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Register a new userExperimental Observation of Efficient Nonreciprocal Mode Transitions via Spatiotemporally-Modulated Acoustic Metamaterials
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In lossless acoustic systems, mode transitions are always time-reversible, consistent with Lorentz reciprocity, giving rise to symmetric sound manipulation in space-time. To overcome this fundamental limitation and break space-time symmetry, nonreciprocal sound steering is realized by designing and experimentally implementing spatiotemporally-modulated acoustic metamaterials. Relying on no slow mechanical parts, unstable and noisy airflow or complicated piezoelectric array, our mechanism uses the coupling between an ultrathin membrane and external electromagnetic field to realize programmable, dynamic control of acoustic impedance in a motionless and noiseless manner. The fast and flexible impedance modulation at the deeply subwavelength scale enabled by our compact metamaterials provides an effective unidirectional momentum in space-time to realize irreversible transition in k-{omega} space between different diffraction modes. The nonreciprocal wave-steering functionality of the proposed metamaterial is elucidated by theoretically deriving the time-varying acoustic response and demonstrated both numerically and experimentally via two distinctive examples of unidirectional evanescent wave conversion and nonreciprocal blue-shift focusing. This work can be further extended into the paradigm of Bloch waves and impact other vibrant domains, such as non-Hermitian topological acoustics and parity-time-symmetric acoustics.
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The recently proposed concept of metamaterials has opened exciting venues to control wave-matter interaction in unprecedented ways. Here we demonstrate the relevance of metamaterials for inducing acoustic birefringence, a phenomenon which has already found its versatile applications in optics in designing light modulators or filters, and nonlinear optic components. This is achieved in a suitably designed acoustic metamaterial which is non-Eulerian, in the sense that at low frequencies, it cannot be homogenized to a uniform acoustic medium whose behavior is characterized by Euler equation. Thanks to the feasibility of engineering its subwavelength structure, such non-Eulerian metamaterial allows one to desirably manipulate the birefringence process. Our findings may give rise to generation of innovative devices such as tunable acoustic splitters and filters.
Roton dispersion relations, displaying a pronounced roton minimum at finite momentum, were firstly predicted by Landau and have been extensively explored in correlated quantum systems at low temperatures. Recently, the roton-like dispersion relations were theoretically extended to classical acoustics, which, however, have remained elusive in reality. Here, we report the experimental observation of roton-like dispersions in acoustic metamaterials with beyond-nearest-neighbour interactions at ambient temperatures. The resulting metamaterial supports multiple coexisting modes with different wavevectors and group velocities at the same frequency and broadband backward waves, analogous to the return flow termed by Feynman in the context of rotons. Moreover, by increasing the order of long-range interaction, we observe multiple rotons on a single dispersion band, which have never appeared in Landaus prediction or any other condensed matter study. The realization of roton-like dispersions in metamaterials could pave the way to explore novel physics and applications on quantum-inspired phenomena in classical systems.
In this paper we develop homogenization theory for spatiotemporally modulated wire medium. We first solve for the modal waves that are supported by this composite medium, we show peculiar properties such as extraordinary waves that propagate at frequencies below the cut-off frequency of the corresponding stationary medium. We explain how these unique solutions give rise to an extreme Fresnel drag that exists already with weak and slow spatiotemporal modulation. Next, we turn to derive the effective material permittivity that corresponds to each of the first few supported modes, and write the average fields and Poyntings vector. Nonlocality, nonreciprocity, and anisotropy due to the spatiotemporal modulation direction, are three inherent properties of this medium, and are clearly seen in the effective material parameters. As a figure of merit, we also derive the effective permittivity of a plasma medium with spatiotemporally modulated plasma frequency. This comparison is interesting since the plasma medium can be considered as the effective medium that is obtained by a stationary wire medium. We validate that homogenization and spatiotemporal variation are not necessarily interchangeable operations. And indeed, in certain parameter regimes the homogenization should be performed directly on spatiotemporally modulated composite medium, rather than first homogenize the stationary medium and then phenomenologically introduce the effect of the space-time modulation.
Slow sound is a frequently exploited phenomenon that metamaterials can induce in order to permit wave energy compression, redirection, imaging, sound absorption and other special functionalities. Generally however such slow sound structures have a poor impedance match to air, particularly at low frequencies, and consequently exhibit strong transmission only in narrow frequency ranges. This therefore strongly restricts their application in wave manipulation devices. In this work we design a slow sound medium that halves the effective speed of sound in air over a wide range of low frequencies, whilst simultaneously maintaining a near impedance match to air. This is achieved with a rectangular array of cylinders of elliptical cross section, a microstructure that is motivated by combining transformation acoustics with homogenization. Microstructural parameters are optimised in order to provide the required anisotropic material properties as well as near impedance matching. We then employ this microstructure in order to halve the size of a quarter-wavelength resonator (QWR), or equivalently to halve the resonant frequency of a QWR of a given size. This provides significant space savings in the context of low-frequency tonal noise attenuation in confined environments where the absorbing material is adjacent to the region in which sound propagates, such as in a duct. We term the elliptical microstructure `universal since it may be employed in a number of diverse applications.
Using both multiple scattering theory and effective medium theory, we find that an acoustic metamaterial consisting of an array of spinning cylinders can possess a host of unusual properties including folded bulk and interface-state bands in the subwavelength regime. The folding of the bands has its origin in the rotation-induced antiresonance of the effective compressibility with its frequency at the angular velocity of the spinning cylinders, as well as in the rotational Doppler effect which breaks the chiral symmetry of the effective mass densities. Both bulk and interface-state bands exhibit remarkable variations as the filling fraction of the spinning cylinders is increased. In particular, a zero-frequency gap appears when exceeds a critical value. The uni-directional interface states bear interesting unconventional characteristics and their robust one-way transport properties are demonstrated numerically.
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