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We study longitudinal and transverse wave propagation in beams with elastic properties that are periodically varying in space and time. Spatiotemporal modulation of the elastic properties breaks mechanical reciprocity and induces one-way propagation. We follow an analytic approach to characterize the non-reciprocal behavior of the structures by analyzing the symmetry breaking of the dispersion spectrum, which results in the formation of directional band gaps and produces shifts of the first Brillouin zone limits. This approach allows us to relate position and width of the directional band gaps to the modulation parameters. Moreover, we identify the critical values of the modulation speed to maximize the non-reciprocal effect. We numerically verify the theoretical predictions by using a finite element model of the modulated beams to compute the transient response of the structure. We compute the two-dimensional Fourier transform of the collected displacement fields to calculate numerical band diagrams, showing excellent agreement between theoretical and numerical dispersion diagrams.

A plasmonic antenna array is used to control the propagation of a light beam across an interface. The precise manipulation of a propagating wave using phase control is a fundamental building block of optical systems. The wavefront of a light beam propagating across an interface can be modified arbitrarily by introducing abrupt phase changes. We experimentally demonstrated unparalleled wavefront control in a broadband optical wavelength range from 1.0 to 1.9 micrometers. This is accomplished by using an extremely thin plasmonic layer (~λ/50) consisting of an optical nanoantenna array that provides subwavelength phase manipulation on light propagating across the interface. Anomalous light-bending phenomena, including negative angles of refraction and reflection, are observed in the operational wavelength range.

Understanding and analysing the complex phenomena related to elastic wave propagation has been the subject of intense research for many years and has enabled application in numerous fields of technology, including structural health monitoring (SHM).

The development of new artificial structures and materials is today one of the major research challenges in optics. In most studies so far, the design of such structures has been based on the judicious manipulation of their refractive index properties. Recently, the prospect of simultaneously using gain and loss was suggested as a new way of achieving optical behaviour that is at present unattainable with standard arrangements. What facilitated these quests is the recently developed notion of ‘parity–time symmetry’ in optical systems, which allows a controlled interplay between gain and loss. Here we report the experimental observation of light transport in large-scale temporal lattices that are parity–time symmetric. In addition, we demonstrate that periodic structures respecting this symmetry can act as unidirectional invisible media when operated near their exceptional points. Our experimental results represent a step in the application of concepts from parity–time symmetry to a new generation of multifunctional optical devices and networks. Using techniques by analogy with parity–time symmetry allows a combination of optical gain and loss in large-scale synthetic lattices, which can lead, for example, to such a lattice being invisible when viewed from one side. Symmetry rules in optical lattices In an impressive conceptual leap, it was recently suggested that the abstract idea of 'parity-time symmetry', part of quantum field theory, could be adapted as inspiration for the design of novel types of artificial optical material with unique properties not seen in natural materials. In practice, this means cleverly exploiting an interplay between optical gain and loss — the parameters that, along with a more readily manipulated refractive index, are the basic ingredients of an optical system. Until now, experimental work in this area has been limited to elemental two-component arrangements, but here Ulf Peschel and colleagues demonstrate light transport in large-scale 'parity-time' synthetic lattices by transferring the concept to the temporal domain. As an example of the unusual optical effects that can be produced in such a system, the authors demonstrate that the medium can become invisible in one direction.

We estimate depth‐dependent azimuthal anisotropy and shear wave velocity structure beneath the Alaska subduction zone by the inversion of a new Rayleigh wave dispersion dataset from 8 to 85 s period. We present a layered azimuthal anisotropy model from the forearc region offshore to the subduction zone onshore. In the forearc crust, we find a trench‐parallel pattern in the Semidi and Kodiak segments, while a trench‐oblique pattern is observed in the Shumagins segment. These fast directions agree well with the orientations of local faults. Within the subducted slab, a dichotomous pattern of anisotropy fast axes is observed along the trench, which is consistent with the orientation of fossil anisotropy generated at the mid‐ocean ridges of the Pacific‐Vancouver and Kula‐Pacific plates that is preserved during subduction. Beneath the subducted slab, a trench‐parallel pattern is observed near the trench, which may indicate the direction of mantle flow. Plain Language Summary The azimuthal anisotropy of seismic waves refers to the directional dependence of the seismic wave propagation speed. We present a comprehensive azimuthal anisotropy model of the Alaska subduction zone to a depth of 200 km, revealing anisotropy caused by local faults and fractures, fossil anisotropy inherited from the oceanic plate within the subducted slab, and sub‐slab mantle flow. The along‐strike variation of crustal anisotropy indicates variations in the stress regime in the forearc region. The along‐strike variation of anisotropy within the subducted slab identifies different origins of the subducted slab. Our model contributes to the understanding of the anisotropic structure and the sources of anisotropy in subduction zones. Key Points A new model of depth‐dependent azimuthal anisotropy of the Alaska subduction zone is built based on a new surface wave dataset The along‐strike variation in the azimuthal anisotropy of the forearc crust is caused by faults and fractures Azimuthal anisotropy within the subducted slab is controlled by fossil anisotropy produced at different mid‐ocean ridges

