Comprehensive refractive manipulation of water waves using electrostriction.

The control of wave propagation based on refraction principles offers unparalleled possibilities as shown by the striking example of optics. This approach is unfortunately limited for water waves as it relies mainly on variations of the liquid depth which, while controlling the wave velocity, also trigger nonlinearities and damping. In this article, we show experimentally that electrostriction allows to implement extensive refraction-based control of water waves in a precise and contactless manner. The setup consists of an electrode under high voltage placed above the grounded conductive water. The waves propagating under the electrode can be slowed down up to approximately half their speed compared to free propagation. We characterize the Snell-Descartes laws of refraction and the total internal reflection for the water waves. We implement emblematic refraction-based devices such as electrically tunable focusing lenses, waveguides without obstacles, and beam splitters based on frustrated internal reflection to perform interference experiments.

Current developments in elastic and acoustic metamaterials science.

The concept of metamaterial recently emerged as a new frontier of scientific research, encompassing physics, materials science and engineering. In a broad sense, a metamaterial indicates an engineered material with exotic properties not found in nature, obtained by appropriate architecture either at macro-scale or at micro-/nano-scales. The architecture of metamaterials can be tailored to open unforeseen opportunities for mechanical and acoustic applications, as demonstrated by an impressive and increasing number of studies. Building on this knowledge, this theme issue aims to gather cutting-edge theoretical, computational and experimental studies on elastic and acoustic metamaterials, with the purpose of offering a wide perspective on recent achievements and future challenges. This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 1)'.

Current developments in elastic and acoustic metamaterials science.

The concept of metamaterial recently emerged as a new frontier of scientific research, encompassing physics, materials science and engineering. In a broad sense, a metamaterial indicates an engineered material with exotic properties not found in nature, obtained by appropriate architecture either at macro-scale or at micro-/nano-scales. The architecture of metamaterials can be tailored to open unforeseen opportunities for mechanical and acoustic applications, as demonstrated by an impressive and increasing number of studies. Building on this knowledge, this theme issue aims to gather cutting-edge theoretical, computational and experimental studies on elastic and acoustic metamaterials, with the purpose of offering a wide perspective on recent achievements and future challenges.This article is part of the theme issue, 'Current developments in elastic and acoustic metamaterials science (Part 2)'.

Nonlocal elastic metasurfaces: Enabling broadband wave control via intentional nonlocality.

While elastic metasurfaces offer a remarkable and very effective approach to the subwavelength control of stress waves, their use in practical applications is severely hindered by intrinsically narrow band performance. In applications to electromagnetic and photonic metamaterials, some success in extending the operating dynamic range was obtained by using nonlocality. However, while electronic properties in natural materials can show significant nonlocal effects, even at the macroscales, in mechanics, nonlocality is a higher-order effect that becomes appreciable only at the microscales. This study introduces the concept of intentional nonlocality as a fundamental mechanism to design passive elastic metasurfaces capable of an exceptionally broadband operating range. The nonlocal behavior is achieved by exploiting nonlocal forces, conceptually akin to long-range interactions in nonlocal material microstructures, between subsets of resonant unit cells forming the metasurface. These long-range forces are obtained via carefully crafted flexible elements, whose specific geometry and local dynamics are designed to create remarkably complex transfer functions between multiple units. The resulting nonlocal coupling forces enable achieving phase-gradient profiles that are functions of the wavenumber of the incident wave. The identification of relevant design parameters and the assessment of their impact on performance are explored via a combination of semianalytical and numerical models. The nonlocal metasurface concept is tested, both numerically and experimentally, by embedding a total-internal-reflection design in a thin-plate waveguide. Results confirm the feasibility of the intentionally nonlocal design concept and its ability to achieve a fully passive and broadband wave control.

Phase-conjugate mirror for water waves driven by the Faraday instability.

