Cold atoms in light fields: from free space optical lattices to multimode optical cavities

The electromagnetic mode density of the vacuum can be dramatically modified by the presence of an optical resonator. In the strong coupling regime, spontaneous emission in a cavity becomes a reversible process and the intracavity photon number undergoes Rabi oscillations. We load up to 200x10^3 ^133Cs atoms into a nearly confocal lossy cavity and reach the collective strong coupling regime. Normal mode splitting, the hallmark of this regime, is observed and cooperativities up to C_coll = (186±5) are measured. In a second experiment we investigate for the first time the multi-mode character of the coupled cavity-atom system. In a confocal cavity the higher-order transverse cavity modes are degenerate in frequency and accessible to the spontaneous emission of the atomic ensemble. We observe an increase of the coupling constant measured via modal decomposed transmission analysis, which could be attributed to the presence of the higher-order modes. Normal mode splitting proportional to the square root of the atom number was visible for all of the different mode components. Furthermore, we observe a redistribution of the relative weights in the modal transmission composition, which scales with the atom number in the cavity mode. In a second set of experiments, ^87Rb atoms were loaded into a dissipative lin ⊥ lin lattice. By driving the lattice with a biharmonic force, transport can be observed when the systems symmetries are broken: the so called ratchet effect. Research in this area is concerned with the appearance of current reversals. We were able to identify dissipation related symmetry breaking as the underlying cause of an observed current reversal, which occurs as a function of the driving frequency. Furthermore, in a second experiment, we use the ratchet effect as a probe of the optical potential depths. We show that an oscillating force with a frequency far above any other system-inherent timescale, can be used to renormalize the optical potential. The ^87Rb atoms experience an average position dependent force, which becomes controllable over the amplitude of the applied driving.