We report the first experimental realization of pattern formation in a spatially extended nonlinear system when the system is alternated between two states, neither of which exhibits patterning. Dynamical equations modeling the system are used for both numerical simulations and a weakly nonlinear analysis of the patterned states. The simulations show excellent agreement with the experiment. The nonlinear analysis provides an explanation of the patterning under alternation and accurately predicts both the observed dependence of the patterning on the frequency of alternation and the measured spatial frequencies of the patterns.

A novel experiment has been devised which provides direct evidence for critical point behavior in the longstanding problem of the transition to turbulence in a pipe. The novelty lies in the quenching of turbulence by reducing the Reynolds number and observing the decay of disordered motion. Divergence of the time scales implies underlying deterministic dynamics which are analogous to those found in boundary crises in dynamical systems. A modulated wave packet emerges from the long term transients and this coherent state provides evidence for connections with recent theoretical developments.

Continuous and pulsed forms of control of a multistable system are compared directly, both theoretically and numerically, taking as an example the switching of a periodically driven class-B laser between its stable and unstable pulsing regimes. It is shown that continuous control is the more energy efficient. This result is illuminated by making use of the close correspondence that exists between the problems of energy-optimal control and the stability of a steady state.

We report the first experimental demonstration of ring-shaped photonic lattices by optical induction and the formation of discrete solitons in such radially symmetric lattices. The transition from discrete diffraction to single-channel guidance or nonlinear self-trapping of a probe beam is achieved by fine-tuning the lattice potential or the focusing nonlinearity. In addition to solitons trapped in the lattice center and in different lattice rings, we demonstrate controlled soliton rotation in the Bessel-like ring lattices.

We studied numerically and experimentally the effects of structural disorder on the performance of ultraviolet photonic crystal slab lasers. Optical gain selectively amplifies the high-quality modes of the passive system. For these modes, the in-plane and out-of-plane leakage rates may be automatically balanced in the presence of disorder. The spontaneous optimization of in-plane and out-of-plane confinement of light in a photonic crystal slab may lead to a reduction of the lasing threshold.

A theory of turbulent transport is presented in two-dimensional magnetohydrodynamics with background shear and magnetic fields. We provide theoretical predictions for the transport of magnetic flux, momentum, and particles and turbulent intensities, which show stronger reduction compared with the hydrodynamic case, with different dependences on shearing rate, magnetic field, and values of viscosity, Ohmic diffusion, and particle diffusivity. In particular, particle transport is more severely suppressed than momentum transport, effectively leading to a more efficient momentum transport. The role of magnetic fields in quenching transport without altering the amplitude of flow velocity and in inhibiting the generation of shear flows is elucidated. Implications of the results are discussed.

We report on the first experimental observation of 2D multicolored transverse arrays in a quadratic nonlinear medium under the pump of two crossly overlapped femtosecond beams. The 2D reproducible patterns are caused by cascaded noncollinear quadratic nonlinear couplings between the input pulses and quadratic spatial solitons originated from spatial breakup of one of the input beams with spatial ellipticity. A seed supercontinuum pulse is then diffracted and amplified with phase preservation, resulting in the formation of up-converted multicolor 2D transverse arrays. By seeding with weak second harmonic pulses, the 2D multicolored transverse patterns are suppressed due to weak beam control of the induced quadratic spatial solitons.

We use pervaporation-based microfluidic devices to concentrate species in aqueous solutions with spatial and temporal control of the process. Using experiments and modeling, we quantitatively describe the advection-diffusion behavior of the concentration field of various solutions (electrolytes, colloids, etc.) and demonstrate the potential of these devices as universal tools for the kinetic exploration of the phases and textures that form upon concentration.

We measure the relation between convective heat flux and temperature gradient in a vertical channel filled with water, the average vertical mass flux being zero. Compared to the classical Rayleigh-Bénard case, this situation has the advantage of avoiding plates and, thus, their neighborhood, in which is usually concentrated most of the temperature gradient. Consequently, inertial processes should control the convection, with poor influence of the viscosity. This idea gives a good account of our observations, if we consider that a natural vertical length, different from the channel width, appears. Our results also suggest that heat fluxes can be deduced from velocity measurements in free convective flows. This confers to our results a wide range of applications.

We study the physical mechanisms of the two-dimensional inverse energy cascade using theory, numerics, and experiment. Kraichnan’s prediction of a $-5/3$ spectrum with constant, negative energy flux is verified in our simulations of 2D Navier-Stokes equations. We observe a similar but shorter range of inverse cascade in laboratory experiments. Our theory predicts, and the data confirm, that inverse cascade results mainly from turbulent stress proportional to small-scale strain rotated by 45°. This “skew-Newtonian” stress is explained by the elongation and thinning of small-scale vortices by large-scale strain which weakens their velocity and transfers their energy upscale.

