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Register a new userEfficient asymmetric transmission of elastic waves in thin plates with lossless metasurfaces
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Requiring neither active components nor complex designs, we propose and experimentally demonstrate a generic framework for undistorted asymmetric elastic-wave transmission in a thin plate just using a layer of lossless metasurface. The asymmetric transmission stems from the uneven diffraction of +1 and -1 orders on opposite sides of the metasurface, respectively. Compared with previous loss-induced strategies, the present metasurface maintains a nearly total transmission for the transportation side, but a total reflection from the opposite side, exhibiting a higher contrast ratio of transmission. Moreover, we illustrate that this strong asymmetric behavior is robust to the frequency, the incident angle and the loss effect. The present work paves new avenues to compact rectification, high resolution ultrasonography, vibration and noise control in elastodynamics and acoustics.
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As 2D materials with subwavelength structures, elastic metasurfaces show remarkable abilities to manipulate elastic waves at will through artificial boundary conditions. However, the application prospects of current metasurfaces may be restricted by their phase-only modulating boundaries. Herein, we present the next generation of elastic metasurfaces by additionally incorporating amplitude-shift modulation. A general theory for target wave fields steered by metasurfaces is proposed by modifying the Huygens-Fresnel principle. As examples, two amplitude-shift metasurfaces concerning flexural waves in thin plates are carried out: one is to transform a cylindrical wave into a Gaussian beam by elaborating both amplitude and phase shifts, and the other one is to focus the incidence by amplitude modulations only. These examples coincide well over theoretical calculations, numerical simulations and experimental tests. This work may underlie the design of metasurfaces with complete control over guided elastic waves, and may extend to more sophisticated applications, such as analog signal processing and holographic imaging.
We present here how a coherent perfect absorber-laser (CPAL) enabled by parity-time ($mathcal{PT}$)-symmetry breaking may be exploited to build monochromatic amplifying devices for flexural waves. The fourth order partial differential equation governing the propagation of flexural waves leads to four by four transfer matrices, and this results in physical properties of the $mathcal{PT}$-symmetry specific to elastic plate systems. We thus demonstrate the possibility of using CPAL for such systems and we argue the possibility of using this concept to detect extremely small-scale vibration perturbations with important outcomes in surface science (imaging of nanometer vibration) and geophysics (improving seismic sensors like velocimeters). The device can also generate finite signals using very low exciting intensities. The system can alternatively be used as a perfect absorber for flexural energy by tailoring the left and right incident wave for energy harvesting applications.
The investigation of topologically protected waves in classical media has opened unique opportunities to achieve exotic properties like one-way phonon transport, protection from backscattering and immunity to imperfections. Contrary to acoustic and electromagnetic domains, their observation in elastic solids has so far been elusive due to the presence of both shear and longitudinal modes and their modal conversion at interfaces and free surfaces. Here we report the experimental observation of topologically protected helical edge waves in elastic media. The considered structure consists of an elastic plate patterned according to a Kagome architecture with an accidental degeneracy of two Dirac cones induced by drilling through holes. The careful breaking of symmetries couples the corresponding elastic modes which effectively emulates spin orbital coupling in the quantum spin Hall effect. The results shed light on the topological properties of the proposed plate waveguide and opens avenues for the practical realization of compact, passive and cost-effective elastic topological waveguides.
Waveguides are critically important components in microwave, THz, and optical technologies. Due to recent progress in two-dimensional materials, metasurfaces can be efficiently used to design novel waveguide structures which confine the electromagnetic energy while the structure is open. Here, we introduce a special type of such structures formed by two penetrable metasurfaces which have complementary isotropic surface impedances. We theoretically study guided modes supported by the proposed structure and discuss the corresponding dispersion properties. Furthermore, we show the results for different scenarios in which the surface impedances possess non-resonant or resonant characteristics, and the distance between the metasurfaces changes from large values to the extreme limit of zero. As an implication of this work, we demonstrate that there is a possibility to excite two modes with orthogonal polarizations having the same phase velocity within a broad frequency range. This property is promising for applications in leaky-wave antennas and field focusing.
We investigate the scattering of elastic waves off a disordered region described by a one-dimensional random-phase sine-Gordon model. The collective pinning results in an effective static disorder potential with universal and non-Gaussian correlations, acting on propagating waves. We find signatures of the correlations in the wave transmission in a wide frequency range, which covers both the weak and strong localization regimes. Our theory elucidates the dynamics of collectively-pinned phases occurring in any natural or synthetic elastic medium. The latter one is exemplified by a one-dimensional array of Josephson junctions, for which we specify our results. The obtained results provide benchmarks for the array-enabled quantum simulations addressing the dynamics in broader and yet-unexplored domains of individual pinning and quantum Bose glass.
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