Cubic metamaterial crystal supporting broadband isotropic chiral phonons
Chiral metamaterials can support chiral phonons leading to acoustical activity, the acoustical counterpart of optical activity. However, the properties of early metamaterial designs have been very highly anisotropic, and chiral acoustical phonons occurred only for selected high-symmetry directions. The authors propose a novel chiral metamaterial based on ``twisting'' a truncated octahedron in a simple-cubic unit cell. Not supported by crystal symmetry alone but rather by a tuned degeneracy, chiral phonons and large broadband acoustical activity are obtained for all phonon propagation directions in 3D. This result is notable because even isotropic achiral acoustical phonons are rare for crys…
Isotropic Chiral Acoustic Phonons in 3D Quasicrystalline Metamaterials.
International audience; The elastic properties of three-dimensional (3D) crystalline mechanical metamaterials, unlike those of amorphous structures, are generally strongly anisotropic—even in the long-wavelength limit and for highly symmetric crystals. Aiming at isotropic linear elastic wave propagation, we therefore study 3D periodic approximants of 3D icosahedral quasicrystalline mechanical metamaterials consisting of uniaxial chiral metarods. Considering the increasing order of the approximants, we approach nearly isotropic effective speeds of sound and isotropic acoustical activity. The latter is directly connected to circularly polarized 3D metamaterial chiral acoustic phonons—for all …
Topologically Protected Twist Edge States for a Resonant Mechanical Laser-Beam Scanner
We design a one-dimensional chain of two different alternating three-dimensional elastic chiral unit cells. The chain’s topological band gap, a result of the alternation of unit cells combined with their chirality and an effective mirror symmetry, guarantees a protected edge state, corresponding to a localized twist mode with an eigenfrequency inside the one-dimensional band gap. A small axial modulation at the one end of the beam can excite this resonant twist mode at the other end of the beam, via evanescent modes in the gap. The topological robustness of the edge state allows us to add a micromirror to the other end of the beam, turning the arrangement into a resonant mechanical laser-be…
On the Schwarzschild Effect in 3D Two‐Photon Laser Lithography
International audience; The two‐photon Schwarzschild effect in photoresists suitable for 3D laser lithography is revisited. The study ranges over seven orders of magnitude in exposure time (from 1 µs to 10 s) and investigates a wide variety of different photoresist compositions. For short exposure times (“regime I”), the laser power at the polymerization threshold can scale with the inverse square root of the exposure time, as naively to be expected for two‐photon absorption. Substantial deviations occur, however, for low photoinitiator concentrations. For intermediate exposure times (“regime II”), a Schwarzschild‐type of behavior is found, as discussed previously. For very long exposure ti…
Observation of topological gravity-capillary waves in a water wave crystal
The discovery of topological phases of matter, initially driven by theoretical advances in quantum condensed matter physics, has been recently extended to classical wave systems, reaching out to a wealth of novel potential applications in signal manipulation and energy concentration. Despite the fact that many realistic wave media (metals at optical frequencies, polymers at ultrasonic frequencies) are inherently dispersive, topological wave transport in photonic and phononic crystals has so far been limited to ideal situations and proof-of-concept experiments involving dispersionless media. Here, we report the first experimental demonstration of topological edge states in a classical water …
Acoustic Topological Circuitry in Square and Rectangular Phononic Crystals
International audience; We systematically engineer a series of square and rectangular phononic crystals to create experimental realizations of complex topological phononic circuits. The exotic topological transport observed is wholly reliant upon the underlying structure which must belong to either a square or rectangular lattice system and not to any hexagonal-based structure. The phononic system chosen consists of a periodic array of square steel bars which partitions acoustic waves in water over a broadband range of frequencies (∼0.5MHz). An ultrasonic transducer launches an acoustic pulse which propagates along a domain wall, before encountering a nodal point, from which the acoustic si…
Stiffer, Stronger and Centrosymmetrical Class of Pentamodal Mechanical Metamaterials
Pentamode metamaterials have been used as a crucial element to achieve elastical unfeelability cloaking devices. They are seen as potentially fragile and not simple for integration in anisotropic structures due to a non-centrosymmetric crystalline structure. Here, we introduce a new class of pentamode metamaterial with centrosymmetry, which shows better performances regarding stiffness, toughness, stability and size dependence. The phonon band structure is calculated based on the finite element method, and the pentamodal properties are evaluated by analyzing the single band gap and the ratio of bulk and shear modulus. The Poisson&rsquo
Thermal cloaking of complex objects with the neutral inclusion and the coordinate transformation methods
We explore the cloaking of a complex shape by either the neutral inclusion or the transformation thermodynamics (TT) methods. Thin cloaks are built and the heat cloaking efficiency is investigated for both the steady-state and the transient regimes. We show that the neutral inclusion cloak is more efficient in both regimes, though it has the drawback that the thermal conductivity of the cloaked shape must be known. In practice, the neutral inclusion method is more flexible and easier to implement than the coordinate transformation method, especially for complex shapes.We explore the cloaking of a complex shape by either the neutral inclusion or the transformation thermodynamics (TT) methods…
Elastic Wave Near-Cloaking
