Willis coupling in acoustic materials defines the cross-coupling between strain and velocity, analogous to bianisotropic phenomena in electromagnetics. While these phenomena have been garnering significant attention in recent years, to date their effects have been considered mostly perturbative. Here, we derive general bounds on the Willis response of acoustic scatterers, show that these bounds can be reached in suitably designed scatterers, and outline a systematic venue for the realistic implementation of maximally bianisotropic acoustic inclusions. We then employ these inclusions to realize acoustic metasurfaces for bending and steering of sound with unitary efficiency.

We propose a scheme to retrieve the size parameters of a nanoparticle on a glass substrate at a scale much smaller than the wavelength. This is achieved by illuminating the particle using two plane waves to create rich and nontrivial local polarization distributions, and observing the far-field scattering pattern into the substrate. By using this illumination to control the induced complex dipole moment, the exponential decay of power radiated into the supercritical region, as well as directional scattering due to spin-orbit coupling can be exploited to retrieve the particle’s shape, size, and position directly from the far-field scattering with high sensitivity and without the need for a complicated and time-consuming optimization algorithm. Our method brings about a far-field superresolution nanometrology scheme based on the interaction of vectorial light with nanoparticles.

Resonant light scattering by metallic and high-index dielectric nanoparticles has received enormous attention and found many great applications. However, low-index dielectric nanoparticles typically do not show resonant scattering behaviors due to poor light confinement caused by small index contrast. This Letter describes a simple and effective approach to drastically enhance the resonance effect of the low-index particles by partial metal dressing. Mie resonances of low-index nanoparticles can now be easily visualized by scattered light. This scattering peak depends on sphere size and has a reasonable linewidth. A size difference as small as 8 nm was resolved by a peak shift or even by color change. The scattering peak is attributed to the enhanced ${\mathrm{TE}}_{11}$ Mie resonance of the low-index nanospheres. The metal dress not only provides a high-reflection boundary, but also functions as an antenna to couple the confined light power to the far field, leading to scattering maxima in the spectra. Additionally, the enhanced ${\mathrm{TE}}_{11}$ Mie resonance in low-index nanoparticles features a considerable magnetic response due to the strong circulating displacement currents induced by the intensified $E$ field despite of a low permittivity (hence low index) of the particles. The enhanced Mie resonances could be used to sense minute changes in size or refractive index of low-index nanoparticles and benefit a wide range of applications.

We study the propagation of surface plasmon polaritons (SPPs) on a metal surface which hosts a thin film of a liquid dielectric. The Ohmic losses that are inherently present due to the coupling of SPPs to conductors’ electron plasma, induce temperature gradients and fluid deformation driven by the thermocapillary effect, which lead to a nonlinear and nonlocal change of the effective dielectric constant. The latter extends beyond the regions of highest optical intensity and constitutes a novel thermally self-induced mechanism that affects the propagation of the SPPs. We derive the nonlinear and nonlocal Schrödinger equation that describes propagation of low intensity SPP beams, and show analytically and numerically that it supports a novel optical spatial soliton excitation.

There are currently three main classes of liquid-repellent surfaces: micro- or nanostructured superhydrophobic surfaces, flat surfaces grafted with “liquidlike” polymer brushes, and lubricated surfaces. Despite recent progress, the mechanistic explanation for the differences in droplet behavior on such surfaces is still under debate. Here, we measure the dissipative force acting on a droplet moving on representatives of these surfaces at different velocities $U=0.01–1\mathrm{mm}/\mathrm{s}$ using a cantilever force sensor with submicronewton accuracy and correlate it to the contact line dynamics observed using optical interferometry at high spatial (micron) and temporal ($<0.1\mathrm{s}$) resolutions. We find that the dissipative force—due to very different physical mechanisms at the contact line—is independent of velocity on superhydrophobic surfaces but depends nonlinearly on velocity for flat and lubricated surfaces. The techniques and insights presented here will inform future work on liquid-repellent surfaces and enable their rational design.

We perform three-dimensional particle tracking measurements on droplets in a turbulent airflow. The droplets display the well-known preferential concentration of inertial particles, with an additional extreme clustering at the smallest scales. We explain this additional clustering phenomenon theoretically based on a Stokes-flow description of two spheres including their mutual hydrodynamic interaction and a perturbative small-inertia expansion.

The Kuramoto model (KM) is a theoretical paradigm for investigating the emergence of rhythmic activity in large populations of oscillators. A remarkable example of rhythmogenesis is the feedback loop between excitatory ($E$) and inhibitory ($I$) cells in large neuronal networks. Yet, although the $EI$-feedback mechanism plays a central role in the generation of brain oscillations, it remains unexplored whether the KM has enough biological realism to describe it. Here we derive a two-population KM that fully accounts for the onset of $EI$-based neuronal rhythms and that, as the original KM, is analytically solvable to a large extent. Our results provide a powerful theoretical tool for the analysis of large-scale neuronal oscillations.

X-ray diffraction from molecules in the ground state produces an image of their charge density, and time-resolved x-ray diffraction can thus monitor the motion of the nuclei. However, the density change of excited valence electrons upon optical excitation can barely be monitored with regular diffraction techniques due to the overwhelming background contribution of the core electrons. We present a nonlinear x-ray technique made possible by novel free electron laser sources, which provides a spatial electron density image of valence electron excitations. The technique, sum frequency generation carried out with a visible pump and a broadband x-ray diffraction pulse, yields snapshots of the transition charge densities, which represent the electron density variations upon optical excitation. The technique is illustrated by ab initio simulations of transition charge density imaging for the optically induced electronic dynamics in a donor or acceptor substituted stilbene.

