Coiling-Up Space Metasurface for High-Efficient and Wide-angle Acoustic Wavefront Steering

The recent advent of acoustic metasurface displays tremendous potential with their unique and flexible capabilities of wavefront manipulations. In this paper, we propose an acoustic metagrating made of binary coiling-up space structures to coherently control the acoustic wavefront steering. The acoustic wave steering is based on the in-plane coherent modulation of waves in different diffraction channels. The acoustic metagrating structure with a subwavelength thickness is realized with 3D printed two coiling-up space metaunits. By adjusting structural parameters of the metaunits, the −1st-order diffraction mode can be retained, and the rest of the diffraction orders are eliminated as much as possible through destructive interference, forming a high-efficiency anomalous reflection in the scattering field. The anomalous reflection performance of the designed metagrating is achieved over a wide range of incident angles with high efficiency.

A Review of Topological Semimetal Phases in Photonic Artificial Microstructures

In the past few years, the concept of topological matter has inspired considerable research in broad areas of physics. In particular, photonic artificial microstructures like photonic crystals and metamaterials provide a unique platform to investigate topologically non-trivial physics in spin-1 electromagnetic fields. Three-dimensional (3D) topological semimetal band structures, which carry non-trivial topological charges, are fundamental to 3D topological physics. Here, we review recent progress in understanding 3D photonic topological semimetal phases and various approaches for realizing them, especially with photonic crystals or metamaterials. We review topological gapless band structures and topological surface states aroused from the non-trivial bulk topology. Weyl points, 3D Dirac points, nodal lines, and nodal surfaces of different types are discussed. We also demonstrate their application in coupling spin-polarized electromagnetic waves, anomalous reflection, vortex beams generation, bulk transport, and non-Hermitian effects.

Conditions for establishing the “generalized Snell’s law of refraction” in all-dielectric metasurfaces: theoretical bases for design of high-efficiency beam deflection metasurfaces

The generalized Snell’s law dictates that introducing a phase gradient at the interface of two media can shape incident light and achieve anomalous reflection or refraction. However, when the introduced phase gradient is realized via the scattering of nanoparticles in the metasurfaces, this law needs to be modified; certain conditions need to be met when the law is established. We present the conditions for establishing the “generalized Snell’s law of refraction” in all-dielectric metasurfaces under the incidence of different polarized light. These conditions can provide theoretical bases for the subsequent design of high-efficiency beam deflection metasurfaces. The relationship between the highest achievable anomalous refraction efficiency and the number of nanoparticles within one period of the metasurface is also summarized. In addition, the generalized refraction should not depend on the polarization states of incident light; however, the previous realization conditions of anomalous refraction were sensitive to the polarization states. Thus, conditions for establishing the polarization-independent generalized Snell’s law of refraction in all-dielectric metasurfaces are presented.

Diffraction in phase gradient acoustic metagratings: multiple reflection and integer parity design

Diffraction occurs when acoustic waves are incident on periodic structures such as graded metasurfaces. While numerous interesting diffraction phenomena have been observed and demonstrated, the underlying mechanism of diffraction in these structures is often overlooked. Here we provide a generic explanation of diffraction in phase gradient acoustic metagratings and relate high-order diffractions to multiple reflections in the unit cells. As such, we reveal that the number of unit cells within the metagrating plays a dominant role in determining the diffraction patterns. It is also found that the integer parity of the metagrating leads to anomalous reflection and refraction with high efficiency. The theory is verified by numerical simulations and experiments on planar metagratings and provides a powerful mechanism to manipulate acoustic waves. We further extend the theory to cylindrical waveguides for the control of sound vortices via topological charge in azimuthal metagratings. The relevance of the theory in achieving asymmetric wave control and high absorption is also discussed and verified both numerically and experimentally.