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Journal of Physics B Atomic Molecular and Optical Physics

The optical piston in a capillary cell with open ends

H. G. C. Werij, Gerard Nienhuis, J. P. Woerdman

Published Sep 28, 1988• 2 citations
Publisher

Abstract

The authors present measurements of the time evolution of the spatial density distribution of Na atoms in an argon buffer gas, inside a capillary cell with open ends. The entrance of the capillary is irradiated with a laser beam that fills its cross section, so that light-induced drift occurs. For sufficiently high intensity, a steep front may build up, which in some cases runs backwards, against the propagation direction of the light, and against the drift velocity. This happens in particular when the initial optical thickness is just above unity. The authors also study the analytical and numerical solution of the macroscopic equations for the optical piston. The steady-state distribution is found to display a sharp variation when the initial optical thickness passes the value unity. They find satisfactory agreement with the measurements.

DOI
10.1088/0953-4075/21/18/015

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Modeling of the optical piston: Density-wave shape and resonance-fluorescence effects

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We present results of a numerical model of the optical piston driven by laser-induced drift (LID). The model is based on a drift-velocity formulation of Lawandy, derived from an asymmetric-random-walk argument, and includes resonance-fluorescence effects. The model is applied primarily to the experimental conditions of H. G. C. Werij and co-workers [Phys. Rev. A 33, 3270 (1986)]. It is shown that both the velocity and the shape of the propagating density wave change continuously throughout the traverse of a (finite) diffusion tube under these conditions. In addition, we have numerically verified the contention of G. Nienhuis [Phys. Rev. A 31, 1636 (1985)] that a ``self-preserving'' form of the density wave is indeed possible for the case of a ``finite number of absorbers in an infinite medium,'' both with and without saturation. Finally, we show that resonance fluorescence slows the piston velocity, broadens the density distribution, and decreases its peak height. It is concluded that resonance fluorescence can play a significant role in determining the time evolution and detailed shape of the optical-piston density distribution in sodium.

Physical Review A•Sep 1, 1988•1 citation
The authors present measurements of the time evolution of the spatial density distribution of Na atoms in an argon buffer gas, inside a capillary cell with open ends. The entrance of the capillary is irradiated with a laser beam that fills its cross section, so that light-induced drift occurs. For sufficiently high intensity, a steep front may build up, which in some cases runs backwards, against the propagation direction of the light, and against the drift velocity. This happens in particular when the initial optical thickness is just above unity. The authors also study the analytical and numerical solution of the macroscopic equations for the optical piston. The steady-state distribution is found to display a sharp variation when the initial optical thickness passes the value unity. They find satisfactory agreement with the measurements.
Na atoms in a buffer gas are compressed to a steep front by a laser beam tuned into the red Doppler wing of the Na DDD transition. The Na densities on either side of the front differ by a factor of 500. The front formation is due to a laser-induced drift which is a consequence of the state dependence of the diffusion cross sections involved. The Na drift velocity is deduced to be \ensuremath{\sim} 4 m/s.
We report an experimental and theoretical study of the optical piston, a consequence of the phenomenon of light-induced drift (LID), observed in binary mixtures of Na vapor and a buffer gas. Good agreement has been found between experiment and existing theory as far as the shape of the Na density profile in the piston is concerned. The piston velocity is anomalously low; this is accounted for by incorporating the effects of adsorption and desorption into the theory. Furthermore, we introduce a rate-equation model for LID which incorporates the fine and hyperfine structure of the level scheme of the Na absorbers; this gives insight into the sublevel velocity distributions and the nonequilibrium part of the total velocity distribution during LID.
In the past, interpretation of light-induced drift (LID) effects in alkali–noble gas mixtures in capillary cells has been severely complicated by surface interactions. As an example, the velocity of the optical piston, a spectacular manifestation of LID, is limited by adsorption–desorption processes to a value in the mm/s range.1 Recently, a breakthrough has been made: by coating the capillary wall with paraffin the surface interaction has been completely eliminated leading to transport velocities in the m/s range and allowing a study of LID in its pure form.2 Here we report the first measurements of the detuning and intensity dependence of the drift velocity in Na–noble gas mixtures and compare these to theory.
Light-induced drift arises when optically excited atoms with a Doppler-selected velocity suffer a diffusive friction that is different from that of ground-state atoms. When the atoms form an optically dense system, this drift can cause a piston effect, which sweeps the atoms in the propagation direction of the field. We derive coupled differential equations for the density of atoms and the light intensity, with an explicit expression for the drift velocity in terms of easily accessible parameters. Solutions of these equations are obtained for the stationary state in a closed cell. The density at the dark end of the cell, considered as the response to the incident-light intensity as its driving force, exhibits a discontinuity resembling a second-order phase transition for an optically dense system. We also obtain solutions with a soliton character, describing a localized density packet moving at uniform speed.
We present results of a numerical model of the optical piston driven by laser-induced drift (LID). The model is based on a drift-velocity formulation of Lawandy, derived from an asymmetric-random-walk argument, and includes resonance-fluorescence effects. The model is applied primarily to the experimental conditions of H. G. C. Werij and co-workers [Phys. Rev. A 33, 3270 (1986)]. It is shown that both the velocity and the shape of the propagating density wave change continuously throughout the traverse of a (finite) diffusion tube under these conditions. In addition, we have numerically verified the contention of G. Nienhuis [Phys. Rev. A 31, 1636 (1985)] that a ``self-preserving'' form of the density wave is indeed possible for the case of a ``finite number of absorbers in an infinite medium,'' both with and without saturation. Finally, we show that resonance fluorescence slows the piston velocity, broadens the density distribution, and decreases its peak height. It is concluded that resonance fluorescence can play a significant role in determining the time evolution and detailed shape of the optical-piston density distribution in sodium.