The polarization property of high harmonics from gallium selenide is investigated using linearly polarized midinfrared laser pulses. With a high electric field, the perpendicular polarization component of the odd harmonics emerges, which is not present with a low electric field and cannot be explained by the perturbative nonlinear optics. A two-dimensional single-band model is developed to show that the anisotropic curvature of an energy band of solids, which is pronounced in an outer part of the Brillouin zone, induces the generation of the perpendicular odd harmonics. This model is validated by three-dimensional quantum mechanical simulations, which reproduce the orientation dependence of the odd-order harmonics. The quantum mechanical simulations also reveal that the odd- and even-order harmonics are produced predominantly by the intraband current and interband polarization, respectively. These experimental and theoretical demonstrations clearly show a strong link between the band structure of a solid and the polarization property of the odd-order harmonics.

Interscale interaction between small-scale structures near the wall and large-scale structures away from the wall plays an increasingly important role with increasing Reynolds number in wall-bounded turbulence. While the top-down influence from the large- to small-scale structures is well known, it has been unclear whether the small scales near the wall also affect the large scales away from the wall. In this Letter we show that the small-scale near-wall structures indeed play a role to maintain the large-scale structures away from the wall, by showing that the Reynolds shear stress is transferred from small to large scales throughout the channel. This is in contrast to the turbulent kinetic energy transport which is from large to small scales. Such an “inverse” interscale transport of the Reynolds shear stress eventually supports the turbulent energy production at large scales.

We present a generalization of turbulent pair dispersion to large initial separations ($\eta <r_0<L$), by introducing a new time scale, ${\tau }_{v_0}$, that reflects the persistence of initial conditions at time $\tau =0$. Results of 3D Lagrangian tracking experiments at moderate Reynolds numbers show that pairs, for which the new time scale is shorter than the eddy turnover time scale, separate as in the Richardson superdiffusive regime, $⟨\mathrm{\Delta }{r}^2⟩\propto {\tau }^3$. The analysis of delay times (time interval to cross $\mathrm{\Delta }r=\rho r_0$) of these conditionally sampled pairs exhibit ${\rho }^{2/5}$ scaling.

General relativity uses curved space-time to describe accelerating frames. The movement of particles in different curved space-times can be regarded as equivalent physical processes based on the covariant transformation between different frames. In this Letter, we use one-dimensional curved metamaterials to mimic accelerating particles in curved space-times. The different curved shapes of structures are used to mimic different accelerating frames. The different geometric phases along the structure are used to mimic different movements in the frame. Using the covariant principle of general relativity, we can obtain equivalent nanostructures based on space-time transformations, such as the Lorentz transformation and conformal transformation. In this way, many covariant structures can be found that produce the same surface plasmon fields when excited by spin photons. A new kind of accelerating beam, the Rindler beam, is obtained based on the Rindler metric in gravity. Very large effective indices can be obtained in such systems based on geometric-phase gradient. This general covariant design method can be extended to many other optical media.

A thermodynamically consistent free energy model for fluid flows comprised of one gas and two liquid components is presented and implemented using the entropic lattice Boltzmann scheme. The model allows a high density ratio, up to the order of $O({10}^3)$, between the liquid and gas phases, and a broad range of surface tension ratios, covering partial wetting states where Neumann triangles are formed, and full wetting states where complete encapsulation of one of the fluid components is observed. We further demonstrate that we can capture the bouncing, adhesive, and insertive regimes for the binary collisions between immiscible droplets suspended in air. Our approach opens up a vast range of multiphase flow applications involving one gas and several liquid components.

We uncover signatures of quantum chaos in the many-body dynamics of a Bose-Einstein condensate-based quantum ratchet in a toroidal trap. We propose measures including entanglement, condensate depletion, and spreading over a fixed basis in many-body Hilbert space, which quantitatively identify the region in which quantum chaotic many-body dynamics occurs, where random matrix theory is limited or inaccessible. With these tools, we show that many-body quantum chaos is neither highly entangled nor delocalized in the Hilbert space, contrary to conventionally expected signatures of quantum chaos.

Nonlinear optical frequency conversion has been challenged to move down to the extreme ultraviolet and x-ray region. However, the extremely low signals have allowed researchers to only perform transmission experiments of the gas phase or ultrathin films. Here, we report second harmonic generation (SHG) of the reflected beam of a soft x-ray free-electron laser from a solid, which is enhanced by the resonant effect. The observation revealed that the double resonance condition can be met by absorption edges for transition metal oxides in the soft x-ray range, and this suggests that the resonant SHG technique can be applicable to a wide range of materials. We discuss the possibility of element-selective SHG spectroscopy measurements in the soft x-ray range.

We propose a scheme for the reconstruction of plasmonic near fields at isolated nanoparticles from infrared-streaked extreme-ultraviolet photoemission spectra. Based on quantum-mechanically modeled spectra, we demonstrate and analyze the accurate imaging of the IR-streaking-pulse-induced transient plasmonic fields at the surface of gold nanospheres and nanoshells with subfemtosecond temporal and subnanometer spatial resolution.

We report a theoretical derivation, an experimental observation and a numerical validation of nonlinear phase domain walls in weakly nonlinear deep water surface gravity waves. The domain walls presented are connecting homogeneous zones of weakly nonlinear plane Stokes waves of identical amplitude and wave vector but differences in phase. By exploiting symmetry transformations within the framework of the nonlinear Schrödinger equation we demonstrate the existence of exact analytical solutions representing such domain walls in the weakly nonlinear limit. The walls are in general oblique to the direction of the wave vector and stationary in moving reference frames. Experimental and numerical studies confirm and visualize the findings. Our present results demonstrate that nonlinear domain walls do exist in the weakly nonlinear regime of general systems exhibiting dispersive waves.