The study presents (a) a 44‐year wintertime climatology of resolved gravity wave (GW) fluxes and forcing in the extratropical stratosphere using ERA5, and (b) their composite evolution around gradual (final warming) and abrupt (sudden warming) transitions in the wintertime circulation, focusing on lateral fluxes. The transformed Eulerian mean equations are leveraged to provide a glimpse of the importance of GW lateral propagation (i.e., horizontal propagation) toward driving the wintertime stratospheric circulation by analyzing the relative contribution of the vertical versus meridional flux dissipation. The relative contribution from lateral propagation is found to be notable, especially in the Austral winter stratosphere where lateral (vertical) momentum flux convergence provides a peak climatological forcing of up to −0.5 (−3.5) m/s/day around 60°S at 40–45 km altitude. Prominent lateral propagation in the wintertime midlatitudes also contributes to the formation of belts of GW activity in both hemispheres. Plain Language Summary Atmospheric Gravity Waves (GWs) are atmospheric disturbances created by processes like convection, thunderstorms, flow over topography, etc. These waves can have wavelengths as small as 1 km to as large as 1,000–2,000 km. Most atmospheric GWs are not resolved in coarse‐resolution climate models. As a result, they are represented in climate models using parameterizations, which are approximate models that can be subject to various idealizations. One such idealization is the assumption of pure vertical propagation of GWs. In this study, we use multidecadal records from ERA5 reanalysis—which combines a high‐resolution model with assimilated observations to produce a close‐to‐observed state of the atmosphere and resolves some of these GWs—to quantify the impact of this assumption on the mean state of the extratropical stratosphere. This is done by extracting GWs from ERA5 data, computing horizontal momentum fluxes carried by these waves, and comparing the net acceleration/deceleration provided by these fluxes on the peak winter stratospheric circulation and key episodes of abrupt changes in the circulation. Analysis using ERA5 reveals that horizontal propagation of GWs can be notable in the midlatitude stratosphere, highlighting the need to develop GW parameterizations that represent this essential property of atmospheric GWs. Key Points Climatology of lateral fluxes from ERA5 shows substantial lateral propagation of gravity waves in both hemispheres Contribution of both lateral and vertical GW fluxes toward zonal wind forcing is the same order of magnitude Abrupt changes in GW forcing in the upper stratosphere around sudden stratospheric warmings persist even 20 days following the event

The present study focuses on the mechanism that controls the transition of the Euro-Atlantic circulation responses to El Niño–Southern Oscillation (ENSO) from early (December) to late winter (February) for the period 1981–2015. A positive phase of ENSO induces a precipitation dipole with increased precipitation in the western and reduced precipitation in the eastern tropical Indian Ocean; this occurs mainly during early winter (December) and less so in late winter (February). It is shown that these interbasin atmospheric teleconnections dominate the response in the Euro-Atlantic sector in early winter by modifying the subtropical South Asian jet (SAJET) and forcing a wavenumber-3 response that projects spatially onto the positive North Atlantic Oscillation (NAO) pattern. On the contrary, during late winter, the response in the Euro-Atlantic sector is dominated by the well-known ENSO wave train from the tropical Pacific region, involving extratropical anomalies that project spatially on the positive phase of the Pacific–North American (PNA) pattern and the negative phase of the NAO pattern. Numerical experiments with an atmospheric model (an AGCM) forced by an Indian Ocean heating dipole anomaly support the hypothesis that the Indian Ocean modulates the SAJET and enforces the Rossby wave propagation to the Euro-Atlantic region in early winter. These phenomena are also investigated using the ECMWF SEAS5 reforecast dataset. In SEAS5, the ENSO interbasin tropical teleconnections and the response of the Euro-Atlantic circulation anomalies and their change from early to late winter are realistically predicted, although the strength of the early winter signal originated from the Indian Ocean is underestimated.