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CRC Press eBooks

Collimation of Atomic Beam

V. I. Balykin, Vladilen S Letokhov

Published Apr 23, 2020• 0 citations
Publisher

Abstract

The need to collimate a beam of particles arises whenever one has to increase their phase space density (the goal is to compress a spatial and a velocity distributions of the atoms) and to increase a spatial coherence of atomic beam. The method of increasing the spatial coherence by using special scheme of collimation is only one so far known for atomic beam. There is optimal laser detuning when the atomic beam collimation is maximum. As the detuning is decreased, the degree of collimation drops drastically. Near zero detuning the broadening of the atomic beam can be clearly seen, and its intensity is decreased. The collimation of atomic beam by the retarding force was first observed with a cesium than with sodium atoms. The collimation of atomic beams by polarization gradient force was demonstrated as in one-dimensional and two-dimensional cases.

DOI
10.1201/9780429333378-3

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The need to collimate a beam of particles arises whenever one has to increase their phase space density (the goal is to compress a spatial and a velocity distributions of the atoms) and to increase a spatial coherence of atomic beam. The method of increasing the spatial coherence by using special scheme of collimation is only one so far known for atomic beam. There is optimal laser detuning when the atomic beam collimation is maximum. As the detuning is decreased, the degree of collimation drops drastically. Near zero detuning the broadening of the atomic beam can be clearly seen, and its intensity is decreased. The collimation of atomic beam by the retarding force was first observed with a cesium than with sodium atoms. The collimation of atomic beams by polarization gradient force was demonstrated as in one-dimensional and two-dimensional cases.
Laser collimation of a Li atomic beam has been studied to create an intense collimated atomic beam. The gain of Li atomic intensity in the beam has been measured as functions of laser collimation parameters, such as laser-atom interaction length, laser-frequency detuning and laser power density. The Li intensity gain depends non-linearly on the laser-atom interaction length due to the longitudinal velocity distribution of the thermal Li beam (550°C). In the high laser power regime, the dependence of the Li intensity gain on the laser power and detuning disagrees with the theoretical prediction of the scattering force. These findings are explained by considering the saturation broadening of the Li absorption line. By optimizing the laser collimation parameters, 3.2-fold increase of the atomic intensity is attained for the collimated Li beam with the transverse velocity spread of±5m/s.
This paper presents the results of experimental and theoretical investigations on the control of atomic beam motion by light pressure of laser radiation. Collimation, focusing, and compression of an atomic beam are considered. Collimation of an atomic beam was achieved under the interaction of laser radiation with atoms when the light pressure force depended only on the atoms’ velocity. A similar regime of atomic beam interaction with radiation was performed under transversal irradiation of a beam by the axisymmetric field. The latter was formed by laser radiation reflected from the conical mirror surface of a reflecting axicon. The axis of an atomic beam coincided with that of the axicon. The collimation regime was reached under negative detuning of the laser radiation frequency from that of the atomic transition one by a value equal to a few homogeneous widths. Under positive detuning by the same value beam decollimation was observed.
We have studied laser collimation of a chromium atomic beam using a transverse polarization gradient cooling scheme. We present detailed measurements of the angular distribution of atoms on the beam axis, over a broad range of laser intensities and detunings, including those that produce significant excitation, and observe collimation angles as small as 0.16\ifmmode\pm\else\textpm\fi{}0.01 mrad (50% quantile). We compare our results with existing calculations based on assumptions of steady-state conditions and low excited-state population.
In the early 1970s, various researchers showed that the force exerted by light on an atomic beam could deflect the beam. In particular, traveling wave light perpendicular to the beam could be used to impart velocity along the wave vector k of the light.1,2 In these experiments, the angular deflection of an atom depended on the interaction time between the atom and field and thus on the longitudinal velocity of the atoms; consequently, the resulting atomic beam had an increased angular divergence. Other experiments have demonstrated that light could be used to collimate an atomic beam.3 In this experiment we show that a 1-D spontaneous optical molasses nearly perpendicular to an atomic beam can be used to both collimate and angularly deflect an atomic beam, where the deflection is insensitive to longitudinal velocity. In fact, this technique decreases rather than increases the angular divergence of the initial beam. In our experiment we started with a 300°C thermal sodium atomic beam with an angular divergence of ~10 mrad. The light was detuned by Δ ≈ − 20 MHz from the 32S1/2 → 32P3/2 transition in sodium, which corresponds to twice the natural linewidth Γ. Using 1-D molasses formed from counterpropagating light beams of the 1/e waist size of the order of 3 mm, we were able to produce an atomic beam with an ~l-mrad divergence and an angular deflection of >10 mrad. The maximum possible angular deflection θ is of the order.
We have laser collimated a gallium atomic beam in one dimension using the linear-perpendicular-linear polarization gradient technique. Operating on the cycling $4{p}^{\phantom{\rule{0.2em}{0ex}}2}{P}_{3∕2}(F=3)\ensuremath{\rightarrow}4{d}^{\phantom{\rule{0.3em}{0ex}}2}{D}_{5∕2}(F=4)$ transition at 294.45nm294.45\phantom{\rule{0.3em}{0ex}}\text{nm}294.45nm, the full angular divergence of the atomic beam was reduced to 0.3mrad0.3\phantom{\rule{0.3em}{0ex}}\text{mrad}0.3mrad, corresponding to a transverse velocity of $\ifmmode\pm\else\textpm\fi{}11\phantom{\rule{0.3em}{0ex}}\text{cm}∕\mathrm{s}$, about one-half the Doppler cooling limit. The dependence of the cooling efficiency on the laser detuning and power was investigated. Optical pumping of the atoms out of the 4p2P3∕2(F=3)4{p}^{\phantom{\rule{0.2em}{0ex}}2}{P}_{3∕2}(F=3)4p2P3∕2​(F=3) state by the cooling laser was observed. Repumping schemes were investigated and found to successfully repopulate the cooled F=3F=3F=3 hyperfine state.
We present the results of an experimental investigation of the control of atomic beam motion by the light pressure of laser radiation. Collimation, focusing, and reflection of the atomic beam are considered. Collimation of the atomic beam is achieved by the interaction of laser radiation with atoms, when the light pressure force depends only on the atom’s velocity. A similar regime of atomic beam interaction with radiation was performed with transversal irradiation of a beam by the axis-symmetrical field. The axis-symmetrical field was formed by laser radiation reflected from the conical mirror surface of a reflecting axicon. The axis of the atomic beam coincided with that of the axicon. The collimation regime was reached under negative detuning of the laser radiation frequency from the atomic transition frequency by a value equal to several homogeneous widths. With positive detuning by the same value the regime of beam decollimation was observed.