The Faraday instability appears on liquid baths submitted to vertical oscillations above a critical value. The pattern of standing ripples at half the vibrating frequency that results from this parametric forcing is usually shaped by the boundary conditions imposed by the enclosing receptacle. Here, we show that the time modulation of the medium involved in the Faraday instability can act as a phase-conjugate mirror--a fact which is hidden in the extensively studied case of the boundary-driven regime. We first demonstrate the complete analogy with the equations governing its optical counterpart. We then use water baths combining shallow and deep areas of arbitrary shapes to spatially localize the Faraday instability. We give experimental evidence of the ability of the Faraday instability to generate counterpropagating phase-conjugated waves for any propagating signal wave. The canonical geometries of a point and plane source are implemented. We also verify that Faraday-based phase-conjugate mirrors hold the genuine property of being shape independent. These results show that a periodic modulation of the effective gravity can perform time-reversal operations on monochromatic propagating water waves, with a remarkable efficiency compared with wave manipulation in other fields of physics.

Confinement of surface spinners in liquid metamaterials.

We show that rotating particles at the liquid-gas interface can be efficiently manipulated using the surface-wave analogue of optical lattices. Two orthogonal standing waves generate surface flows of counter-rotating half-wavelength unit cells, the liquid interface metamaterial, whose geometry is controlled by the wave phase shift. Here we demonstrate that by placing active magnetic spinners inside such metamaterials, one makes a powerful tool which allows manipulation and self-assembly of spinners, turning them into vehicles capable of transporting matter and information between autonomous metamaterial unit cells. We discuss forces acting on a spinner carried by a nonuniform flow and show how the forces confine spinners to orbit inside the same-sign vortex cells of the wave-driven flow. Reversing the spin, we move the spinner into an adjacent cell. By changing the spinning frequency or the wave amplitude, one can precisely control the spinner orbit. Multiple spinners within a unit cell self-organize into stable patterns, e.g., triangles or squares, orbiting around the center of the cell. Spinners having different frequencies can also be confined, such that the higher-frequency spinner occupies the inner orbit and the lower-frequency one circles on the outer orbit, while the orbital motions of both spinners are synchronized.

Non-Layered Gold-Silicon and All-Silicon Frequency-Selective Metasurfaces for Potential Mid-Infrared Sensing Applications.

We report simulations on the spectral behavior of non-layered gold-silicon and all-silicon frequency-selective metasurfaces in an asymmetric element configuration in the mid-infrared spectral window of 5-5.8 µm. The non-layered layout is experimentally feasible due to recent technological advances such as nano-imprint and nano-stencil lithography, and the spectral window was chosen due to the multitude of applications in sensing and imaging. The architecture exhibits significant resonance in the window of interest as well as extended tunability by means of variation of cell element sizes and relative coordinates. The results indicate that the proposed metasurface architecture is a viable candidate for mid-infrared absorbers, sensors and imaging systems.

Uncovering and Experimental Realization of Multimodal 3D Topological Metamaterials for Low-Frequency and Multiband Elastic Wave Control.

Topological mechanical metamaterials unlock confined and robust elastic wave control. Recent breakthroughs have precipitated the development of 3D topological metamaterials, which facilitate extraordinary wave manipulation along 2D planar and layer-dependent waveguides. The 3D topological metamaterials studied thus far are constrained to function in single-frequency bandwidths that are typically in a high-frequency regime, and a comprehensive experimental investigation remains elusive. In this paper, these research gaps are addressed and the state of the art is advanced through the synthesis and experimental realization of a 3D topological metamaterial that exploits multimodal local resonance to enable low-frequency elastic wave control over multiple distinct frequency bands. The proposed metamaterial is geometrically configured to create multimodal local resonators whose frequency characteristics govern the emergence of four unique low-frequency topological states. Numerical simulations uncover how these topological states can be employed to achieve polarization-, frequency-, and layer-dependent wave manipulation in 3D structures. An experimental study results in the attainment of complete wave fields that illustrate 2D topological waveguides and multi-polarized wave control in a physical testbed. The outcomes from this work provide insight that will aid future research on 3D topological mechanical metamaterials and reveal the applicability of the proposed metamaterial for wave control applications.

Geometric Phase in Twisted Topological Complementary Pair.

