OPTOMECHANICAL COLLECTIVE EFFECTS USING COLD ATOMS IN FREE SPACE: COLLECTIVE ATOMIC RECOIL LASING & OPTICAL BINDING

This theoretical doctoral thesis investigates the collective effects that emerge in cold atomic systems caused by light-scattering in free space. Two specific cases are investigated: the collective atomic recoil laser (CARL) effect in a cold gas, without optical cavity, and a novel cooperative cooling effect via optical binding (OB) with cold atoms. As a main objective, this theoretical project investigates the spatial grating structures and the backward radiation that appears in a cold atomic cloud when it is irradiated by a single far-detuned laser beam, also known as CARL effect. While this effect has traditionally been described using a ring cavity, the study is performed here in free space, in the absence of such a cavity. Both 2D and 3D clouds show a transition from single-atom isotropic scattering to collective directional scattering. The effect is shown by the derivation and numerical solution of a set of multi-particle motion equations coupled by a self-consistent optical field, which is inspected with both a scalar model and a vectorial model. New original approaches are used to address the numerical study of the dynamics of the atomic system, such as molecular dynamics (MD) algorithms. A second system emerged, from the attempt to understand the main objective, where a few atoms rearrange themselves into crystalline atomic structures, with a periodicity between particles close to the optical wavelength. The atomic system is initially confined into a 2D plane (or 1D string) using two (or four) counter-propagating laser beams. Due to the multiple scattering experienced by all the particles in the system, a dipole-dipole force arises among them, generating a non-trivial dynamical trapping potential landscape that compels the atoms, to self-organize at distances multiple of the light wavelength. When atoms are rearranged into an atomic crystal, the force acting on each particle depends on the position of the others, thus allowing to study the stability of such optically bound structures. In addition, it turns out that a non-conservative force is generated from the dipole-dipole interaction, allowing the system to be cooled by controlling the value of certain parameters. This new phenomenon arises as a direct consequence of the use of cold atoms instead of dielectric nanoparticles in an OB system. Therefore, besides the atomic external motion, internal degrees of freedom (DOF) of the atoms are considered by treating each atom as a dipole. This latter aspect is investigated using the coupled dipole equations. When multiple atoms are set in line, the cooling mechanism is collectively enhanced, generating a novel cooperative cooling effect.