In simulations of decaying two-dimensional turbulence, Clercx and co-workers discovered the unexpected phenomenon of “spin-up.” (This is the spontaneous acquisition of angular momentum by a turbulent two-dimensional fluid in a rigid container.) Here we show that this phenomenon can readily be interpreted in terms of statistical models of two-dimensional turbulence. When the net vorticity is zero in a bounded system, there are two distinct types of statistical equilibrium. The first has the expected property that its angular momentum is zero. However, the second type has a nonzero angular momentum even though its circulation vanishes. The relative probability of the two types of equilibrium depends strongly on the shape of the boundary and weakly on the energy. The angular momentum predicted for the second type of equilibrium is in good agreement with that found in simulations at high Reynolds number.

We study theoretically the dispersion of plasmonic honeycomb lattices and find Dirac spectra for both dipole and quadrupole modes. Zigzag edge states derived from Dirac points are found in ribbons of these honeycomb plasmonic lattices. The zigzag edge states for out-of-plane dipole modes are closely analogous to the electronic ones in graphene nanoribbons. The edge states for in-plane dipole modes and quadrupole modes, however, have rather unique characters due to the vector nature of the plasmonic excitations. The conditions for the existence of plasmonic edge states are derived analytically.

We consider an overdamped Brownian particle, exposed to a two-dimensional, square lattice potential and a rectangular ac drive. Depending on the driving amplitude, the linear response to a weak dc force along a lattice symmetry axis consist in a mobility in basically any direction. In particular, motion exactly opposite to the applied dc force may arise. Upon changing the angle of the dc force relatively to the square lattice, the particle motion remains predominantly opposite to the dc force. The basic physical mechanism consists in a spontaneous symmetry breaking of the unbiased deterministic particle dynamics.

The classic “Ewald-sphere construction” for determining the structure of crystalline objects from x-ray and neutron diffraction experiments is generalized to determine the correlation functions of scattering potentials of stationary random media from scattering experiments.

We experimentally demonstrate two-beam coupling between nearly identical filament-forming beams intersecting in air. A 7% amplification of one beam occurs at the energy expense of the other in a single interaction, controllable by adjusting their relative delay by tens of femtoseconds. The data are consistent with the impulsive Raman nonlinear response of the air molecules as the coupling mechanism. The filament conical emission is controllably enhanced or suppressed by the interaction, indicating that two-beam coupling may be an effective means for filament regeneration and control.

Electromagnetic waves carry angular momenta, and, due to spin-orbit interaction, an encounter with a gradient of refractive index leads to transport of spin similar to the electronic spin Hall effect. We show here that transversal spin transport is possible even when the symmetry of optical interaction is of higher dimensionality. We demonstrate that for a wave in a pure state of polarization, the spin-orbit interaction results in a spiraling power flow that is determined by the extent of the interaction. As a result, the spin transport can be resonantly enhanced in a spherical geometry. Our results open the possibility for developing new functionalities for photonic devices.

Two intersecting beams of internal gravity waves will generically create two wave packets by nonlinear interaction. The frequency of one packet will be the sum and that of the other packet will be the difference of the frequencies of the intersecting beams. In principle, each packet should form an “$X$” pattern, or “St. Andrew’s cross” consisting of four beams outgoing from the point of intersection. Here we derive selection rules and show that most of the expected nonlinear beams are forbidden. These rules can also be applied to the reflection of a beam from a boundary.

Injected power $P$ and pressure $p$ fluctuations in a swirling flow of polymer solutions in a wide range of polymer concentrations $c$ in elastic turbulence regime show non-Gaussian statistics that strongly resemble statistical behavior of $P$ and $p$ in hydrodynamic turbulence. Together with this fact, weak dependence of the statistics of rescaled variables on $c$ may suggest that there are universal mechanisms determining the intermittent statistics of $P$ and $p$. We also show that the study of the statistics of $p$ provides a way to study statistics of the elastic stresses in elastic turbulence otherwise currently unattainable.

We report measurements of turbulent heat transport in samples of ethane (${\mathrm{C}}_2{\mathrm{H}}_6$) heated from below while the applied temperature difference $\Delta T$ straddled the liquid-vapor coexistence curve $T_{\varphi }(P)$. When the sample top temperature $T_t$ decreased below $T_{\varphi }$, droplet condensation occurred and the latent heat of vaporization $H$ provided an additional heat-transport mechanism. The effective conductivity ${\lambda }_{\mathrm{eff}}$ increased linearly with decreasing $T_t$, and reached a maximum value ${\lambda }_{\mathrm{eff}}^{*}$ that was an order of magnitude larger than the single-phase ${\lambda }_{\mathrm{eff}}$. As $P$ approached the critical pressure, ${\lambda }_{\mathrm{eff}}^{*}$ increased dramatically even though $H$ vanished. We attribute this phenomenon to an enhanced droplet-nucleation rate as the critical point is approached.

We discuss two independent, large scale experiments performed in two wave basins of different dimensions in which the statistics of the surface wave elevation are addressed. Both facilities are equipped with a wave maker capable of generating waves with prescribed frequency and directional properties. The experimental results show that the probability of the formation of large amplitude waves strongly depends on the directional properties of the waves. Sea states characterized by long-crested and steep waves are more likely to be populated by freak waves with respect to those characterized by a large directional spreading.

