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We present a method to design a single-layer, all-metallic angular stable metasurface (ASM) filter unit cell for nearly the entire angular spectrum. We formulate the optimization criterion using the energy balance condition derived from Poynting’s Theorem. The proposed unit cell has a “cloverleaf” geometry, and we show that by adjusting the properties of the cloverleaf, we manipulate the in-plane spatial dispersion, thus realizing an angularly stable resonance that extends to nearly 90° incidence. After optimization, we revisit the energy balance criterion and show, using a spatially dispersive admittance expansion, how optimization at three points is enough to obtain the required balance for nearly the entire angular spectrum. The proposed ASM was fabricated from a bare aluminum sheet and measured for transverse electric (TE) and transverse magnetic (TM) polarization, with angularly stable performance that matches the theory and simulation. The proposed design offers new capabilities for radar and antenna design applications, with the simple, single-layer, all-metallic structure being particularly useful in aerospace and satellite applications.

Twisted two-dimensional bilayer materials exhibit many exotic electronic phenomena. Manipulating the ‘twist angle’ between the two layers enables fine control of the electronic band structure, resulting in magic-angle flat-band superconductivity1,2, the formation of moiré excitons3–8and interlayer magnetism9. However, there are limited demonstrations of such concepts for photons. Here we show how analogous principles, combined with extreme anisotropy, enable control and manipulation of the photonic dispersion of phonon polaritons in van der Waals bilayers. We experimentally observe tunable topological transitions from open (hyperbolic) to closed (elliptical) dispersion contours in bilayers of α-phase molybdenum trioxide (α-MoO3), arising when the rotation between the layers is at a photonic magic twist angle. These transitions are induced by polariton hybridization and are controlled by a topological quantity. At the transitions the bilayer dispersion flattens, exhibiting low-loss tunable polariton canalization and diffractionless propagation with a resolution of less than λ0/40, where λ0is the free-space wavelength. Our findings extend twistronics10and moiré physics to nanophotonics and polaritonics, with potential applications in nanoimaging, nanoscale light propagation, energy transfer and quantum physics.

Recent advances in twistronics of low-dimensional materials, such as bilayer graphene and transition-metal dichalcogenides, have enabled a plethora of unusual phenomena associated with moiré physics. However, several of these effects require demanding manipulation of superlattices at the atomic scale, such as the careful control of rotation angle between two closely spaced atomic lattices. Here, we study moiré hyperbolic plasmons in pairs of hyperbolic metasurfaces (HMTSs), unveiling analogous phenomena at the mesoscopic scale. HMTSs are known to support confined surface waves collimated toward specific directions determined by the metasurface dispersion. By rotating two evanescently coupled HMTSs with respect to one another, we unveil rich dispersion engineering, topological transitions at magic angles, broadband field canalization, and plasmon spin-Hall phenomena. These findings open remarkable opportunities to advance metasurface optics, enriching it with moiré physics and twistronic concepts.

The cross-coupling between strain and velocity in acoustic materials, known as Willis coupling, has recently been receiving increasing attention within the broad acoustics community. Willis coupling can provide a new degree of freedom to control sound propagation, which has been enabling several novel applications. In this work, based on the general constitutive relations of Willis media and the acoustic Poynting theorem, we study the constraints stemming from fundamental symmetries – parity, time-reversal, reciprocity and energy conservation – in these media. The wave features, such as wave-vectors, wave impedance and orthogonality are also generally investigated. In addition, we put forward a nonlocal model that unveils the relation between Willis media and nonlocal materials, and how Willis phenomena stem from weak forms of nonlocality. • We relate wave properties in Willis media to physical symmeties: parity, time-reversal, reciprocity and energy conservation. • We generally prove orthogonality relations, wave numbers and impedance for eigen-waves in Willis media. • We highlight the exotic responses emerging in purely nonreciprocal Willis media. • We derive an equivalent description of Willis media in terms of weak nonlocality. [ABSTRACT FROM AUTHOR]

Optical metasurfaces are assemblies of subwavelength artificially designed inclusions forming planarized devices with unprecedented capabilities to manipulate electromagnetic waves and facilitate multiple functionalities. Recent topical efforts in this research area have been focused on pushing forward the frontiers of enhanced light-matter interactions exploiting new concepts and fabrication capabilities. In this context, layered and twisted metasurfaces have showcased interesting features, including flexibility and tunability in manipulating light, contributing to the development of moiré physics for light. Here, we provide an overview on the state-of-art layered and stacked moiré metasurfaces. Free-space multifunctional metadevices are first discussed, followed by recent discoveries in polaritonics and twistronics for twisted hyperbolic metasurface bilayers. We conclude with a discussion on recent advances in the nanofabrication of moiré metasurfaces, and remark on their future potential applications.