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arXiv:1505.01567

High-brightness ultra-cold metastable neon-beam

Fujio Shimizu

Published May 7, 2015• 0 citations
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Abstract

This paper presents detailed characteristics of an ultra-cold bright metastable neon atomic beam which we have been using for atom-interferometric applications. The basis of the device is an atomic beam released from a magneto-optical trap (MOT) which is operated with a high intensity trapping laser, high magnetic quadrupole field, and large laser detuining. Mainly due to the complex structure of three dimensional magnetic field and laser beams, a bright small spot of atoms is formed near the center of the quadrupole magnetic field under an appropriate operating condition. We obtained the minimum trap diameter of 50 micron meter, the atomic density nearly 10^{13}cm^{-3}, and the atomic temperature slightly less than the Doppler limited temperature of 200 micro-K. By releasing trapped atoms we obtained an bright cold atomic beam which is not far from the collision limited atomic density.

DOI
10.48550/arxiv.1505.01567

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This paper presents detailed characteristics of an ultra-cold bright metastable neon atomic beam which we have been using for atom-interferometric applications. The basis of the device is an atomic beam released from a magneto-optical trap (MOT) which is operated with a high intensity trapping laser, high magnetic quadrupole field, and large laser detuining. Mainly due to the complex structure of three dimensional magnetic field and laser beams, a bright small spot of atoms is formed near the center of the quadrupole magnetic field under an appropriate operating condition. We obtained the minimum trap diameter of 50 micron meter, the atomic density nearly 10^{13}cm^{-3}, and the atomic temperature slightly less than the Doppler limited temperature of 200 micro-K. By releasing trapped atoms we obtained an bright cold atomic beam which is not far from the collision limited atomic density.
This thesis presents an in-depth study into the characterization and enhancement of a metastable neon laser cooled and trapped atomic beam. The apparatus consists of a standard Zeeman slowed atomic beam loaded into a magneto-optical trap and was designed for applications to electron scattering experiments and photoionization. The efficiency of the metastable neon atomic source was investigated to determine the ideal cathode type for maximum metastable production and optimal atomic beam velocity haracteristics. A series of characterization measurements were performed on the MOT, and the trap volume and population were investigated for a range of trapping and slowing laser intensities and detunings, together with the MOT and Zeeman slower magnetic fields. The volume measurements were compared to standard Doppler theory and it was found that the Doppler model inadequately explained the trap behaviour. It was found that the MOT population characteristics were governed by two processes: two-body losses that limit the trap population at high densities, and the efficiency of the atom capture process which limits the operational range of the MOT over the various parameters. The trap temperature was determined to be 1.3mK via a time-of-flight technique. This was nearly twice that predicted by Doppler theory and the lack of agreement once again suggests the inadequacies in the Doppler theory to correctly model the experiment. The application of the MOT to the photoionization cross-section measurement of the (2p53p)3D3 state of neon was investigated. The MOT decay technique was utilized to measure cross-section values of o351 = 2.9+0.2 -0.3 x 10 -18cm2 and o363 = 3.1 +0.3 -0.4 x 10-18cm2 at the wavelengths of 351nm and 363nm respectively. This is an increase in accuracy of around a factor of five from previous measurements and it was found that the results agreed well with the values predicted by current theories.
We have developed a ``beam brightener'' in which several different laser cooling methods produce a highly monochromatic, unidirectional and intense beam of metastable Ne(3s;^3P_2) atoms. The beam brightener consists of four main stages. From an exited neon source we capture in a collimating section 5× 10^11 Ne(3s)/s from which ≈ 50 % is slowed to 100 m/s in a Zeeman slower. The final velocity distribution shows a width of 3.3 m/s uc(RMS,) corresponding to a temperature of 12 mK. A fraction of ≈ 25 % of these cold atoms is captured by a magneto-optic compressor and molded into a 0.7 mm uc(FWHM) wide atomic beam that contains up to 5× 10^10 Ne(3s)/s. The resulting density is 1.1× 10^9 Ne(3s)/cm^3. In the final step, a Doppler cooler is used to minimize the divergence of the beam to 10 mrad. This value will further improve once the Doppler cooler is replaced by a sub-Doppler cooler. We plan to use this ``bright beam'' to investigate the ionization probabilities in Ne(3s)+Ne(3s) collisions. Emphasis will be on the suppression of ionization when the atoms are spin-polarized. This has important implications for the possibilities of using metastable atoms for BEC-type experiments. Present adress: NIST
We present an experimental study of the loading of a magneto-optical trap (MOT) from a brightened and slowed beam of metastable neon atoms. The unprecedented high numbers of $9\ifmmode\times\else\texttimes\fi{}{10}^{920}\mathrm{Ne}$ and $3\ifmmode\times\else\texttimes\fi{}{10}^{9}$ 22Ne{}^{22}\mathrm{Ne}22Ne metastable atoms are trapped under unconventional trap conditions as compared to metastable helium traps, such as low intensity and small detuning. These cause the MOT to have an extraordinarily large volume on the order of 1cm31 {\mathrm{cm}}^{3}1cm3 and a typical peak density of 1010{10}^{10}1010 atoms/cm3.{\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}.atoms/cm3. A simple Doppler model is discussed which explains why the optimum is found under these conditions. The model includes the seventh beam necessary for the last slowing step before loading.