We report that the classical phenomenon of optical activity, which is traditionally associated with chirality (helicity) of organic molecules, proteins, and inorganic structures, can be observed in artificial planar media which exhibit neither 3D nor 2D chirality. We observe the effect in the microwave and optical parts of the spectrum at oblique incidence to regular arrays of nonchiral subwavelength metamolecules in the form of strong circular dichroism and birefringence indistinguishable from those of chiral three-dimensional media.

A numerical continuation method has been carried out seeking solutions for two distinct flow configurations, planar Couette flow (PCF) and laterally heated flow in a vertical slot (LHF). We found that the spanwise vortex solution in LHF identifies a new solution in PCF. The vortical structure of our new solution has the shape of a hairpin observed ubiquitously in high-Reynolds-number turbulent flow, and we believe this discovery may provide the paradigm for a hierarchical organization of coherent structures in turbulent shear layers.

We use terahertz time-domain spectroscopy to study the transmission properties of metallic films perforated with aperture arrays having deterministic or stochastic fractal morphologies (“plasmonic fractals”), and compare them with random aperture arrays. All of the measured plasmonic fractals show transmission resonances and antiresonances at frequencies that correspond to prominent features in their structure factors in $\mathbf{k}$ space. However, in sharp contrast to periodic aperture arrays, the resonant transmission enhancement decreases with increasing array size. This property is explained using a density-density correlation function, and is utilized for determining the underlying fractal dimensionality, $D(<2)$. Furthermore, a sum rule for the transmission resonances and antiresonances in plasmonic fractals relative to the transmission of the corresponding random aperture arrays is obtained, and is shown to be universal.

The manipulation of optical waves in the subwavelength scale is limited by diffraction. In the vicinity of a narrow aperture, the amplitude of the electric field is approximately uniform and the transmissivity is extremely low. Here we show that despite these fundamental constraints it may be possible to transport and redirect a complex electromagnetic image through a tiny subwavelength hole with diameter considerably smaller than the diameter of the image, without losing the subwavelength details. The proposed concepts hold promise for an unprecedented manipulation of the electromagnetic and optical fields in the nanoscale with potential applications in imaging and sensing.

Heat convection in water can be suppressed by adding a small amount of highly thermophilic nanoparticles. We show that such suppression is not effective when a suspension with uniform concentration of nanoparticles is suddenly heated from below. At Rayleigh numbers smaller than a sample dependent threshold ${\mathrm{Ra}}^{*}$ we observe transient oscillatory convection. Unexpectedly, the duration of convection diverges at ${\mathrm{Ra}}^{*}$. Above ${\mathrm{Ra}}^{*}$ oscillatory convection becomes permanent and the heat transferred exhibits bistability. Our results are explained only partially and qualitatively by existing theories.

A theoretical description for the equilibrium states of a large class of models of two-dimensional and geophysical flows is presented. A statistical ensemble equivalence is found to exist generically in these models, related to the occurrence of peculiar phase transitions in the flow topology. The first example of a bicritical point (a bifurcation from a first toward two second order phase transitions) in the context of systems with long-range interactions is reported. Academic ocean models, the Fofonoff flows, are studied in the perspective of these results.

The coalescence of a drop with a flat liquid surface pinches off a satellite from its top, in the well-known coalescence cascade, whereas the coalescence of two equally sized drops does not appear to leave such a satellite. Herein we perform experiments to identify the critical diameter ratio of two drops, above which a satellite is produced during their coalescence. We find that the critical parent ratio is as small as 1.55, but grows monotonically with the Ohnesorge number. The daughter size is typically about 50% of the mother drop. However, we have identified novel pinch-off dynamics close to the critical size ratio, where the satellite does not fully separate, but rather goes directly into a second stage of the coalescence cascade, thus generating a much smaller satellite droplet.

We report detailed measurements of the high-harmonic spectra generated from argon atoms. The spectra exhibit a deep minimum that is shown to be independent of the laser intensity, and is thus a clear measure of the electronic structure of the atom. We show that exact field-free continuum wave functions reproduce the minimum, but plane wave and Coulomb wave functions do not. This remarkable observation suggests that electronic structure can be accurately determined in high-harmonic experiments despite the presence of the strong laser field. Our results clarify the relation between high-harmonic generation and photoelectron spectroscopy. The use of exact continuum functions also resolves the ambiguity associated with the choice of the dispersion relation.

The response of a noisy nonlinear system to deterministic input signals can be enhanced by cooperative phenomena. We show that when one presents two square waves as input to a two-state system, the response of the system can produce a logical output ($\mathrm{NOR}/\mathrm{OR}$) with a probability controlled by the noise intensity. As one increases the noise (for fixed threshold or nonlinearity), the probability of the output reflecting a $\mathrm{NOR}/\mathrm{OR}$ operation increases to unity and then decreases. Changing the nonlinearity (or the thresholds) of the system changes the output into another logic operation ($\mathrm{NAND}/\mathrm{AND}$) whose probability displays analogous behavior. The interplay of nonlinearity and noise can yield logic behavior, and the emergent outcome of such systems is a logic gate. This “logical stochastic resonance” is demonstrated via an experimental realization of a two-state system with two (adjustable) thresholds.

We present a local capacitor model that enables a simple yet quantitatively accurate description of lightning rod effect in nanoplasmonics. A notion of $\lambda$-zone capacitance is proposed and applied to predict the strongly induced electric field by a light source near nanoscale metal edges such as metal tip or metal gap. The enhancement factor, calculated from the local capacitor model, shows excellent agreement with more rigorous results. The $\lambda$-zone capacitor allows a blockwise treatment of nano-optical devices and constitutes a basic element of optical nanocircuits.