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Metasurfaces, the ultrathin media with extraordinary wavefront modulation ability, have shown versatile potential in manipulating waves. However, existing acoustic metasurfaces are limited by their narrow-band frequency-dependent capability, which severely hinders their real-world applications that usually require customized dispersion. To address this bottlenecking challenge, we report ultra-broadband achromatic metasurfaces that are capable of delivering arbitrary and frequency-independent wave properties by bottom-up topology optimization. We successively demonstrate three ultra-broadband functionalities, including acoustic beam steering, focusing and levitation, featuring record-breaking relative bandwidths of 93.3%, 120% and 118.9%, respectively. All metasurface elements show novel asymmetric geometries containing multiple scatters, curved air channels and local cavities. Moreover, we reveal that the inversely designed metasurfaces can support integrated internal resonances, bi-anisotropy and multiple scattering, which collectively form the mechanism underpinning the ultra-broadband customized dispersion. Our study opens new horizons for ultra-broadband high-efficiency achromatic functional devices on demand, with promising extension to the optical and elastic achromatic metamaterials., Comment: 21 pages, 5 figures

Acoustic negative-index metamaterials show promise in achieving superlensing for diagnostic medical imaging. In spite of the recent progress made in this field, most metamaterials suffer from deficiencies such as low spatial symmetry, sophisticated labyrinth topologies and narrow-band features, which make them difficult to be utilized for symmetric subwavelength imaging applications. Here, we propose a category of robust multi-cavity metamaterials and reveal their common double-negative mechanism enabled by multi-polar (dipole, quadrupole and octupole) resonances in both two-dimensional (2D) and three-dimensional (3D) scenarios. In particular, we discover explicit relationships governing the double-negative frequency bounds from equivalent circuit analogy. Moreover, broadband single-source and double-source subwavelength imaging is realized and verified by 2D and 3D superlens. More importantly, the analogical 3D superlens can ensure the subwavelength imaging in all directions. The proposed multi-polar resonance-enabled robust metamaterials and design methodology open horizons for easier manipulation of subwavelength waves and realization of practical 3D metamaterial devices., Comment: 10 pages, 4 figures

Double-negative acoustic metamaterials (AMMs) offer the promising ability of superlensing for applications in ultrasonography, biomedical sensing and nondestructive evaluation. Here, under the simultaneous increasing or non-increasing mechanisms, we develop a unified topology optimization framework considering the different microstructure symmetries, minimal structural feature sizes and dispersion extents of effective parameters. Then we apply the optimization framework to furnish the heuristic resonance-cavity-based and space-coiling metamaterials with broadband double negativity. Meanwhile, we demonstrate the essences of double negativity derived from the novel artificial multipolar LC and Mie resonances which can be induced by controlling mechanisms in optimization. Furthermore, abundant numerical simulations validate the double negativity, negative refraction, enhancements of evanescent waves and subwavelengh imaging for the optimized AMMs. Finally, we experimentally show the desired broadband subwavelengh imaging using the 3D-printed optimized space-coiling metamaterial. The present methodology and broadband metamaterials provide the ideal strategy of constructing AMMs for subwavelengh imaging technology., Comment: 36pages, 13 figures, 1 appendix

Previously reported acoustic metasurfaces that consist of fixed channels, are untunable to meet the broadband requirement and alterable functionalities. To overcome this limitation, we propose screw-and-nut mechanism of tunability and design a type of continuously tunable acoustic metasurface with unit components of helical cylinders which are screwed into a plate. The spiral channel length can be tuned continuously by the screwed depth; and then a metasurface with continuously tunable acoustic phase at independent pixels is attained. Different distributions of the unit components can shape an arbitrary metasurface profile. We also developed an approximate equivalent medium model to predict the tunability of the unit component. As an example, we present the design details of a circular tunable metasurface for three-dimensional acoustic focusing of different airborne sound sources in a wide frequency region. A sample of the metasurface is manufactured by poly lactic acid (PLA) plastic with the helical cylinders being 3D-printed. Experiments of sound focusing are performed. It is shown that the results of the equivalent medium model, the finite element simulation and the experiments are in a good agreement.

Hyperbolic metamaterials are strongly anisotropic artificial composite materials at a subwavelength scale and can greatly widen the engineering feasibilities for manipulation of wave propagation. However, limited by the empirical structure topologies, the previously reported hyperbolic elastic metamaterials (HEMMs) suffer from the limitations of relatively narrow frequency width, inflexible adjusting operating subwavelength scale and being difficult to further ameliorate imaging resolution. Here, we develop an inverse-design approach for HEMMs by topology optimization based on the effective medium theory. We successfully design two-dimensional broadband HEMMs supporting multipolar resonances, and theoretically demonstrate their deep-subwavelength imagings for longitudinal waves. Under different prescribed subwavelength scales, the optimized HEMMs exhibit broadband negative effective mass densities. Moreover, benefiting from the extreme enhancement of evanescent waves, an optimized HEMM at the ultra-low frequency can yield a super-high imaging resolution (~{\lambda}/64), representing the record in the field of elastic metamaterials. The proposed computational approach can be easily extended to design hyperbolic metamaterials for other wave counterparts. The present research may provide a novel design methodology for exploring the HEMMs based on unrevealed resonances and serve as a useful guide for the ultrasonography and general biomedical applications., Comment: 23 pages, 13 figures

Aiming at the promising superlensing for the medical ultrasonic and detection, the double-negative metamaterials which possess the negative mass density and elastic modulus simultaneously can be acted as the ideal superlens for breaking the diffraction limit. In this paper, we use topology optimization to design the two-dimensional single-phase anisotropic elastic metamaterials with broadband double-negative indices and numerically demonstrate the superlensing at the deep-subwavelength scale. We also discuss the impact of several parameters adopted in the objective function and constraints on the optimized results. Unlike all previous reported mechanisms, our optimized structures exhibit the new quadrupolar or multipolar resonances for the negative mass density, negative longitudinal and shear moduli. In addition, we observe the negative refraction of transverse waves in a single-phase material. Most structures can serve as the anisotropic zero-index metamaterials for the longitudinal or transverse wave at a certain frequency. The cloaking effect is demonstrated for both the longitudinal and transverse waves. Moreover, with the particular constraints in optimization, we design a super-anisotropic metamaterial exhibiting the double-negative and hyperbolic dispersions along two principal directions, respectively. Our optimization work provides a robust computational approach to negative index engineering in elastic metamaterials and guides design of other kinds of metamaterials, including the electromagnetic and acoustic metamaterials. The unusual properties of our optimized structures are likely to inspire new ideas and novel applications including the low-frequency vibration attenuation, flat lens and ultrasonography for elastic waves in the future., Comment: 37 pages, 18 figures