Fig. 1. Schematic of the experimental arrangement to produce the 1D CO2 laser optical lattice

  

 

                                           

 Fig. 2.  Absorption image of atoms trapped in the two counter-propagating trapping laser beams before alignment for creating the optical lattice. Atoms trapped in the orthogonal single beam dipole trap crossing with the two counter-propagating trapping beams is also shown.

 

 

                                                                       

Fig. 3. Image of rubidium atoms trapped in (a) central part of 1D CO2 laser optical lattice (b) single beam dipole trap, after being transferred from an orthogonal single beam dipole trap. The trap atoms remain well-localized in case of the transfer to the optical lattice whereas it spreads out in case of the transfer to the single beam dipole trap.

Note:  The spacing between adjacent lattice sites of the 1D optical lattice is 5.3 μm whereas the optical resolution of our imaging system is more than 8 μm, hence the individual lattice sites of the optical lattice is not resolved in our experiment.

             

                                                                        

Fig. 4. Image of rubidium atoms trapped in (a) CO2 laser 1D optical lattice (b) single beam dipole trap (when retro-reflected beam required to create the optical lattice is blocked). Due to the large differential ac-Stark shift because of  the overlap of the focus of the two counter-propagating beams creating the optical lattice, very few atoms can be loaded in the trap centre. We circumvent this problem by proper detuning of the MOT cooling laser beams and a two-step loading process.

 

 

                                                                  

Fig. 5. Formation of a Bose-Einstein Condensate (BEC) in an 1D optical lattice after loading from an orthogonal single beam dipole trap. The plots show the absorption images and the corresponding atomic density profiles at various stages of evaporative cooling having the following final trapping powers: (a) 3W  (b) 750 mW  (c) 270 mW. Plot (c) without significant thermal component indicates that most of the atoms are in the BEC phase. Field of view is 300 μm. Bose-Einstein condensate in the optical lattice was produced with 82000 atoms in the condensate after evaporative cooling for 1 second to 150 nK temperature.

 Signature of BEC: For non-interacting ideal gas trapped in a harmonic trap, the ground state wave function in the harmonic potential is a Gaussian. Due to the presence of weak repulsive mean-field interaction between the atoms (positive s-wave scattering length of 87Rb) the ground state wave function no longer remains a Gaussian. If the kinetic energy of the atoms can be neglected as compared to the interaction energy (Thomas-Fermi limit), which is the case for atoms in the BEC, the ground-state wave function takes the shape of the bottom of the harmonic trap, which is parabolic. This non-Gaussian nature of the density profile of atoms in the condensate is the main distinction of the atoms in the BEC as compared to the atoms in the thermal, incoherent state which follow the Gaussian thermal distribution. It is observed that the density profile at the centre is enhanced beyond the thermal Bose-Einstein distribution as the evaporative cooling progresses as shown in the Fig. 5. This is an important signature of the formation of Bose-Einstein condensate.

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