Local Field Effect In Ultracold Atom-Light Interaction

Optical lattice is a versatile tool in current atom and molecule physics which has various important applications, including quantum simulation of condensed matter physics and quantum non-equilibrium dynamics, diffraction of matter waves, deceleration of a supersonic molecular beam, as well as precision measurement of gravity and fine structure constants. In these researches, there is a widely used assumption that two counter-propagating far-off resonant optical fields will generate a perfect interference fringe within the gases, in other words, the light propagation is free of the gases perturbation, and thus a periodic dipole potential will be produced. However, our theoretical research demonstrate that the optical lattice in an inhomogeneous ultracold atomic gas could be deformed by the local field effect (LFE), that is, the light propagation could be affected by local variations of the density dependent refraction index. We call such a deformable optical lattice a soft lattice. We find the importance of the LFE by studying an experimental phenomenon about asymmetric Bragg diffraction of a Bose-Einstein condensate (BEC). It lays the groundwork for our following research. Then we find the quantum dynamics of an ultracold atomic gas within an optical lattice produced by splitting an input light is different from that by retro-reflecting the input light by a mirror due to the LFE. In the latter case, asymmetric momentum distribution could arise. Then we focus on symmetric systems. In the next work, we investigate the ground state of a collisionless BEC trapped in a soft one-dimensional optical lattice. We show that stable photon-atomic (polaritonic) lattices solitons, including an optical component, in the form of the deformation of the optical lattice, in a combination with a localized matter-wave component, are generated in the blue-detuned setting. These self-trapped modes are essentially different from the gap solitons. After that, we compare the Bragg diffraction of atomic matter waves within a soft optical lattice to which within a rigid optical lattice. We find the soft lattice could deform the Gaussian distribution of the split wavepacket and produce red-blue detuning asymmetric diffraction. In the last work, we study the superradiant Rayleigh scattering from an atomic grating. Our analysis shows that when the atom number is low, superradiance is induced. However, when the atom number increases further, the superradiance is suppressed.Our works imply that the LFE could be essential for getting a better quantitative analysis of other optical lattice experiments and shed new light on photon-atom joint control.