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Physical Review Letters

Demonstration of a Semipermeable Optical Piston

H. G. C. Werij, J. P. Woerdman, J.J.M. Beenakker, Ivan Kuščer

Published Jun 18, 1984• 78 citations
Publisher

Abstract

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 $D$ 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.

DOI
10.1103/physrevlett.52.2237

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SEPARATION OF A GAS MIXTURE BY A SEMIPERMEABLE OPTICAL PISTON

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We have observed effective separation of Na vapor from a buffer gas using the principle of laser induced drift1. The basic idea is as follows. A travelling laser beam produces velocity-selective saturation of optically absorbing atoms, by means of the Doppler effect. This yields opposing fluxes of excited and unexcited atoms. Both fluxes feel different diffusion resistances in the buffer gas, since the collision cross sections σ* and σ of the absorbing atoms are generally state-dependent. This results in a net flux of the absorbing atoms. It should be noted that this mechanical action of the light field is completely different from (and much stronger than) the well known radiation pressure.2

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Light-induced drift of optically active atoms in a buffer gas is a consequence of the unequal diffusive frictions suffered by the excited and the ground-state atoms. This drift can be used to create an ``optical piston,'' in which the active atoms are pushed forward by light through the semipermeable membrane of the buffer gas. Normally, optical piston effect is studied when the active atoms are neutral in a confined one-dimensional situation. We present a detailed theory of this phenomenon when the active atoms are charged and a magnetic field is applied for ``tuning'' the direction as well as the magnitude of the drift, thus removing the necessity of confinement. Our study is in different geometries of the light beam and the magnetic field, first in the weak collision model, and then for the strong collision and the Boltzmann-Lorentz models.

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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.

Journal of the Optical Society of America B•Feb 1, 1987•0 citations
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 have observed effective separation of Na vapor from a buffer gas using the principle of laser induced drift1. The basic idea is as follows. A travelling laser beam produces velocity-selective saturation of optically absorbing atoms, by means of the Doppler effect. This yields opposing fluxes of excited and unexcited atoms. Both fluxes feel different diffusion resistances in the buffer gas, since the collision cross sections σ* and σ of the absorbing atoms are generally state-dependent. This results in a net flux of the absorbing atoms. It should be noted that this mechanical action of the light field is completely different from (and much stronger than) the well known radiation pressure.2
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.
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.
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.
Light-induced drift of optically active atoms in a buffer gas is a consequence of the unequal diffusive frictions suffered by the excited and the ground-state atoms. This drift can be used to create an ``optical piston,'' in which the active atoms are pushed forward by light through the semipermeable membrane of the buffer gas. Normally, optical piston effect is studied when the active atoms are neutral in a confined one-dimensional situation. We present a detailed theory of this phenomenon when the active atoms are charged and a magnetic field is applied for ``tuning'' the direction as well as the magnitude of the drift, thus removing the necessity of confinement. Our study is in different geometries of the light beam and the magnetic field, first in the weak collision model, and then for the strong collision and the Boltzmann-Lorentz models.
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.