Multimode Collective Atomic Recoil Lasing in Free Space

Cold atomic clouds in collective atomic recoil lasing are usually confined by an optical cavity, which forces the light-scattering to befall in the mode fixed by the resonator. Here we consider the system to be in free space, which leads into a vacuum multimode collective scattering. We show that the presence of an optical cavity is not always necessary to achieve coherent collective emission by the atomic ensemble and that a preferred scattering path arises along the major axis of the atomic cloud. We derive a full vectorial model for multimode collective atomic recoil lasing in free space. Such a model consists of multi-particle equations capable of describing the motion of each atom in a 2D/3D cloud. These equations are numerically solved by means of molecular dynamic algorithms, usually employed in other scientific fields. The numerical results show that both atomic density and collective scattering patterns are applicable to the cloud’s orientation and shape and to the polarization of the incident light.

Wave-Kinetic Approach to Collective Atomic Emission

We study the collective scattering of radiation by a large ensemble of Na≫1 atoms, in the presence of a pump field. We use the wave-kinetic approach where the center-of-mass position of the moving atoms is described by a microscopic discrete distribution, or alternatively, by a Wigner distribution. This approach can include thermal effects and quantum recoil in a natural way, and even consider atomic ensembles out of equilibrium. We assume two-level atoms with atomic transition frequency ωa very different from the frequency ω0 of the pump field. We consider both the quasi-classical and quantum descriptions of the center-of-mass motion. In both cases, we establish the unstable regimes where coherent emission of radiation can take place.

Diffusing wave microrheology of highly scattering concentrated monodisperse emulsions

Motivated by improvements in diffusing wave spectroscopy (DWS) for nonergodic, highly optically scattering soft matter and by cursory treatment of collective scattering effects in prior DWS microrheology experiments, we investigate the low-frequency plateau elastic shear moduli Gp′ of concentrated, monodisperse, disordered oil-in-water emulsions as droplets jam. In such experiments, the droplets play dual roles both as optical probes and as the jammed objects that impart shear elasticity. Here, we demonstrate that collective scattering significantly affects DWS mean-square displacements (MSDs) in dense colloidal emulsions. By measuring and analyzing the scattering mean free path as a function of droplet volume fraction φ, we obtain a φ-dependent average structure factor. We use this to correct DWS MSDs by up to a factor of 4 and then calculate Gp′ predicted by the generalized Stokes–Einstein relation. We show that DWS-microrheological Gp′(φ) agrees well with mechanically measured Gp′(φ) over about three orders of magnitude when droplets are jammed but only weakly deformed. Moreover, both of these measurements are consistent with predictions of an entropic–electrostatic–interfacial (EEI) model, based on quasi-equilibrium free-energy minimization of disordered, screened-charge–stabilized, deformable droplets, which accurately describes prior mechanical measurements of Gp′(φ) made on similar disordered monodisperse emulsions over a wide range of droplet radii and φ. This very good quantitative agreement between DWS microrheology, mechanical rheometry, and the EEI model provides a comprehensive and self-consistent view of weakly jammed emulsions. Extensions of this approach may improve DWS microrheology on other systems of dense, jammed colloids that are highly scattering.