Spontaneously Sliding Multipole Spin Density Waves in Cold Atoms.

We report on the observation of spontaneously drifting coupled spin and quadrupolar density waves in the ground state of laser driven Rubidium atoms. These laser-cooled atomic ensembles exhibit spontaneous magnetism via light mediated interactions when submitted to optical feedback by a retroreflecting mirror. Drift direction and chirality of the waves arise from spontaneous symmetry breaking. The observations demonstrate a novel transport process in out-of-equilibrium magnetic systems.

Synchronization of Bloch oscillations by a ring cavity.

We consider Bloch oscillations of ultracold atoms stored in a one-dimensional vertical optical lattice and simultaneously interacting with a unidirectionally pumped optical ring cavity whose vertical arm is collinear with the optical lattice. We find that the feedback provided by the cavity field on the atomic motion synchronizes Bloch oscillations via a mode-locking mechanism, steering the atoms to the lowest Bloch band. It also stabilizes Bloch oscillations against noise, and even suppresses dephasing due to atom-atom interactions. Furthermore, it generates periodic bursts of light emitted into the counter-propagating cavity mode, providing a non-destructive monitor of the atomic dynamics. All these features may be crucial for future improvements of the design of atomic gravimeters based on recording Bloch oscillations.

Quantum threshold for optomechanical self-structuring in a Bose-Einstein condensate.

Theoretical analysis of the optomechanics of degenerate bosonic atoms with a single feedback mirror shows that self-structuring occurs only above an input threshold that is quantum mechanical in origin. This threshold also implies a lower limit to the size (period) of patterns that can be produced in a condensate for a given pump intensity. These thresholds are interpreted as due to the quantum rigidity of Bose-Einstein condensates, which has no classical counterpart. Above the threshold, the condensate self-organizes into an ordered supersolid state with a spatial period self-selected by optical diffraction.

Self-organization in cold atomic gases: a synchronization perspective.

We study non-equilibrium spatial self-organization in cold atomic gases, where long-range spatial order spontaneously emerges from fluctuations in the plane transverse to the propagation axis of a single optical beam. The self-organization process can be interpreted as a synchronization transition in a fully connected network of fictitious oscillators, and described in terms of the Kuramoto model.

Kinetic theory for transverse optomechanical instabilities.

We investigate transverse symmetry-breaking instabilities emerging from the optomechanical coupling between light and the translational degrees of freedom of a collisionless, damping-free gas of cold, two-level atoms. We develop a kinetic theory that can also be mapped on to the case of an electron plasma under ponderomotive forces. A general criterion for the existence and spatial scale of transverse instabilities is identified; in particular, we demonstrate that monotonically decreasing velocity distribution functions are always unstable.

Dissipative solitons in the coupled dynamics of light and cold atoms.

We investigate the coupled dynamics of light and cold atoms in a unidirectional ring cavity, in the regime of low saturation and linear single-atom response. As the dispersive opto-mechanical coupling between light and the motional degrees of freedom of the atoms makes the dynamics nonlinear, we find that localized, nonlinearity-sustained and bistable structures can be encoded in the atomic density by means of appropriate control beams.

Rapid coherent optical modulation of atomic momenta via pseudoresonances.

We show that modulation of an optical field injected into a cavity containing a dilute Bose-Einstein condensate is transformed into a modulation of the population of the atomic momentum states due to pseudoresonances of the resolvent which describes the linearized evolution of the atom-cavity system. This effect is related to the way the atomic momentum states and the cavity optical field are dynamically coupled. The results presented offer new possibilities for rapid modulation of atomic momentum state populations up to 3 orders of magnitude faster than modulation of magnetic trapping potentials.

Collective atomic recoil lasing with a partially coherent pump.

We investigate the effect of pump phase noise on the collective backscattering of light by a cold, collisionless atomic gas. We show that for a partially coherent pump field, the growth rate of the backscattered field is reduced relative to that for a coherent pump, but the backscattered intensity can be increased. Our results demonstrate that fluctuations and noise can play a counterintuitive role in nonlocally coupled many-body systems.

Harmonic lasing in a free-electron-laser amplifier.

A method is demonstrated that allows a planar wiggler high-gain Free-Electron-Laser (FEL) amplifier to lase so that the interaction with an odd harmonic of the radiation field dominates that of the fundamental. This harmonic lasing of the FEL is achieved by disrupting the electron interaction with the usually dominant fundamental while allowing that of a harmonic interaction to evolve unhindered. The disruption is achieved by a series of relative phase changes between the electrons and the ponderomotive potentials of both the fundamental and harmonic fields. Such phase changes are relatively easy to implement and some current FEL designs would require little or no structural modification to test the scheme.

Four-wave mixing with self-phase matching due to collective atomic recoil.

We describe a method for nondegenerate four-wave-mixing in a cold sample of four-level atoms. An integral part of the four-wave-mixing process is a collective instability which spontaneously generates a periodic density modulation in the cold atomic sample with a period equal to half of the wavelength of the generated high-frequency optical field. Because of the generation of this density modulation, phase matching between the pump and scattered fields is not a necessary initial condition for this wave-mixing process to occur; rather the density modulation acts to "self-phase match" the fields during the course of the wave-mixing process. We describe a one-dimensional model of this process, and suggest a proof-of-principle experiment which would involve pumping a sample of cold Cs atoms with three infrared pump fields to produce blue light.

Two-beam free-electron laser.

