We experimentally investigate the sub-Fourier behavior of a $\delta$-kicked-rotor resonance by performing a measurement of the fidelity or overlap of a Bose-Einstein condensate exposed to a periodically pulsed standing wave. The temporal width of the fidelity resonance peak centered at the Talbot time and zero initial momentum exhibits an inverse cube pulse number ($1/N^3$)-dependent scaling compared to a $1/N^2$ dependence for the mean energy width at the same resonance. A theoretical analysis shows that for an accelerating potential the width of the resonance in acceleration space depends on $1/N^3$, a property which we also verify experimentally. Such a sub-Fourier effect could be useful for high precision gravity measurements.

We investigate anomalous energy transport processes in the Fermi-Pasta-Ulam $\beta$ lattice, in particular, the maximum sound velocity of the relevant weakly damped energy carriers. That velocity is numerically resolved by measuring the propagating fronts of the correlation functions of energy-momentum fluctuations at different times. We use fixed boundary conditions and stochastic heat baths. The numerical results are compared with the theoretical predictions of the sound velocities for solitons and effective (renormalized) phonons, respectively. Excellent agreement has been found for the prediction of effective long wavelength phonons, giving strong evidence that the energy carriers should be effective phonons rather than solitons.

We show that an arbitrary body or aggregate can be made perfectly absorbing at discrete frequencies if a precise amount of dissipation is added under specific conditions of coherent monochromatic illumination. This effect arises from the interaction of optical absorption and wave interference and corresponds to moving a zero of the elastic $S$ matrix onto the real wave vector axis. It is thus the time-reversed process of lasing at threshold. The effect is demonstrated in a simple Si slab geometry illuminated in the 500–900 nm range. Coherent perfect absorbers act as linear, absorptive interferometers, which may be useful as detectors, transducers, and switches.

We demonstrate the formation of long-lived resonances via external coupling in open nanostructures. In the examples of three dielectric nanorods or strips, external coupling of modes is induced by tuning of the rod spacing and increases the lifetime over an order of magnitude. Such an enhancement results from the destructive interference of fields that minimizes light leakage. Our results illustrate an effective way of storing light in nanostructures, and have potential applications to nanoscale photonic devices.

We propose a novel forcing technique of spatially extended self-oscillatory systems able to excite phase bistability and the dissipative structures associated with it. The forcing is time periodic at a frequency close to the oscillators’ frequency and is spatially modulated. The effects of this type of forcing are demonstrated analytically and numerically in a directly driven complex Ginzburg-Landau equation. Both spatially periodic and spatially random drives prove to be effective.

We present the results of a combined experimental and theoretical investigation of rolling elastic ribbons. Particular attention is given to characterizing the steady shapes that arise in static and dynamic rolling configurations. In both cases, above a critical value of the forcing (either gravitational or centrifugal), the ribbon assumes a two-lobed, peanut shape similar to that assumed by rolling droplets. Our theoretical model allows us to rationalize the observed shapes through consideration of the ribbon’s bending and stretching in response to the applied forcing.

We observe the transient formation of a ringed pattern state during spin up of an evaporating fluid on a time scale of order a few Ekman spin up times. The ringed state is probed using infrared thermometry and particle image velocimetry and it is demonstrated to be a consequence of the transient balance between Coriolis and viscous forces which dominate inertia, each of which are extracted from the measured velocity field. The breakdown of the ringed state is quantified in terms of the antiphasing of these force components which drives a Kelvin-Helmholtz instability and we show that the resulting vortex grid spacing scales with the ring wavelength. This is the fundamental route to quasi-two-dimensional turbulent vortex flow and thus may have implications in astrophysics and geophysics wherein rotating convection is ubiquitous.

Large-scale motions, important in turbulent shear flows, are frequently attributed to the interaction of structures at smaller scales. Here we show that, in a turbulent channel at ${\mathrm{Re}}_{\tau }\approx 550$, large-scale motions can self-sustain even when smaller-scale structures populating the near-wall and logarithmic regions are artificially quenched. This large-scale self-sustained mechanism is not active in periodic boxes of width smaller than $L_z\approx 1.5h$ or length shorter than $L_x\approx 3h$ which correspond well to the most energetic large scales observed in the turbulent channel.

We present time-resolved studies and Fourier transform spectroscopy of inner-shell excited states undergoing Auger decay and doubly excited autoionizing states, utilizing coherent extreme-ultraviolet (XUV) radiation continua. Series of states spanning a range of $\sim 4\mathrm{eV}$ are excited simultaneously. An XUV probe pulse tracks the oscillatory and decaying evolution of the formed wave packet. The Fourier transform of the measured trace reproduces the spectrum of the series. The present work paves the way for ultrabroadband XUV spectroscopy and studies of ultrafast dynamics in all states of matter.

We periodically modulate a turbulent wind-tunnel flow with an active grid. We find a resonant enhancement of the mean turbulent dissipation rate at a modulation frequency which equals the large-eddy turnover rate. Thus, we find the best frequency to inject energy into a turbulent flow. The resonant response is characterized by the emergence of vortical structures in the flow and depends on the spatial mode of the stirring grid.