We report on the direct observation of an attosecond pulse train with a mode-resolved autocorrelation technique. The chirp among the three harmonic fields is specified by analyzing two-photon above-threshold ionization spectra of electrons, resulting in a pulse duration that should be shorter than 450 as, which is, to our knowledge, the first determination of the chirp in the attosecond pulse train with an autocorrelation technique. These results will open the way to full characterization of an attosecond pulse train with its envelope.

Resonantly absorbing media supporting electromagnetically induced transparency may give rise to specific periodic patterns where a light probe is found to experience a fully developed photonic band gap yet with negligible absorption everywhere. In ultracold atomic samples the gap is found to arise from spatial regions where Autler-Townes splitting and electromagnetically induced transparency alternate with one another and detailed calculations show that accurate and efficient coherent optical control of the gap can be accomplished. The remarkable experimental simplicity of the control scheme would ease quantum nonlinear optics applications.

The three-step model (TSM) of high-harmonic generation (HHG) is generalized to atomic and molecular many-electron systems. Using many-body perturbation theory, corrections to the standard TSM due to exchange and electron-electron correlations are derived. It is shown that canonical Hartree-Fock orbitals represent the most appropriate set of one-electron states for calculating the HHG spectrum. To zeroth order in many-body perturbation theory, a HHG experiment allows direct access, in general, to a combination of occupied Hartree-Fock orbitals rather than to the highest occupied molecular orbital by itself.

We show that a metal-dielectric-metal structure can function as a negative refraction lens for surface plasmon waves on a metal surface. The structure is uniform with respect to a plane of incidence and operates at the optical frequency range. Using three-dimensional finite-difference time-domain simulations, we demonstrate the imaging operation of the structure with realistic material parameters including dispersions and losses. Our design should facilitate the demonstration of many novel effects associated with negative refraction on chip at optical wavelength ranges. In addition, this structure provides a new way of controlling the propagation of surface plasmons, which are important for nanoscale manipulation of optical waves.

We present a solution to the problem of reflection and refraction of a polarized Gaussian beam on the interface between two transparent media. The transverse shifts of the beams’ centers of gravity are calculated. They always satisfy the total angular momentum conservation law for beams, but, in general, do not satisfy the conservation laws for individual photons as a consequence of the lack of the “which path” information in a two-channel wave scattering. The field structure for the reflected and refracted beams is analyzed. In the scattering of a linearly polarized beam, photons of opposite helicities are accumulated at the opposite edges of the beam: this is the spin Hall effect for photons, which can be registered in the cross-polarized component of the scattered beam.

Since the work of Ebbesen et al. [Nature (London) 391, 667 (1998)], there has been immense interest in the optical properties of subwavelength holes in metal layers. While the enhanced transmission observed is generally associated with surface plasmon polaritons (SPPs), theoretical predictions suggest a similar response with perfectly conducting materials. However, Pendry et al. [Science 305, 847 (2004)] proposed that, if textured on a subwavelength scale, even perfect conductors support surface modes. Here, using microwave radiation incident upon an array of metal waveguides, we observe peaks in the transmissivity below cutoff and confirm the crucial role of these SPP-like modes in the mechanism responsible.

By controlling the irradiance of an extended quasimonochromatic, spatially incoherent source, an optical field is generated that exhibits spatial coherence with phase singularities, called coherence vortices. A simple optical geometry for direct visualization of coherence vortices is proposed, and the local properties and the spatial evolution of coherence vortex are experimentally investigated. To our knowledge, this is the first direct and quantitative experimental measurement of a generic coherence vortex.

We employ numerical computations of the full Navier-Stokes equations to investigate non-Boussinesq convection in a rotating system using water as the working fluid. We identify two regimes. For weak non-Boussinesq effects the Hopf bifurcation from steady to oscillating (whirling) hexagons is supercritical and typical states exhibit defect chaos that is systematically described by the cubic complex Ginzburg-Landau equation. For stronger non-Boussinesq effects the Hopf bifurcation becomes subcritical and the oscillations exhibit localized chaotic bursting, which is modeled by a quintic complex Ginzburg-Landau equation.

We put forward the existence of surface gap solitons at the interface between uniform media and an optical lattice with defocusing nonlinearity. Such new type of solitons forms when the incident and reflected waves at the interface of the lattice experience Bragg scattering, and feature a combination of the unique properties of both surface waves and gap solitons. We discover that gap surface solitons exist only when the lattice depth exceeds a threshold value, that they can be made completely stable, and that they can form stable bound states.

Numerical and analytical evidence is presented to show that hydrodynamic Lyapunov modes (HLMs) do exist in lattices of coupled Hamiltonian and dissipative maps. More importantly, we find that HLMs in these two classes of systems are different with respect to their spatial structure and their dynamical behavior. To be concrete, the corresponding dispersion relations of Lyapunov exponent versus wave number are characterized by $\lambda \sim k$ and $\lambda \sim k^2$, respectively. The HLMs in Hamiltonian systems are propagating, whereas those of dissipative systems show only diffusive motion. Extensive numerical simulations of various systems confirm that the existence of HLMs is a very general feature of extended dynamical systems with continuous symmetries and that the above-mentioned differences between the two classes of systems are universal in large extent.