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Conference on Lasers and Electro-Optics

Light-induced drift with low-power spectrally shaped lasers: efficient isotope separation and high drift velocities

E. R. Eliel, Alan D. Streater, J. P. Woerdman

Published Apr 25, 1988• 0 citations
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Abstract

The use of paraffin-coated cells has led to a breakthrough in the experimental study of light-induced drift (LID) of alkalis in noble gases. In recent experiments isotope separation of the two naturally occurring Rb isotopes through the LID effect has been achieved (enrichment of the rare isotope up to 95%), 1,2 and direct measurements of the drift velocity of Na in noble gases as a function of various experimental parameters have been made.3 In these experiments a single single-mode dye laser, tuned within the Doppler profile of the resonance line, was used to excite a specific velocity group of atoms (isotope selective in the case of Rb). Optical pumping between the ground-state hyperfine levels F = I+½ and F=I−½ plays a dominant (limiting) role in determining the magnitude of the LID effects by strongly depleting the resonantly excited hyperfine substate (of a specific isotope).

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EFFICIENT ISOTOPE SEPARATION USING SEMICONDUCTOR LASERS

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Recently separation of the two naturally occurring Rb isotopes (28% 87Rb and 72% 85Rb) has been achieved1 using the light-induced drift (LID) effect;2 enrichment up to 95% 87Rb has been observed. In those experiments a high-power (~ 150mW) single-frequency dye laser was used. In the case of single-frequency excitation such power is required since most of the Rb atoms are trapped in the non- resonant hyperfine (hf) ground level due to optical hf pumping.3 Here we demonstrate that the Rb isotopes can be separated equally well using a low-power (3.3 mW) AlGaAs diode laser which has strong relaxation oscillation sidebands. The presence of these sidebands greatly increases LID by eliminating optical hf pumping.

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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.

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We present a model for light-induced drift (LID) in the Na--noble-gas system which should enable direct comparison with experiment. In contrast to previous theories of LID based on a two-level description of the optical absorbers and on a simplified collision treatment, the present model is based on a realistic description of laser-driven Na atoms immersed in a buffer gas. Starting from the generalized Bloch equations, we introduce a rate-equation model for the velocity distributions in the four important Na levels. The velocity-changing and fine-structure-changing collisions are described using composite Keilson-Storer collision kernels in which all adjustable parameters have been eliminated by using available literature data. We apply the model in numerical calculations of LID as a function of all experimentally accessible parameters. It is found that the ground-state hyperfine splitting can have large effects on LID, whereas the excited-state fine-structure splitting has not. The paper establishes criteria for optimum LID effects; when using a single-frequency laser the maximum attainable drift velocity is predicted to be 13.8 m/s. Using a proper set of boundary conditions, we find a pressure dependence of LID qualitatively different from the predictions based on previous work. Finally, the influence of the collision model is investigated. We find that LID is independent of the shape of the collision kernel, indicating that a strong-collision model is always valid.

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The influence of the bandwidth of the radiation source on light-induced drift (LID) of Na is studied experimentally. Broadband radiation can be used to eliminate optical hyperfine pumping on the one hand, and to provide more efficient excitation due to an increased velocity coverage, on the other hand. These aspects are highlighted in two separate experiments. An increase of the drift velocity of Na by a factor of 4 compared to monochromatic excitation has been measured. A frequency-modulated (FM) ring dye laser is used as a broadband radiation source, having a bandwidth continuously variable from single mode to multimode with a bandwidth of 10 GHz. Contrary to passive multimode lasers, the spectrum of such a laser is well defined and stable. Various modulation frequencies are used to study the dependence of the drift velocity on the mode spacing of the multimode laser. Only small differences are found. All experimental results are compared with results of a four-level rate-equation model for LID of Na, in which the excited-state hyperfine structure of Na and the detailed shape of the FM spectrum are taken into account. Good agreement between the model and the experimental data is found. The model is also used to show that the FM spectrum yields almost the same values for the drift velocity as a rectangular spectrum, which so far has been considered optimal for LID.