We investigate both experimentally and theoretically the traffic of particles flowing in microfluidic obstacle networks. We show that the traffic dynamics is a nonlinear process: the particle current does not scale with the particle density even in the dilute limit where no particle collision occurs. We demonstrate that this nonlinear behavior stems from long-range hydrodynamic interactions. Importantly, we also establish that there exists a maximal current above which no stationary particle flow can be sustained. For higher current values, intermittent traffic jams form, thereby inducing the ejection of the particles from the initial path and the subsequent invasion of the network. Eventually, we put our findings in the broader context of the transport processes of driven particles in low dimension.

We demonstrate in the soft x-ray regime a novel technique for high-resolution lensless imaging based on differential holographic encoding. We have achieved superior resolution over x-ray Fourier transform holography while maintaining the signal-to-noise ratio and algorithmic simplicity. We obtain a resolution of 16 nm by synthesizing images in the Fourier domain from a single diffraction pattern, which allows resolution improvement beyond the reference fabrication limit. Direct comparisons with iterative phase retrieval and images from state-of-the-art zone-plate microscopes are presented.

A subcritical transition to turbulence in magnetized Keplerian shear flows is investigated by using a statistical approach. Three-dimensional numerical simulations of the shearing box equations with zero net magnetic flux are employed to determine the transition from decaying to sustained turbulence as a function of the magnetic Reynolds number $R_m$. The results reveal no clear transition to sustained turbulence as the average lifetime of the transients grows as an exponential function of $R_m$, in accordance with a type-II supertransient law.

We analyze the reversals of the large-scale flow in Rayleigh-Bénard convection both through particle image velocimetry flow visualization and direct numerical simulations of the underlying Boussinesq equations in a (quasi-) two-dimensional, rectangular geometry of aspect ratio 1. For medium Prandtl number there is a diagonal large-scale convection roll and two smaller secondary rolls in the two remaining corners diagonally opposing each other. These corner-flow rolls play a crucial role for the large-scale wind reversal: They grow in kinetic energy and thus also in size thanks to plume detachments from the boundary layers up to the time that they take over the main, large-scale diagonal flow, thus leading to reversal. The Rayleigh vs Prandtl number space is mapped out. The occurrence of reversals sensitively depends on these parameters.

We discuss how two subparts of a randomly stirred scalar mixture interact to form the overall concentration distribution. We derive, in particular, the appropriate composition laws in the absence and presence of a strong correlation between the fields. The resulting concentration distributions compare favorably with several distinct experiments, illustrating the two limits. The initial relative spatial position of the subparts plays a crucial role on the nature of their subsequent entanglement.

An imaging technique is here proposed to overcome the classical “diffraction limit” by using helical beams. This technique and the analysis presented are valid for all kinds of waves (either optical or acoustical) as long as the field can be considered as scalar. We show that the stable structure of such phase singularities turns out to be appropriate to measure both the position and the diameter of subdiffraction circular apertures. The property used is a shift of the scattered vortex. Its location is obtained with a very high resolution thanks to a nonclassical correlation method exploiting the superoscillating property of a vortex near its axis. This theoretical analysis is supported by acoustic experiments performed underwater evidencing subdiffraction imaging.

Isomeric molecules that only differ in the spatial orientation of their constituents are called stereoisomers. We demonstrate that different stereoisomers are transported typically at different velocities in a fluid streaming through a straight microchannel. As the underlying mechanism, we identify a translation-rotation coupling in the motion of the molecules which is specific for the molecule structure. This effect can be exploited for the separation of stereoisomers, a task of immense importance in biotechnology and pharmaceutics.

We put forward a new approach to generate stable, fully three-dimensional light bullets, which is based on the matching of the intrinsic material dispersion with a suitable effective diffraction. The matching is achieved in adequate waveguide arrays whose refractive index is periodically modulated along the direction of light propagation. We show that by using nonconventional, out-of-phase longitudinal modulation of the refractive index of neighboring channels, it is possible to tune the effective diffraction to match the intrinsic material group velocity dispersion. Three-dimensional light bullets are shown to form at reduced energy levels, in settings where the dispersion would be far too weak to generate bullets in the absence of array.

Granular flows involving liquid-coated solids are ubiquitous in nature (pollen capture, avalanches) and industry (filtration, pharmaceutical mixing). In this Letter, three-body collisions between liquid-coated spheres are investigated experimentally using a “Stokes’s cradle,” which resembles the popular desktop toy Newton’s cradle (NC). Surprisingly, previous work shows that every possible outcome was observed in the Stokes’s cradle except the traditional NC outcome. Here, we experimentally achieve NC via guidance from a theory, which revealed that controlling the liquid-bridge volume connecting two target particles is the key in attaining the NC outcome. These three-body experiments also provide direct evidence that the fluid resistance upon rebound cannot be completely neglected due to presumed cavitation; this resistance also influences two-body systems yet cannot be isolated experimentally in such systems.

We report magnetic field control of the quantum chaotic dynamics of hydrogen analogues in an anisotropic solid state environment. The chaoticity of the system dynamics was quantified by means of energy level statistics. We analyzed the magnetic field dependence of the statistical distribution of the impurity energy levels and found a smooth transition between the Poisson limit and the Wigner limit, i.e., transition between regular Poisson and fully chaotic Wigner dynamics. The effect of the crystal field anisotropy on the quantum chaotic dynamics, which manifests itself in characteristic transitions between regularity and chaos for different field orientations, was demonstrated.