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Annual Meeting Optical Society of America

Vorticity and suppressed heating in optical molasses

Luís Gustavo Marcassa, Marcos Oriá, Gregory I. Surdutovich, Gregory P. Lafyatis, Sérgio Carlos Zílio, Vanderlei Salvador Bagnato

Published Jan 1, 1992• 0 citations
Publisher

Abstract

We present results of numerical simulation that demonstrated the existence of long-lived optical molasses (super-molasses). The model is based on the presence of a vortex force, which is mainly due to the misalignment and the nonuniform transverse profiles of the laser beams. A computer simulation shows an increase in the molasses storage time, which qualitatively agrees with previous experimental observations. A few different aspects and explanations arising from this model are discussed.

DOI
10.1364/oam.1992.thgg3

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We present results of numerical simulation that demonstrated the existence of long-lived optical molasses (super-molasses). The model is based on the presence of a vortex force, which is mainly due to the misalignment and the nonuniform transverse profiles of the laser beams. A computer simulation shows an increase in the molasses storage time, which qualitatively agrees with previous experimental observations. A few different aspects and explanations arising from this model are discussed.
Recent theoretical investigations have indicated that rapid optical cycling should be feasible in complex polyatomic molecules with diverse constituents, geometries, and symmetries.However, as a composite molecular mass grows, so does the required number of photon scattering events necessary to decelerate and confine molecular beams using laser light.Utilizing coherent momentum exchange between light fields and molecules can suppress spontaneous emission and significantly reduce experimental complexity for slowing and trapping.Working with BaH as a test species, we have identified a robust, experimentally viable configuration to achieve large molasses-like cooling forces for molecules using polychromatic optical fields addressing both X -A and X -B electronic transitions, simultaneously.Using numerical solutions of the time-dependent density matrix as well as Monte Carlo simulations, we demonstrate that creation of suppressed emission rate (SupER) molasses with large capture velocities (∼40 m/s) is generically feasible for polyatomic molecules of increasing complexity that have an optical cycling center.Proposed SupER molasses are anticipated to not only extend quantum control to novel molecular species with abundant vibrational decay channels, but also significantly increase trapped densities for previously laser-cooled diatomic and triatomic species.
We present an analytical treatment of the force on a two-level atom interacting with a three-dimensional optical molasses. We show that, for small misalignment of the molasses light fields, corresponding to the so-called supermolasses configuration, there can be a dynamically induced stabilization of the atomic motion. This stabilization increases the time required for the atom to diffuse out of the molasses region and therefore provides a novel explanation of experimental observations. We describe the stabilization effect by analogy with the stabilization of the Kapitza pendulum, which is subject to a harmonic modulation of the pendulum pivot point.
A theoretical analysis is given for laser cooling of a two-level atom with magnetic sublevels in the presence of polarization gradients. The optical Bloch equations for the multilevel system are solved numerically for four combinations of polarizations in one-dimensional optical molasses. The light-pressure force on the atom as given by a simple two-level theory is recovered in the absence of polarization gradients, whereas a spatial variation of the polarization is found to lead to a strong cooling force for slow atoms. The increased cooling force is responsible for the recent observations of atoms cooled in optical molasses to temperatures an order of magnitude below the Doppler limit.
We measured the temperature of Cs atoms in an optical molasses after a sudden intensity reduction of the cooling beams. The temperature exhibits an interesting temporal behaviour in that the decay follows a series of exponential curves which can include a few damped oscillations. We present the results of a one-dimensional model which reproduces the features observed experimentally. The complex decay of the temperature is understood as a result of frictional forces in the molasses and collective oscillatory motion of the atoms. This is interpreted as showing a spatial localization of the atoms in the optical quasi-lattice formed in a six-beam molasses.
We have studied the expansion of a bright rubidium three-dimensional optical molasses realized with an intense laser light. In this regime, in addition to the laser friction force, the atoms strongly repel each other due to the radiation trapping force, which is analogous to the Coulomb force. The experimental observations show that the cloud volume increases linearly in time and that, simultaneously, the cloud temperature decreases following the law $(1+t/\ensuremath{\tau}{)}^{\ensuremath{-}4/3}.$ These time evolutions are explained using a physical picture of Coulomb explosion in a viscous medium.
Using six optical beams (+ and − directions on three orthogonal axes) tuned slightly to the red of resonance it is possible to cool sodium atoms to a few hundred microkelvin. The cooling occurs because the laser radiation pressure creates a damping force which reduces the atomic velocities to a rms value of ~60 cm/s, determined by random scattering. In this optical molasses, the laser beams propagate in all directions and the light acts as a viscous photon fluid which damps the atomic velocity. The resulting motion of the sodium atoms is diffusive until the atoms reach the edge of the laser beams where they escape. The process of atomic confinement by optical molasses can be modeled as a random walk in a viscous medium with a boundary. For a spherical volume of 0.2 cm3 the resulting confinement time is ~0.1 s for half of the atoms to escape. Direct time-of-flight measurement of the atomic temperature gives ~240 μK, which agrees with predictions of quantum limited cooling by resonance radiation pressure. With confinement times of 0.1 s and temperatures of ~240 μK this method provides a source of atoms sufficiently slow for even the highest resolution optical spectroscopy, collision studies, or optical traps.