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Alan D. Streater, Jaap J. Mooibroek, J. P. Woerdman

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Applied Physics Letters•Feb 22, 1988•25 citations
The use of paraffin-coated cells has led to a breakthrough in the experimental study of light-induced drift (LID) of alkalis in noble gases. In recent experiments isotope separation of the two naturally occurring Rb isotopes through the LID effect has been achieved (enrichment of the rare isotope up to 95%), 1,2 and direct measurements of the drift velocity of Na in noble gases as a function of various experimental parameters have been made.3 In these experiments a single single-mode dye laser, tuned within the Doppler profile of the resonance line, was used to excite a specific velocity group of atoms (isotope selective in the case of Rb). Optical pumping between the ground-state hyperfine levels F = I+½ and F=I−½ plays a dominant (limiting) role in determining the magnitude of the LID effects by strongly depleting the resonantly excited hyperfine substate (of a specific isotope).
Recently separation of the two naturally occurring Rb isotopes (28% 87Rb and 72% 85Rb) has been achieved1 using the light-induced drift (LID) effect;2 enrichment up to 95% 87Rb has been observed. In those experiments a high-power (~ 150mW) single-frequency dye laser was used. In the case of single-frequency excitation such power is required since most of the Rb atoms are trapped in the non- resonant hyperfine (hf) ground level due to optical hf pumping.3 Here we demonstrate that the Rb isotopes can be separated equally well using a low-power (3.3 mW) AlGaAs diode laser which has strong relaxation oscillation sidebands. The presence of these sidebands greatly increases LID by eliminating optical hf pumping.
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 (LID) is a transport phenomenon in a gas mixture with dramatic manifestations such as the optical piston1 and applications in atomic isotope separation.2 The induced drift velocity is proportional to the excited state fraction, and for Na in Xe high drift velocities have been achieved using two lasers.3 Here one laser excites atoms from one hyperfine ground state and the second laser atoms from the other, avoiding hyperfine pumping. Excited in this way, the Na atom represents a Λ type three-level system, where coherent effects can be important.
We present a model for light-induced drift (LID) in the Na--noble-gas system which should enable direct comparison with experiment. In contrast to previous theories of LID based on a two-level description of the optical absorbers and on a simplified collision treatment, the present model is based on a realistic description of laser-driven Na atoms immersed in a buffer gas. Starting from the generalized Bloch equations, we introduce a rate-equation model for the velocity distributions in the four important Na levels. The velocity-changing and fine-structure-changing collisions are described using composite Keilson-Storer collision kernels in which all adjustable parameters have been eliminated by using available literature data. We apply the model in numerical calculations of LID as a function of all experimentally accessible parameters. It is found that the ground-state hyperfine splitting can have large effects on LID, whereas the excited-state fine-structure splitting has not. The paper establishes criteria for optimum LID effects; when using a single-frequency laser the maximum attainable drift velocity is predicted to be 13.8 m/s. Using a proper set of boundary conditions, we find a pressure dependence of LID qualitatively different from the predictions based on previous work. Finally, the influence of the collision model is investigated. We find that LID is independent of the shape of the collision kernel, indicating that a strong-collision model is always valid.
The influence of the bandwidth of the radiation source on light-induced drift (LID) of Na is studied experimentally. Broadband radiation can be used to eliminate optical hyperfine pumping on the one hand, and to provide more efficient excitation due to an increased velocity coverage, on the other hand. These aspects are highlighted in two separate experiments. An increase of the drift velocity of Na by a factor of 4 compared to monochromatic excitation has been measured. A frequency-modulated (FM) ring dye laser is used as a broadband radiation source, having a bandwidth continuously variable from single mode to multimode with a bandwidth of 10 GHz. Contrary to passive multimode lasers, the spectrum of such a laser is well defined and stable. Various modulation frequencies are used to study the dependence of the drift velocity on the mode spacing of the multimode laser. Only small differences are found. All experimental results are compared with results of a four-level rate-equation model for LID of Na, in which the excited-state hyperfine structure of Na and the detailed shape of the FM spectrum are taken into account. Good agreement between the model and the experimental data is found. The model is also used to show that the FM spectrum yields almost the same values for the drift velocity as a rectangular spectrum, which so far has been considered optimal for LID.