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Europhysics Letters (EPL)

Magnetically Induced Laser Cooling for Ne*: Approaching the Recoil Limit

Maarten D. Hoogerland, Herman C. W. Beijerinck, K. A. H. van Leeuwen, P. van der Straten, H. Metcalf

Published Aug 15, 1992• 12 citations
Publisher

Abstract

Magnetically induced laser cooling to temperatures close to the recoil limit is investigated in one dimension. For a metastable neon beam, we present high-precision measurements investigating the actual temperature limit in this cooling process. Using time-of-flight techniques to reduce the effect of the longitudinal velocity spread, we observe cooling at small magnetic field toward ν = 0 with an r.m.s. width of the distribution of 5.4 cm/s, well below the Doppler limit. At a larger magnetic field (0.4 Gauss) the velocity-selective resonances are extremely sharp. Here we find the r.m.s. width of the distribution to be 3.4 cm/s, only 1.1 times the recoil speed ℏ k/M , corresponding to a temperature T = 2.7 μK.

DOI
10.1209/0295-5075/19/8/002

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Magnetically induced laser cooling to temperatures close to the recoil limit is investigated in one dimension. For a metastable neon beam, we present high-precision measurements investigating the actual temperature limit in this cooling process. Using time-of-flight techniques to reduce the effect of the longitudinal velocity spread, we observe cooling at small magnetic field toward ν = 0 with an r.m.s. width of the distribution of 5.4 cm/s, well below the Doppler limit. At a larger magnetic field (0.4 Gauss) the velocity-selective resonances are extremely sharp. Here we find the r.m.s. width of the distribution to be 3.4 cm/s, only 1.1 times the recoil speed ℏ k/M , corresponding to a temperature T = 2.7 μK.
We have observed magnetically induced laser cooling (MILC) on the J = 1 {yields} 2 component of the 2{sup 3}S {yields} 2{sup 3}P transition at {lambda} = 1.083 {mu}m in metastable helium (He*). At low magnetic field the measured widths of our atomic velocity distributions are as narrow as 2{Dirac_h}k/M corresponding to a temperature {approx_equal} 3 {mu}K (1-D Doppler limit {approx_equal} 26 {mu}K). At higher fields ({approximately}150 mG) the velocity distribution splits into two peaks of sub-Doppler width whose separation is proportional to the Zeeman splitting. Our measurements are consistent with model calculations based on the time evolution of quantum density matrices. Our experiments explore the transition regime between the quantum mechanical and semiclassical models of momentum exchange in atom-light interactions because of the large ratio (0.4) of the recoil velocity {Dirac_h}k/M to the rms of the Doppler-limited velocity distribution {radical}{Dirac_h}{gamma}/3M for this transition. We use a supersonic LN{sub 2} cooled nozzle source excited by a dc discharge for He*, and we excite the 2{sup 3}P state with a home-built diode-laser-pumped LNA laser.
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 report a new cooling scheme used to actively collimate an atomic beam of 85Rb^{85}\mathrm{Rb}85Rb with one-dimensional optical molasses. The atoms' transverse velocity is damped by a standing wave of circularly polarized light in a magnetic field of \ensuremath{\sim}20 \ensuremath{\mu}T to 2 cm/s (rms), well below the Doppler velocity vD{v}_{D}vD​=(7\ensuremath{\Elzxh}\ensuremath{\gamma}/20M)1/2{)}^{1/2})1/2\ensuremath{\sim}10 cm/s for one-dimensional optical molasses. The minimum measured velocity is limited only by the experimental geometry. We have developed a model in which the mixing by the field and optical pumping combine to form a cycle that cools below the Doppler limit, in agreement with our measurements.
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 propose a laser cooling mechanism based on velocity-selective double-dark resonance that leads to a temperature significantly lower than the one-photon recoil limit. This mechanism benefits from sharp and high-contrast spectra which are induced by the two electromagnetically induced transparency windows due to the interacting dark-state resonances. It is theoretically demonstrated that four-level atoms illuminated by two counter-propagating probe beams and two additional beams directed perpendicularly to the other two exhibit new cooling effects; for red detuned probe lasers, atoms can be subject to a strong viscous force with an extremely small diffusion, characteristic of heating caused by the stochastic nature of spontaneous emission processes. By quantum mechanical simulations, we then find that the lowest temperature in EIT windows approaches 1 nK for the case of mercury. However, the different broadening mechanisms can destroy the cooling, so that the lowest temperature can increase to the recoil temperature.
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.