochromatic and bichromatic standing waves. A quasi resonant monochromatic laser field creates a spatially periodic potential field with period equal to half of the wavelength. If the atomic kinetic energy is less than the height of the potential wells then atoms could be trapped in the wells. The motion of atoms in a standing wave is governed by the gradient force, friction force, and momentum diffusion. We studied one dimensional localization of sodium atoms in such a standing wave, as well as the influence of friction and diffusion on the motion of these atoms. The effect of accumulation of atoms at the bottom of potential wells (nodes of a standing wave) was observed. This leads to a new scheme of laser cooling of atoms with a minimum temperature below the Doppler limit.
The effect of the dipole force and its fluctuation on the motion of Li atoms in an intense, one-dimensional, near-resonant standing light wave has been investigated, both theoretically and experimentally. The duration of the interaction of the atoms with the standing wave was varied from several tens of spontaneous emission lifetimes to several hundred. For a standing wave frequency blue-detuned from resonance, diffusive heating can dominate the time-averaged dissipative dipole force so that there is no steady-state momentum distribution. However, for sufficiently large blue detunings the rate of diffusion is so slow that the resulting distribution approaches a quasi-steady state. For red detunings the diffusion is balanced with the force and a true steady-state is achieved. A Monte Carlo method based on the density matrix equations in the dressed-state representation is applied to simulate the atomic motion. The dynamics of atom channeling is studied. One of the important applications of this investigation is adiabatic laser cooling of atoms. Lithium atoms channeled in the nodes of an intense standing wave radiation field are cooled to near the recoil limit, a single photon's momentum hbark\\hbar{k}hbark, by adiabatically reducing the radiation intensity. The final momentum distribution has a narrow component with a root mean squared momentum of 2 hbark\\hbar{k}hbark in one-dimension. The data are compared with the results obtained by the Monte Carlo method. This process may be useful for cooling and increasing the phase-space density of atoms confined in a magnetic trap.
A laser cooling scheme for atoms with an effective three-level \ensuremath{\Lambda} configuration is presented. The cooling force is obtained by using two standing light waves, each interacting only with one of the two atomic transitions and both detuned red with respect to the transition. For a given laser power, the force exceeds the largest force on a two-level atom by a factor of 2 or more and the atomic velocity capture range is at the same time significantly increased. The spatially averaged force as well as the spatially averaged momentum diffusion coefficient, as a function of the atomic velocity, is calculated. The semiclassical steady-state momentum distributions are presented for some specific cases and an experimental realization of the cooling scheme is proposed for the case of metastable helium.
An atom moving in a strong standing wave radiation field may be translationally cooled by the induced or dipole force.1 The dipole force is proportional to I1/2, where I is the laser intensity, whereas the Doppler (usual) cooling force saturates with increasing I. Since laser sources can provide intensities which are much greater than the saturation intensities of many atomic transitions, cooling via the dipole force can be much more efficient than Doppler cooling. The dipole force has been used to cool the transverse2,3 and the longitudinal4 velocities of atoms in an atomic beam.
We present a general theoretical description of laser cooling of three-level atoms in a Λ configuration in two standing waves. Detunings, intensity, and spatial phase difference of the two waves are arbitrary. We demonstrate that, for a zero spatial phase shift, sub-Doppler cooling requires equal detunings. The basic mechanism is then coherent population trapping. For different detunings sub-Doppler cooling occurs only when the spatial phases of the two standing waves are different. This effect can be traced back to a mechanism of the Sisyphus type. These schemes of sub-Doppler cooling require neither polarization gradients nor magnetic fields.
The focusing of an atomic beam with the use of a two-dimensional magneto-optical trap in order to increase the number of atoms in the region of their laser cooling and localization near an atom chip is discussed. Two regimes of the interaction of atoms with a focusing laser field are considered: (i) the Doppler interaction regime, which occurs at small detunings of the laser field from the atomic resonance, and (ii) the sub-Doppler interaction regime, which occurs at large detunings of the laser field from the atomic resonance. The efficiency of focusing in the first case is low because of the momentum diffusion. It has been shown that the momentum diffusion in the sub-Doppler cooling mechanism is insignificant and, as a result, the broadening of the transverse velocity distribution of atoms is small. The sharp focusing of the atomic beam is possible in this interaction regime.