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par Julien Dugué - 1er août 2007

I) Recent photoassociation experiments

Production of giant helium dimers
Study of frequency shifts in photoassociation spectra : a new measurement of the s-wave scattering length
Two photon photoassociation using dark resonances for an accurate determination of the scattering length
Lifetime of exotic helium molecules

II) Work in progress

Transformation of the experiment
Crossed dipole trap and HELAT

I) Recent photoassociation experiments

We completed a photoassociation (PA) study in a gas of ultracold metastable helium atoms, just above the critical temperature of Bose-Einstein condensation. For PA we use a laser beam which frequency is detuned to the red of the 2³S₁-2³P_0,1,2 helium transition around 1083 nm. The detuning can be varied from a few MHz up to about 1.5 GHz in order to create ultracold molecules in the bound states of the S-P and S-S molecular potentials. The intensity of the PA pulse is weak enough in order not to destroy the sample through the light-assisted Penning ionization process. The molecular lines are observed optically, measuring the atom losses by absorption of a resonant probe beam.

Fig.1 - Experimental setup.

1 - Production of giant helium dimers

We observed a large number of molecular lines in the different S-P molecular potentials. Interesting results were obtained in the vicinity of the 2S-2P₀ transition where we observed 5 narrow lines in the purely long range potential 0_u⁺, the position of which was also calculated from the weakly attractive long-range potential C₃/R³. The minimum internuclear distance in these dimers is unprecedently large (150 a₀). Therefore, retardation clearly influences the dipole-dipole interactions.

Fig.2 - Resonances corresponding to the purely longe range molecular states.

2- Study of frequency shifts in photoassociation spectra : a new measurement of the s-wave scattering length

We studied frequency shifts of photoassociation lines under light irradiation. The photo-association laser excites a pair of spin polarized metastable helium atoms to a molecular bound state in the purely long range 0_u⁺ potential. The frequency displacements are proportional to the laser intensity.

Fig.3 - Two resonance curves obtained for two different intensities of the photoassociation laser, (a) : 9 mW/cm2, (b) : 5W/cm2 . The signal shows the temperature raise of the atomic cloud as a function of the frequency detuning of the photoassociation laser from the 2³S₁-2³P₀ atomic transition. The resonance peak (b) at high intensity is shifted ; the frequency shift is proportional to the laser intensity.

The frequency shifts arises from the optical coupling of the excited molecular state with the continuum of scattering states and the bound states of the two colliding 2³S₁ atoms. The comparison between the predicted and measured shifts leads to a value of the s-wave scattering length for the spin polarized helium atoms : a = 7.2 ± 0.6 nm, significantly lower than values previously measured by other methods.

Fig.4 - Ratio of the frequency shifts for photoassociation lines obtained with 2³S₁ helium atoms excited to v=0,1 and 2 molecular levels in the 0_u⁺ potential, plotted as a function of the value of the s-wave scattering length a . Curves are theoretical ; grey areas represent the experimental uncertainties ; (a) : with σ- polarization, (b) : with linear polarization. Comparison between theory and experiment provides : a = 7.2 ± 0.6 nm.

3- Two-photon photoassociation using dark resonances for an accurate determination of the scattering length

Using the value of a determined from the light shifts, we were able to make a much more precise measurement by a two-photon photoassociation experiment. We determined the position of the least bound state (v=14) in the interaction potential ⁵Σ_g⁺ between the two spin-polarized metastable atoms, using a 3-level scheme involving the excited molecular bound state v=0 of the potential 0_u⁺ .

Fig.5 - Principle of a two-photon photoassociation experiment. A first laser excites two free atoms towards a molecular bound state in the 0_u⁺ potential. This level is connected to the bound molecular state v = 14 of the ground potential by a second laser L2.

A very narrow atom-molecule dark resonance is observed, which position is related to the energy of the v=14 state. This energy depends on the s-wave scattering length and the following value of a can be deduced : a = 7.512 ± 0.005 nm.

Fig.6 - Two-photon photoassociation experiment, according to the scheme of Fig 4. Laser L2 is at fixed frequency, laser L1 is scanned. (a) : L2 is set close to resonance, with a large intensity. One can identify a double-peak structure corresponding to an Autler-Townes splitting. (b) : L2 is slightly off-resonance and a Raman peak appears with a Fano profile. (c) : L2 is again close to resonance but with a weak intensity. The narrow, central peak is attributed to an atom-molecule dark resonance.

4 - Lifetime of exotic helium molecules

Two-photon photoassociation experiments operating with ultracold helium atoms in the 2³S₁ metastable state allow us to prepare exotic helium molecules in the least vibrational bound state (v=14) of the ⁵Σ_g⁺ interaction potential. Information on the lifetime of these molecules are given from the linewidths of the dark-resonance and the Raman signals. Figure 6 (left) shows a typical Raman peak, obtained for a 10 MHz detuning of laser L2. The line shape of such signals was calculated and their width recorded as a function of our experimental parameters. Figure 6 (right) shows the width as a function of the temperature of the cloud. We finally obtained a lifetime for the exotic molecules of the order of 1.5 μs (S. Moal et al, Phys.Rev.A 75, 033415, 2007). We attempted to interpret this value in terms of Penning ionization induced by spin-dipole coupling. So far it significantly differs from two recent calculations (T. J. Beams et al, Phys.Rev.A 74, 014702, 2006 and M. Portier and G. V. Shlyapnikov, to be submitted to Phys.Rev.A). The limitation of the lifetime by atom-molecule inelastic collisions was investigated and ruled out.

Fig.7.

II) Work in progress

Since January 2006, our experiment undergoes a substantial reconstruction. The most important changes include :

Modifications of the ultra-high vacuum system (completed): We added 2 CF-gate valves and 2 turbo-molecular pumps were replaced. We also modified the geometry of the system to adapt to the demands of the HELAT project (see below).
Rebuilding of the optical setup (completed): The entire optical setup has been rebuilt using commercial optomechanics. The experimental installation is now entirely fiber-coupled in order to improve the reliability of alignment. Vertical and horizontal platforms are used and their stability was tested for the project of trapping Helium atoms in an optical lattice (see below) .
New computer control (completed): The sequences of our experiment are controlled by a computer using industrial DAQ-type hardware. This new tool offers script-based interface and fully automatic operation with micro-second resolution and nanosecond synchronization.
Ion/Electron flux detection (completed): Metastable helium atoms can decay into the ground state via a spin-relaxation process with subsequent ionization (Penning ionization). Our new Channel-Electron Multiplier allows for the detection of the ion or electron flux resulting from this ionization process.
Dipole Trap (in progress): We are implementing an optical setup for creating a crossed dipole trap. The lasers at 1.5 μm are being installed. It will serve as a preliminary test for the future installation of a 3D optical lattice (see below). Several experiments in such a trap are planned (optical Feschbach, photoassociation,...)
Atoms in optical lattice (future): We designed a new generation of magnetic trap, based on a cloverleaf scheme, which is being built. The new design offers improved optical access, switching speed and stability, allowing for the creation of a MOT, a magnetic trap, an optical dipole trap and a 3D optical lattice in the same spatial region. Very high-current circuits have been designed and are ready for testing.
After this new magnetic trap will be installed, the optical access quality will allow injection of 6 MOT-beams, 6 Optical Lattice beams, 1 Zeeman-slowing beam and 1 absorption imaging beam without any of the optical paths being used for two beams. Once completed, the Penning ionization process will allow for real-time monitoring of the Superfluid - Mott Insulator transition dynamics, using the Channel-Electron Multiplier. (C. Buggle et al, to be submitted to Rev. of. Scient. Inst.).