Two-dimensional Bose-Einstein condensate in an optical surface trap

This thesis reports on the realization of a two-dimensional Bose-Einstein condensation of cesium atoms in an optical surface microtrap, where the condensate is produced few micro meters above a dielectric surface. The optical micro trap is a combination of a repulsive evanescent wave and a focussed 1064 nm-laser beam to provide the horizontal confinement. To load the microtrap we release the cesium atoms from a magneto-optical trap into the gravito-optical surface trap, which prepares a cold atom reservoir, then the atoms are loaded by thermalization into the conservative microtrap potential. After loading we optically pump the atoms in the lowest spin state F=3,mF=3, where cesium offers very favorable scattering properties and the scattering length can be tuned to positive values. To introduce forced evaporative cooling we ramp down the power of the red-detuned beam by more than three orders of magnitude to few milliwatts within 5 seconds. After the evaporation we observe a release energy of 1/2*kB*16 nK, which is well bellow the vibrational energy quantum h 61557;=kB 26 nK and close to the zero point energy of 1/2* h 61557;. This shows, that a two-dimensional gas is realized. To proof the presence of a condensate we use a method, which relies on the fact that the condensate collapse at negative scattering length. Our second method to proof the condensate relies on the horizontal expansion of the condensate, which is slowed down when a condensate is present. With this two methods we can observe the onset of the condensate. We see that the transition takes place at about 100 nK in a 3D regime, where about 5 quantum states of the vertical motion are occupied. For lower final values of the evaporation we reach the 2D regime, where the vertical motion is dominated by the zero-point energy. (author)