Dipole and quadrupole patterns in cold atoms via light induced interactions

摘要

Summary form only given. Recent years have witnessed considerable interest in simulating the dynamics of complex many-body systems by well-controlled cold atom model systems. Among others, magnetic ordering phenomena draw a lot of attention. We demonstrated an unconventional approach to light mediated atomic interactions using laser light undergoing diffraction [1]. Diffractive propagation of periodically modulated light fields leads to an exchange between phase and amplitude modulated planes (Talbot effect) which can be used to couple atomic degrees of freedom. In a feedback scheme this can lead to the spontaneous formation of coupled lattices in the light field and the atomic degrees of freedom. After adiabatic elimination of the dynamics of the light field, one is left with light mediated atomic interactions. The coupling can occur via optomechanical nonlinearities [1] or via optical pumping [2]. In the latter case, the result is a spontaneous magnetic ordering in the atomic cloud, a spin or a magnetic dipole pattern [2]. We will provide further details on the latter as well as evidence for novel alignment patterns, corresponding to quadrupole states.In our experiment a cold atom cloud placed near a retro-reflecting virtual mirror is driven by a detuned pump laser (Fig. 1a). A cloud of 1011 8,Rb atoms (temperature of 200 μK, optical thickness of 200) is loaded into a magneto-optical trap. After turning off the gradient B-field, the cooling beams are turned off and the cloud is pumped by a linearly polarized pump beam for 150-1000 μs, red detuned to the F=2 to F'=3 transition. A constant external B-field with a chosen magnitude and direction can be switched on during the pattern formation. Pattern imaging is performed by monitoring two polarization channels of the transmitted light with CCD cameras, producing near and far field images of the field intensity profile inside the cloud.