Binary Metasurface Research Articles

Efficient asymmetric acoustic transmission based on the combination of a binary metasurface and a gradient index metasurface

We design and demonstrate the asymmetric acoustic transmission based on the combination of a binary metasurface (BM) and a gradient index metasurface (GIM). It yields efficient asymmetric acoustic transmission for two opposite incident directions: nearly total transmission for positive incidence but nearly total reflection for negative incidence. The underlying mechanism is ascribed to the diffraction characteristics of the BM and the GIM. We exemplify the BM and the GIM by coating unit cells consisting of three-layers of acoustic labyrinthine metamaterials, which provide full phase control and total transmission. Numerical simulations agree well with the theoretical analysis.

A Low-Temperature Annealing Method for Alloy Nanostructures and Metasurfaces: Unlocking a Novel Degree of Freedom.

The material and exact shape of a nanostructure determine its optical response, which is especially strong for plasmonic metals. Unfortunately, only a few plasmonic metals are available, which limits the spectral range where these strong optical effects can be utilized. Alloying different plasmonic metals can overcome this limitation, at the expense of using a high-temperature alloying process, which adversely destroys the nanostructure shape. Here, a low-temperature alloying process is developed where the sample is heated at only 300°C for 8 h followed by 30min at 450°C and Au-Ag nanostructures with a broad diversity of shapes, aspect ratios, and stoichiometries are fabricated. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses confirm the homogeneous alloying through the entire sample. Varying the alloy stoichiometry tunes the optical response and controls spectral features, such as Fano resonances. Binary metasurfaces that combine nanostructures with different stoichiometries are fabricated using multiple-step electron-beam lithography, and their optical function as a hologram or a Fresnel zone plate is demonstrated at the visible wavelength of λ= 532nm. This low-temperature annealing technique provides a versatile and cost-effective way of fabricating complex Au-Ag nanostructures with arbitrary stoichiometry.

The generation of acoustic Airy beam with selective band based on binary metasurfaces: Customized on demand

In this work, we present a type of binary metasurface (BM) to generate an acoustic Airy beam in air. Two coding bits, a rectangular cavity (bit “0”) and a waveguide with seven Helmholtz resonators (bit “1”), are adopted to construct the acoustic structure, which offers degrees of freedom to manipulate the transmitted field. The operating band is capable of customizing in an ultrabroadband of 3000–15 000 Hz owing to the linear-like phase shift and high transmittance of the coding bits. To verify the feasibility of the design, a BM with a certain parameter (w = 5) is fabricated with photosensitive resin via stereolithography, and the working band is customized as 4000–5500 Hz. The experiment results show that the apparent self-bending beam is able to be generated in a broadband, which agree well with the numerical simulation. In addition, we further demonstrate that self-focusing can be realized by taking advantage of two symmetrical BMs conveniently, which improve the functionality of the coding bits. These results may provide potential application in biomedical ultrasound and nondestructive testing.

Tunable asymmetric acoustic transmission via binary metasurface and zero-index metamaterials

The pursuit of tunable asymmetric sound transmission has been a long-term topic since it could contribute to providing more flexibilities in many areas of acoustic engineering. The interference effect can be a feasible approach in which two waves with the same frequency superposed to form the resultant wave with manipulated amplitude according to the relative phase difference between them. However, strictly speaking, restricted by the spatial variance of phase, the manipulated domain created by the specific phase difference is always limited to a spot with dimensions much smaller than the wavelength. Here, we proposed a design to break this barrier that can realize the tunable asymmetric transmission via the combination of zero-index metamaterials and the binary metasurface. The zero-index metamaterial can provide the effective extremely large speed to shrink the infinite domain into a spot acoustically and the binary metasurface can be used to tune the specific phase difference. Numerical simulations and experimental measurements have good agreement and show that the acoustic waves impinged from the side of metasurface will be manipulated to have controllable transmission, while the acoustic waves impinged from the side of zero-index metamaterials will keep a high transmission. We think the proposed design is full of physical significance, which may find potential applications in many fields, like noise cancelation, acoustic imaging, and ultrasound therapy.

Topological-Insulator-Based Gap-Surface Plasmon Metasurfaces

Topological insulators (TIs) have unique highly conducting symmetry-protected surface states while the bulk is insulating, making them attractive for various applications in condensed matter physics. Recently, topological insulator materials have been tentatively applied for both near- and far-field wavefront manipulation of electromagnetic waves, yielding superior plasmonic properties in the ultraviolet (UV)-to-visible wavelength range. However, previous reports have only demonstrated inefficient wavefront control based on binary metasurfaces that were digitalized on a TI thin film or non-directional surface plasmon polariton (SPP) excitation. Here, we numerically demonstrated the plasmonic capabilities of the TI Bi2Te3 as a material for gap–surface plasmon (GSP) metasurfaces. By employing the principle of the geometric phase, a far-field beam-steering metasurface was designed for the visible spectrum, yielding a cross-polarization efficiency of 34% at 500 nm while suppressing the co-polarization to 0.08%. Furthermore, a birefringent GSP metasurface design was studied and found to be capable of directionally exciting SPPs depending on the incident polarization. Our work forms the basis for accurately controlling the far- and near-field responses of TI-based GSP metasurfaces in the visible spectral range.

Ultrasonic sharp autofocusing with acoustic metasurface

Ultrasound focusing manifests itself as the accumulation of ultrasound energy on the target. It attracts extensive interest owing to its substantial advantage in engineering detection and biomedical science. However, the conventional focusing technique leads to the undesirable off-target influence in the nonfocusing region, hindering its application such as ultrasound surgery. Here we propose an ultrasound beam with idiosyncratic sharp autofocusing properties and implement it with the binary metasurface. The beam shows a sharp accumulation at the focus with an abrupt increase of intensity by three orders of magnitude while keeping the unintended region intact. The sharp autofocusing is realized by the controlled transverse self-acceleration and collapse of caustic at the focus in a nonlinear fashion. The intensity contrast, width, and position of the beam are highly tunable to realize the freewheeling focusing with high efficiency. Remarkably, the possibility to synthesize the abrupt autofocusing by the phase-only modulation is revealed. As an implementation, we demonstrate that the precise control of the sharp converging beam can be realized by an extremely simple metasurface with binary elements. The sharp autofocusing beams would open an avenue to realize the diverse ultrasound focusing with promising applications in medical ultrasound imaging, therapy, and nondestructive evaluation.