We study the transfer of energy from an optical potential to atomic and molecular gases and demonstrate that this process is analogous to collisionless Landau damping of electrostatic potentials in plasmas and gravitational potentials observed on astrophysical scales. We show that a signficant fraction of the light attenuation within a cavity can be attributed to this mechanism when the cavity is filled with a gas at high density. The resulting motion of particles created by optical Landau damping can be used to induce transport when a periodic potential produced by two counterpropagating high-intensity pulsed optical fields is used. Bulk drift of the gas also appears feasible even when the mean kinetic energy is much greater that the maximum optical potential.
We study the transfer of energy from an optical potential to atomic and molecular gases and demonstrate that this process is analogous to collisionless Landau damping of electrostatic potentials in plasmas and gravitational potentials observed on astrophysical scales. We show that a signficant fraction of the light attenuation within a cavity can be attributed to this mechanism when the cavity is filled with a gas at high density. The resulting motion of particles created by optical Landau damping can be used to induce transport when a periodic potential produced by two counterpropagating high-intensity pulsed optical fields is used. Bulk drift of the gas also appears feasible even when the mean kinetic energy is much greater that the maximum optical potential.
Analysis of the trapped and untrapped motion of particles, combined with direct numerical simulation of the 1D non-stationary Boltzmann equation, demonstrates that atoms and molecules initially at room temperature can be accelerated to velocities in the 10 to 100 km/s range over distances of 100's microns using an optical lattice. The quantity, final velocity, and velocity/energy spread of the accelerated distribution controlled by tailoring the fluence, duration and frequency chirp of the laser beam that make the lattice, indicating the potential for a compact, versatile, and configurable hyper-thermal accelerator/decelerator. Using similar lattice potentials, differential transport of atomic and molecular species with different polarizability/mass ratios leads towards the possibility of gas mixture separation in short pulses or in hollow fibers. A bulk drift can be induced in a gas by the lattice forces, even when the mean kinetic energy is much greater that the maximum dipole potential of the optical field. In this process the transfer of energy from the slowly traveling optical lattice to the gas is analogous to Landau damping of a plasma wave by charged particles.
This thesis includes both experimental and theoretical investigations of fluctuation-induced transport phenomena, presented in a series of nine papers, by studies of the dynamics of cold atoms in dissipative optical lattices. With standard laser cooling techniques about 108 cesium atoms are accumulated, cooled to a few μK, and transferred into a dissipative optical lattice. An optical lattice is a periodic light-shift potential, and in dissipative optical lattice the light field is sufficiently close to resonance for incoherent light scattering to be of importance. This provides the system with a diffusive force, but also with a friction through laser cooling mechanisms. In the dissipative optical lattices the friction and the diffusive force will eventually reach a steady state. At steady state, the thermal energy is low enough, compared to the potential depth, for the atoms to be localized close to the potential minima, but high enough for the atoms to occasionally make inter-well flights. This leads to a Brownian motion of the atoms in the optical lattices. In the normal case these random walks average to zero, leading to a symmetric, isotropic diffusion of the atoms. If the optical lattices are tilted, the symmetry is broken and the diffusion will be biased. This leads to a fluctuation-induced drift of the atoms. In this thesis an investigation of such drifts, for an optical lattice tilted by the gravitational force, is presented. We show that even though the tilt over a potential period is small compared to the potential depth, it clearly affect the dynamics of the atoms, and despite the complex details of the system it can, to a good approximation, be described by the Langevin equation formalism for a particle in a periodic potential. The linear drifts give evidence of stop-and-go dynamics where the atoms escape the potential wells and travel over one or more wells before being recaptured. Brownian motors open the possibility of creating fluctuation-induced drifts in the absence of bias forces, if two requirements are fulfilled: the symmetry has to be broken and the system has to be brought out of thermal equilibrium. By utilizing two distinguishable optical lattices, with a relative spatial phase and unequal transfer rates between them, these requirements can be fulfilled. In this thesis, such a Brownian motor is realized, and drifts in arbitrary directions in 3D are demonstrated. We also demonstrate a real-time steering of the transport as well as drifts along pre-designed paths. Moreover, we present measurements and discussions of performance characteristics of the motor, and we show that the required asymmetry can be obtained in multiple ways.
The optomechanical coupling and transverse stability of a co-propagating monochromatic electromagnetic wave and mono-energetic beam of two-level atoms is investigated in the collisionless regime. The coupled dynamics are studied through a Landau stability analysis of the coupled gas- kinetic and paraxial wave equations, including the effect of the electronic nonlinearity. The resulting dispersion relation captures the interaction of kinetic and saturation effects and shows that for blue detuning the combined nonlinear interaction is unstable below a critical wavenumber which reduces to the result of Bespalov and Talanov in the limit of a negligible kinetic nonlinearity. For red detuning we find that under a saturation parameter threshold exists whereby the system stabilizes unconditionally. With negligible saturation, an optomechanical form of Landau damping stabilizes all wavenumbers above a critical wavenumber determined by the combined strength of the kinetic and refractive optomechanical feedback. The damping is mediated primarily by atoms traveling along the primary diagonals of the Talbot carpet.
With very few exceptions (such as the centre-of-mass dipole mode), collective oscillations in trapped superfluid Bose gases are damped. In the “collisionless” region the damping is second order in the interaction strength. There are three possible components. One is Beliaev damping, which is due to the decay of a single excitation into two excitations; this can occur even at T = 0. In addition, there is Landau damping, which is due to a collective mode scattering from thermally excited excitations. This process only occurs at finite temperatures but quickly becomes the dominant damping mechanism as the temperature increases. Both Landau and Beliaev damping arise naturally from the imaginary part of the Beliaev second-order self-energies, as given in (5.40) in the case of a uniform Bose gas. Finally there is the damping that arises from the C 22 and C 12 collision processes; this is discussed in Chapters 8, 12 and 19. In Chapter 12 we calculated the damping of various condensate modes at finite temperatures using direct numerical simulations of the ZNG equations. These numerical results were generally in very good agreement with the available experimental data. From a theoretical perspective, one advantage of the simulations is that the Landau damping contribution can be isolated simply by setting the C 12 and C 22 collision terms to zero. After providing an introduction to Landau damping in uniform Bose gases in Section 13.1, we present in Section 13.2 a detailed discussion of Landau damping based on a general formula in terms of Bogoliubov–Popov excitations. This discussion makes it clear that the Landau damping of condensate oscillations arises from the interaction with a thermal cloud of excitations.
In this paper the effects of coupling a non‐resonant optical lattice to a gas is studied. When the lattice induces a periodic modulation of the gas density, strong Bragg diffraction of light can occur. If a significant fraction of the light used to make the periodic structure is diffracted it can potentially limit the achievable intensities inside the lattice limiting the ability to manipulate the gas with the light. We calculate the evolution of the light fields by finding solutions to the wave equation with a periodic refractive index induced by the lattice. The local gas density perturbation induced by the lattice is determined by solution of the Boltzmann kinetic equation in the collisional regime. This allows us to predict the limits of gas transport within optical lattice. We discuss the maximum energy and momentum transfer to the gas from the lattice.