In this thesis, the study of cold gases of neon atoms in different metastable states is described. It contains measurements of the collisional parameters for both the 3s[3/2]2 and the 3s'[1/2]0 metastable state and the dependence of the inelastic loss on external fields. Furthermore, the investigation of frequency dependent laser-induced collisions, and the possibility to excite photoassociation resonances is presented. Based on previous measurements we have selected 22Ne for evaporative cooling. Although, we can experimentally achieve an increase in phase-space density with evaporative cooling, the relatively high inelastic collision parameters prevent the realization of a Bose-Einstein condensate in neon. For the measurements described here, neon atoms have been confined in a magneto-optical trap, in a magnetostatic trap, or in an optical dipole trap, respectively. By laser cooling inside the magnetic trap, atomic samples with more than 95 percent occupation of the magnetic substate m=+2 could be prepared. They have a typical temperature of 0.5 mK, central densities up to 1e11 cm^-3, and a central phase-space density of up to 2.2e-7. After loading the optical dipole trap from the magnetic trap, 2.5e6 atoms with typical temperatures of 0.1 mK, and central densities up to 5e10 cm^-3 were trapped. By evaporative cooling of the atoms in the magnetic trap we could increase the phase-space density by a factor of 200 to 5e-5. Also simulations of optimized evaporation for our experimental parameters show clearly that we are limited to a phase-space density on the order of 1e-5. From these simulations it became clear that a 5-fold increase in the good-to-bad ratio for evaporative cooling suffices to reach the quantum degenerate regime. Investigating the frequency dependence of laser-induced collisions did not reveal an experimental signature for the excitation of photoassociation resonances. For the 3D3 line a frequency dependence of laser enhanced Penning ionization was observed, which is interesting in itself. The absence of the collisional enhancement effect by laser light for the transition to the 3D2 line is intriguing and for an explanation calculations are required. Measurement of the two-body loss coefficient as function of the magnetic field showed a field dependence of the inelastic loss. These losses increase towards both small and large offset fields. In the magnetic trap, we are limited to offset fields < 50 G. In this range of fields, the two-body losses are too large to achieve a Bose-Einstein condensate of magnetically trapped metastable neon. The implementation of an optical dipole trap allowed us to trap the 3P0 metastable state. From the trap loss measurements we determined the two-body loss coefficient of the 3P0 metastable state for both bosonic isotopes 20Ne and 22Ne. For 20Ne we obtained s=6(+5,-4)e-10 cm^3/s and for 22Ne s=11(+7,-6)e-10 cm^3/s. These large two-body losses make it extremely unlikely to reach degeneracy with this metastable state. Nevertheless it is important that the 3P0 metastable state can be trapped to investigate other interesting physical effects. For example, it is essential to apply the STIRAP technique for trapped atoms and to realize the proposed coherent control of collisions. There is also a large interest in a precise determination of the lifetime of this metastable state, which is of importance for the verification of QED. We can also trap neon atoms in their energetically lowest magnetic substate 3P2(m=-2) with the perspective of reducing inelastic collisions in the energetically lowest state.
The calculated upper limit ${\mathcal{K}}_{i}^{\mathrm{pol}}<~{10}^{\ensuremath{-}14}{\mathrm{cm}}^{3}/\mathrm{s}$ of the rate constant for suppressed ionization in a gas of metastable Ne(3P2)\mathrm{Ne}{(}^{3}{\mathrm{P}}_{2})Ne(3P2​) atoms in the fully aligned ∣J=2,mJ=2〉|{J=2,m}_{J}=2〉∣J=2,mJ​=2〉 state is used as input to investigate the prospects for achieving Bose-Einstein condensation (BEC). The heating rate of the trap population by secondary collisions of the hot products of the process of ionization---i.e., ground-state atoms, ions, and dimer-ions---with cold trapped metastable atoms is discussed in terms of a semiclassical model. An important step lies in limiting the depth of the magnetic trap to a value of a few millikelvin, to limit the range of small-angle scattering that contributes to heating. Also, a tight radial confinement reduces the probability for secondary collisions. At a trap depth of 10 mK, a radial dimension of $3\ensuremath{\mu}\mathrm{m},$ and a density of $2\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{\ensuremath{-}3}$ the heating rate is $1.4\ensuremath{\mu}\mathrm{K}/\mathrm{s},$ which should be compared to the transition temperature to BEC of $0.6\ensuremath{\mu}\mathrm{K}.$ The collisional heating is dominated by ion---metastable-atom collisions, due to their long-range charge-induced dipole interaction. Keeping the evaporative cooling switched on at T=TC{T=T}_{C}T=TC​ reduces the heating a hundredfold. Using a bright beam of laser cooled neon atoms, an initial population of > 1010>~{10}^{10}> 1010 atoms can be loaded into a magneto-optical trap in one second. Tight magnetic traps are easy to achieve for metastable neon atoms, due to their magnetic moment of $3{\ensuremath{\mu}}_{B}.$ We conclude that achieving BEC is feasible for metastable neon. This also holds for triplet metastable helium, once the loading rate of traps has been improved. Finally, the semiclassical model used for calculating the heating rate is applicable to a wide range of inelastic collisions in trapped alkali gases and/or collisions with background gas.