13 research outputs found
Atomic selfordering in a ring cavity with counterpropagating pump
The collective dynamics of mobile scatterers and light in optical resonators
generates complex behaviour. For strong transverse illumination a phase
transition from homogeneous to crystalline particle order appears. In contrast,
a gas inside a single-side pumped ring cavity exhibits an instability towards
bunching and collective acceleration called collective atomic recoil lasing
(CARL). We demonstrate that by driving two orthogonally polarized counter
propagating modes of a ring resonator one realises both cases within one
system. The corresponding phase diagram depending on the two pump intensities
exhibits regions in which either a generalized form of self-ordering towards a
travelling density wave with constant centre of mass velocity or a CARL
instability is formed. Controlling the cavity driving then allows to accelerate
or slow down and trap a sufficiently dense beam of linearly polarizable
particles.Comment: 5 page
The pervasive presence of oxygen in ZrC
Based on the recent interest in oxy-carbide materials in catalysis, we employ a thin film model concept to highlight that variation of key reaction parameters in the reactive magnetron sputtering of zirconium carbide films (sputtering power, template temperature or reactive plasma environment) under realistic preparation and application conditions often results in zirconium oxy-carbide films of varying stoichiometry. The composition of the films grown on silicon wafers and in vacuo - cleaved NaCl (001) single crystal facets was confirmed by depth profiling X-ray photoelectron spectroscopy and electron microscopy analysis. A correlation between methane-to-argon ratio, excess carbon and template temperature with elemental composition emphasizes the exclusive presence of oxygen-containing zirconium carbides. To generalize the approach, we also show that embedding of highly ordered Cu particles with uniform sizes in zirconium oxy-carbide matrices yields well-defined metal / oxy-carbide interfaces. As the presence of an oxy-carbide and its reactivity has been inextricably linked to enhanced activity and selectivity in a variety of processes, including hydrogenation, oxidation or reduction reactions, our model thin film approach provides the necessary well-defined catalysts to derive mechanistic details and to study the decomposition/re-carburization cycles of oxy-carbides. We have exemplified the concept for zirconium oxy-carbide, but deliberate extension to similar systems is easily possible
Raman superradiance and spin lattice of ultracold atoms in optical cavities
We investigate synthesis of a hyperfine spin lattice in an atomic
Bose-Einstein condensate, with two hyperfine spin components, inside a
one-dimensional high-finesse optical cavity, using off-resonant superradiant
Raman scattering. Spatio-temporal evolution of the relative population of the
hyperfine spin modes is examined numerically by solving the coupled
cavity-condensate mean field equations in the dispersive regime. We find,
analytically and numerically, that beyond a certain threshold of the transverse
laser pump, Raman superradiance and self-organization of the hyperfine spin
components simultaneously occur and as a result a magnetic lattice is formed.
The effects of an extra laser pump parallel to the cavity axis and the
time-dependence of the pump strength on the synthesis of a sharper lattice are
also addressed.Comment: Accepted for publication in New Journal of Physics. 16 pages and 6
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Multimode mean-field model for the quantum phase transition of a Bose-Einstein condensate in an optical resonator
We develop a mean-field model describing the Hamiltonian interaction of
ultracold atoms and the optical field in a cavity. The Bose-Einstein condensate
is properly defined by means of a grand-canonical approach. The model is
efficient because only the relevant excitation modes are taken into account.
However, the model goes beyond the two-mode subspace necessary to describe the
self-organization quantum phase transition observed recently. We calculate all
the second-order correlations of the coupled atom field and radiation field
hybrid bosonic system, including the entanglement between the two types of
fields.Comment: 10 page
A Vlasov approach to bunching and selfordering of particles in optical resonators
We develop a Vlasov type continuum density description for the coupled nonlinear dynamics of polarizable particles moving in the light field of a high Q optical resonator. The intracavity light field, which exerts optical forces on the particles, depends itself on the dynamics of the particle density, which constitutes a time dependent refractive index. This induces mode frequency shifts, losses and coupling. For typical geometries we find solid analytic criteria for the stability of an initial homogeneous particle density for a wide class of initial velocity distributions including thermal distributions. These agree with previously found bunching and self-ordering instabilities but are extended to a wider range of parameters and initial conditions. Using a linear perturbation expansion we calculate the growth exponents of small density perturbations in the parameter region beyond this instability threshold. Numerical solutions of the full equations as well as simulations of the underlying many particle trajectories confirm these results. In addition the equations allow to extract analytical scaling laws to extrapolate cavity cooling and selfordering dynamics to higher particle numbers