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Laser & Infrared

3-D simulation analysis of laser-collimating Cr atomic beam

HE Fe

Published Jan 1, 2015• 1 citation

Abstract

In order to acquire high collimating neutral Cr atomic beam,a laser collimating field is designed which is composed of three groups of laser beams to collimate high temperature Cr atomic beam. To reflect better the uncertainty of atomic motion in actual status and the influence of atomic isotopes on experiments,Monte Carlo random method is used to simulate the atomic initial state in the actual state. Based on stress distribution of atoms in the laser collimating field,motion characteristics of neutral Cr atomic beam in the actual state are quantitatively analyzed under the laser collimating field,and this motion path in the three-dimensional space and three-dimensional distribution of atomic landing point status are analyzed. The results show that the maximum divergency angle decreases to 1. 5 mrad in x direction and decreases to 1. 6 mrad in y direction after collimating Cr atomic beam,and atomic flux increases twice.Study results provide more detailed data for experiment and more ideal cold atomic beam for further more precise nanometer grating and nanodots.

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In order to acquire high collimating neutral Cr atomic beam,a laser collimating field is designed which is composed of three groups of laser beams to collimate high temperature Cr atomic beam. To reflect better the uncertainty of atomic motion in actual status and the influence of atomic isotopes on experiments,Monte Carlo random method is used to simulate the atomic initial state in the actual state. Based on stress distribution of atoms in the laser collimating field,motion characteristics of neutral Cr atomic beam in the actual state are quantitatively analyzed under the laser collimating field,and this motion path in the three-dimensional space and three-dimensional distribution of atomic landing point status are analyzed. The results show that the maximum divergency angle decreases to 1. 5 mrad in x direction and decreases to 1. 6 mrad in y direction after collimating Cr atomic beam,and atomic flux increases twice.Study results provide more detailed data for experiment and more ideal cold atomic beam for further more precise nanometer grating and nanodots.

In order to study the effect of isotopes except for 52Cr on the transverse position distribution of Cr atomic beam,simulation was carried out for 1-D transverse laser cooling with a three-aperture pre-collimating slit by means of Monte Carlo method.The results show that isotopes except for 52Cr can make the background increased and affect characteristics of each part of the atomic beam: the central maximum value decreases 9.5% and its full width at half maximum(FWHM)increases 2.9% in the center;while the maximum value increases 25% with little increase of the FWHM in either side.It concludes that the existence of the isotopes except for 52Cr has little effect on the center maximum value and FWHM of the center part atomic beam.

A well collimated atomic beam plays an important role in laser focused Cr atom deposition. The simulations of one-dimensional (1D) transverse laser cooling of Cr atomic beam is performed with the Monte Carlo method. Taking into account the stochastic initial conditions of each atom, isotopes other than 52Cr, longitudinal velocity distribution and transverse divergence, the atomic beam cooled by laser is optimized and evaluated. The obtained results are consistent with experimental. In addition, with picking out the trajectories of isotopes other than 52Cr that are not cooled by the laser the center maximum value can be reduced by 9.3%, and the FWHM is increased by 11% of the transverse distribution.

Transverse velocity distributions of Cr atomic beam truncated by a novel three-aperture pre-collimating slit, with and without one-dimensional transverse laser cooling, are presented. The simulations ware performed using Monte Carlo particle-optics approach, taking into account of the isotopes, distribution of longitudinal velocity and transverse divergence. The results showed that the atomic beam had a velocity distribution in the range of ±3m/s and ±1m/s, without and with laser cooling. Consequently, the transverse divergence is 0.77mrad and 0.27mrad, respectively. In addition, with an assumption that only52Cr atoms interact with the cooling laser, the existence of other isotopes will decrease the maximum value of the velocity, and increase the background.

The characteristics of neutral chromium atoms in the standing wave field are discussed. Based on a semi-classical model, the motion equation of neutral atoms in the laser standing wave field is analyzed, and the trajectories of the atoms are obtained by simulations with the different divergence angles of the atomic beam. The simulation results show that the full width at half maximum (FWHM) of the stripe is 2.75 nm and the contrast is 38.5: 1 when the divergence angle equals 0 mrad, the FWHM is 24.1 nm and the contrast is 6.8:1 when the divergence angle equals 0.2 mrad and the FWHMs are 58.6 and 137.8 nm, and the contrasts are 3.3: 1 and 1.6: 1 when the divergence angles equal 0.5 and 1.0 mrad, respectively.

Laser collimation of Cr atomic beam was studied using a transverse Doppler cooling scheme.Collimation occured for a laser detuned to the red side of the 7S3→7P04 transition.Many optimized parameters taken from repetitious experiments and calculations are given,and the misalignment between the probe beam was gotten and the collimation beam will lead to the artificial broadening of detected transverse linewidth.Finally the collimation result is got that the transverse divergence of Cr beam is 0.48 mrad,and the transverse temperature is 265 μK.

In order to study the 3-D features of Cr atoms in laser cooling field,the interaction process of Cr atom and laser cooling field was analyzed.Based on the semi-classical theory of Doppler and polarization gradient force,the fourth-order Runge-Kutta algorithm was adopted to simulate 3-D trajectory of Cr atoms,at the same time,three-dimension distribution of them was also obtained.After passing through laser cooling field,the divergence angle of Cr atomic beam becomes smaller,and the corresponding velocity also decreases.The results show that the three-dimension features of Cr atoms agree with the two-dimension cases,and can provide more comprehensive,richer cooling information.