Geometric phase enabled by spin-orbit coupling has attracted enormous interest in optics over the past few decades. However, it is only applicable to circularly-polarized light and encounters substantial challenges when applied to wave fields lacking the intrinsic spin degree of freedom. Here, a new paradigm is presented for achieving geometric phase by elucidating the concept of topological complementary pair (TCP), which arises from the combination of two compact phase elements possessing opposite intrinsic topological charge. Twisting the TCP leads to the generation of a linearly-varying geometric phase of arbitrary order, which is quantified by the intrinsic topological charge. Notably distinct from the conventional spin-orbit coupling-based theories, the proposed geometric phase is the direct result of the cyclic evolution of orbital-angular-momentum transformation in mode space, thereby exhibiting universality across classical wave systems. As a proof of concept, the existence of this geometric phase is experimentally demonstrated using scalar acoustic waves, showcasing the remarkable ability in the precise manipulation of acoustic waves at subwavelength scales. These findings engender a fresh understanding of wave-matter interaction in compact structures and establish a promising platform for exploring geometric phase, offering significant opportunities for diverse applications in wave systems.

Triangular-wave controlled amplitude-modulated flow analysis for extending dynamic range to saturated signals.

We have applied our amplitude-modulated flow analysis concept to extend the dynamic range to saturated analytical signals. Sample solution, the flow rate FS of which is periodically varied with a triangular control signal Vc, is merged with a reagent solution delivered at a constant flow rate FR. While the total flow rate FT is kept constant, a diluent with FT-(FS + FR) flow rate is aspirated from the third channel. The analytical signal Vd obtained downstream is processed by fast Fourier transform to obtain the amplitudes of the wave components in Vd. When the sample concentration CS is low, Vd shows a triangular profile like Vc; the sum of the amplitudes ΣA is proportional to CS. When CS is high, Vd shows a trapezoidal profile because of the Vd saturation. A linear calibration curve can also be obtained for such saturated signals by plotting CSΣA against CS. The proof of the concept is validated by applying it to spectrophotometric determinations of a dye (Methylene Blue) and colored complexes of Fe2+ - phenanthroline and Fe3+ - Tiron.

Phase Change Metasurfaces by Continuous or Quasi-Continuous Atoms for Active Optoelectronic Integration.

In recent decades, metasurfaces have emerged as an exotic and appealing group of nanophotonic devices for versatile wave regulation with deep subwavelength thickness facilitating compact integration. However, the ability to dynamically control the wave-matter interaction with external stimulus is highly desirable especially in such scenarios as integrated photonics and optoelectronics, since their performance in amplitude and phase control settle down once manufactured. Currently, available routes to construct active photonic devices include micro-electromechanical system (MEMS), semiconductors, liquid crystal, and phase change materials (PCMs)-integrated hybrid devices, etc. For the sake of compact integration and good compatibility with the mainstream complementary metal oxide semiconductor (CMOS) process for nanofabrication and device integration, the PCMs-based scheme stands out as a viable and promising candidate. Therefore, this review focuses on recent progresses on phase change metasurfaces with dynamic wave control (amplitude and phase or wavefront), and especially outlines those with continuous or quasi-continuous atoms in favor of optoelectronic integration.

Introduction of Air-Segmentation Approach to Flow Titration by Feedback-based and Subsequent Fixed Triangular Wave-controlled Flow Ratiometry.

An air-segmentation approach has been introduced to a feedback-based and subsequent fixed triangular wave-controlled flow ratiometry to suppress axial dispersion in flow titration. The flow rate of a base solution containing an indicator is linearly varied with a control signal, Vc, supplied by a computer. The solution is merged with an acid solution under a constant total flow rate. Air is introduced to the merged solution in order to segment the solution with air bubbles. Both phases are led to a UV/Vis detector without phase separation. Air signals are removed by signal processing. The effect of the lag time between the merging of solutions upstream and the sensing of the corresponding signal downstream is offset by feedback-based upward and downward Vc scans, and thus the Vc that gives the equivalence composition is determined. Subsequently, fixed triangular wave control is applied to a narrower Vc range with a higher scan rate to enhance the throughput rate (maximally 11.8 titrations/min). Air-segmentation has been found to be effective to reduce axial dispersion and to preserve the titrand/titrant composition upon their just being merged. Consequently, the applicable range is extended especially to lower titrand concentration. The proposed method has been successfully applied to various acid-base titrations, including the nonaqueous titration of the Japanese Pharmacopoeia drug.