A model of a free electron laser (FEL) amplifier operating simultaneously with two electron beams of different energy is presented. The electron beam energies are chosen so that the fundamental resonance of the higher energy beam is at a harmonic of the lower energy beam. By seeding the lower energy FEL interaction with its fundamental radiation wavelength, an improved coherence of the unseeded higher energy FEL emission is predicted. This method may offer an important alternative to those seeding proposals for FELs currently under development in the xuv and x-ray regions of the spectrum.

Self-synchronization and dissipation-induced threshold in collective atomic recoil lasing.

Networks of globally coupled oscillators exhibit phase transitions from incoherent to coherent states. Atoms interacting with the counterpropagating modes of a unidirectionally pumped high-finesse ring cavity form such a globally coupled network. The coupling mechanism is provided by collective atomic recoil lasing, i.e., cooperative Bragg scattering of laser light at an atomic density grating, which is self-induced by the laser light. Under the rule of an additional friction force, the atomic ensemble is expected to undergo a phase transition to a state of synchronized atomic motion. We present the experimental investigation of this phase transition by studying the threshold behavior of this lasing process.

Superfluorescent rayleigh scattering from suspensions of dielectric particles.

We demonstrate that superfluorescent scattering of light can occur when laser light is incident on a collection of dielectric Rayleigh particles suspended in a viscous medium. Using a linear stability analysis, an expression for the spatiotemporal evolution of the scattered (probe) field is derived. An approximate condition for the progression of the interaction into the nonlinear regime is deduced and it is shown that, in the nonlinear regime, the scattered field intensity shows the characteristic quadratic dependence on particle density expected from a superfluorescent or superradiant process, once the effects of pump depletion are accounted for.

Modulational instability arising from collective Rayleigh scattering.

It is shown that under certain conditions a collection of dielectric Rayleigh particles suspended in a viscous medium and enclosed in a bidirectional ring cavity pumped by a strong laser field can produce a new modulational instability transverse to the wave-propagation direction. The source of the instability is collective Rayleigh scattering i.e., the spontaneous formation of periodic longitudinal particle-density modulations and a backscattered optical field. Using a linear stability analysis a dispersion relation is derived which determines the region of parameter space in which modulational instability of the backscattered field and the particle distribution occurs. In the linear regime the pump is modulationally stable. A numerical analysis is carried out to observe the dynamics of the interaction in the nonlinear regime. In the nonlinear regime the pump field also becomes modulationally unstable and strong pump depletion occurs.

Self-amplified coherent spontaneous emission in the planar wiggler free-electron laser.

Coherent spontaneous emission (CSE) is a potentially important self-generated source of seed radiation in a free-electron laser (FEL) amplifier. A model is derived that describes CSE at the fundamental resonant frequency and its harmonics in a planar wiggler FEL. The subsequent self-amplification of the CSE is investigated in the nonlinear regime for a FEL amplifier configuration.

Effect of electron-beam momentum spread on cyclotron resonance maser operation at two resonant frequencies.

We present a theoretical analysis of cyclotron resonance maser (CRM) operation at two resonant frequencies including the effects of momentum spread in the electron beam. A linear analysis of the system equations is presented in the limit of small momentum spreads. Numerical solutions to the system equations are also given and are in agreement with the linear theory. The results predict that for realistic momentum spreads, operation of the CRM at the higher of the two resonant frequencies should be possible, extending its operating frequency range. An experiment currently under development at Strathclyde University is described and modeled numerically.

Theory and design of a free-electron maser with two-dimensional feedback driven by a sheet electron beam.

The use of two-dimensional Bragg resonators of planar geometry, realizing two-dimensional (2D) distributed feedback, is considered as a method of producing spatially coherent radiation from a large sheet electron beam. The spectrum of eigenmodes is found for a 2D Bragg resonator when the sides of the resonator are open and also when they are closed. The higher selectivity of the open resonator in comparison with the closed one is shown. A time-domain analysis of the excitation of an open 2D Bragg resonator by a sheet electron beam demonstrates that a single-mode steady-state oscillation regime may be obtained for a sheet electron beam of width 100-1000 wavelengths. Nevertheless, for a free-electron maser (FEM) with a closed 2D Bragg resonator, a steady-state regime can also be realized if the beam width does not exceed 50-100 wavelengths. The parameters for a FEM with a 2D planar Bragg resonator driven by a sheet electron beam based on the U-2 accelerator (INP RAS, Novosibirsk) are estimated and the project is described.

Second free flaps in head and neck reconstruction.

Over the past decade, free-tissue transfer has greatly improved the quality of oncology-related head and neck reconstruction. As this technique has developed, second free flaps have been performed for aesthetic improvement of the reconstructed site. This study evaluated the indications for and the success of second free flaps. Medical files for patients who underwent second free flaps for head and neck reconstruction at the University of Texas M.D. Anderson Cancer Center, from May 1, 1988 to November 30, 1996, were reviewed. The flaps were classified as being either immediate (done within 72 hr) or delayed (done within 2 years) reconstructions. Indications, risk factors, recipient vessels, outcome, and complications were analyzed. Of the 28 patients included in this study, 12 had immediate (nine as salvage after primary free flap failure, and three for reconstruction of a soft-tissue defect), and 16 had delayed second free flaps (two for reconstruction of a defect resulting from excision of recurrent tumors, and 14 for aesthetic improvement). Reconstruction sites included the oral cavity in 18 patients; the midface in six; the skull base in two; and the scalp in two. The success rate for the second free flaps was 96 percent. Five patients had significant wound complications. In a substantial number of cases, identical recipient vessels were used for both the first and second free flaps. The authors conclude that second free flaps can play an important role in salvaging or improving head and neck reconstruction in selected patients. In many cases, the same recipient vessels can be used for both the first and second flaps.