Cloaking elastic waves has, in contrast to the cloaking of electromagnetic waves, remained a fundamental challenge: the latter successfully uses the invariance of Maxwell's equations, from which the field of transformational optics has emerged, whereas the elastic Navier equations are not invariant under coordinate transformations. Our aim is to overcome this challenge, at least in practical terms, and thereby unlock applications in mechanics, ultrasound, vibration mitigation, non-destructive evaluation and elastic wave control. We achieve near-cloaking by recognising that, despite the lack of invariance, a decoupling into a system of form invariant potential equations together with a quant…
Optical pulling and pushing forces in bilayer PT-symmetric structures
Photons are massless, yet can exert force on small particles. This $r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n$ $p\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e$, though discussed by Kepler, still needs investigation for modern systems. This study reveals that the optical force exerted on a parity-time-symmetric bilayer with balanced gain and loss can be $a\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}y\phantom{\rule{0}{0…
Mapping acoustical activity in 3D chiral mechanical metamaterials onto micropolar continuum elasticity
Abstract We compare the phonon band structures and chiral phonon eigenmodes of a recently experimentally realized three-dimensional (3D) cubic chiral metamaterial architecture to results from linear micropolar elasticity, an established generalization of classical linear Cauchy elasticity. We achieve very good qualitative agreement concerning the anisotropies of the eigenfrequencies, the anisotropies of the eigenmode properties of the acoustic branches, as well as with respect to the observed pronounced sample-size dependence of acoustical activity and of the static push-to-twist conversion effects. The size dependence of certain properties, that is, the loss of scale invariance, is a finge…
Poroelastic metamaterials with negative effective static compressibility
We suggest a three-dimensional metamaterial structure exhibiting an isotropic expansion in response to an increased hydrostatic pressure imposed by a surrounding gas or liquid. We show that this behavior corresponds to a negative absolute (rather than only differential) effective compressibility under truly static and stable conditions. The poroelastic metamaterial is composed of only a single ordinary constituent solid. By detailed numerical parameter studies, we find that a pressure increase of merely one bar can lead to a relative increase in the effective volume exceeding one percent for geometrical structure parameters that should be accessible to fabrication by 3D printing.
Static chiral Willis continuum mechanics for three-dimensional chiral mechanical metamaterials
International audience; Recent static experiments on twist effects in chiral three-dimensional mechanical metamaterials have been discussed in the context of micropolar Eringen continuum mechanics, which is a generalization of linear Cauchy elasticity. For cubic symmetry, Eringen elasticity comprises nine additional parameters with respect to linear Cauchy elasticity, of which three directly influence chiral effects. Here, we discuss the behavior of the static case of an alternative generalization of linear Cauchy elasticity, the Willis equations. We show that in the homogeneous static cubic case, only one additional parameter with respect to linear Cauchy elasticity results, which directly…
Characteristics of mechanical metamaterials based on buckling elements
Metamaterials are composed of structural elements and derive their properties mainly from the inner structure of the elements, rather than the properties of their constituent material. By designing an unstable structural element as the building block of a metamaterial, many interesting effective material properties can be obtained. The deformation and dissipation mechanisms of such a material built from unstable structural elements is studied in detail. To do so a combination of analytical, semi-analytical, and numerical models are applied to a single buckling element, a periodic cell, and finite size combinations of buckling elements including gradients in the properties of the building bl…
Experiments on the Parallel Hall Effect in Three-Dimensional Metamaterials
The usual Hall effect in a semiconductor leads to a voltage perpendicular to an applied static magnetic field. The authors significantly extend their recent work and demonstrate $e\phantom{\rule{0}{0ex}}x\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}y$ that not only the sign but also the direction of the Hall field can be tailored by a metamaterial's microstructure. They show that, with judicious engineering, the Hall voltage can be $p\phantom{\rule{0}{0…
3D Metamaterials with Negative Thermal Expansion and Negative Effective Compressibility
Materials with negative thermal expansion are desired for controlling thermal stresses, but unusual in nature. With two-component metamaterials it is possible to tune the thermal expansion from positive over zero to negative values, even if both components have positive thermal expansion. Using gray-tone laser lithography we fabricate three-dimensional two-component polymer microlattices, exhibiting zero or negative thermal expansion [1].
Three-dimensional poroelastic metamaterials with extremely negative or positive effective static volume compressibility
Abstract Recently, three-dimensional poroelastic metamaterials have been introduced that show an unusual isotropic increase of their effective volume when increasing the hydrostatic pressure of the surrounding air. This behavior corresponds to a negative effective static volume compressibility. Here, we present significantly simplified metamaterial architectures, which are composed of just one rather than eight hollow sealed functional elements within each cubic unit cell. On cubic symmetry microstructured polymer samples made by 3D laser printing, we measure a negative effective compressibility of about κ eff = − 4 . 7 % ∕ bar under pressure control. This value is six times larger than pre…
Elastodynamic behavior of mechanical cloaks designed by direct lattice transformations
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When size matters
That the unit cell of a metamaterial can't be considered vanishingly small like in ordinary crystals has long been deemed more burden than opportunity. The emergence of a characteristic length scale in metamaterial chains may change that trend.
Experimental observations of topologically guided water waves within non-hexagonal structures
International audience; We investigate symmetry-protected topological water waves within a strategically engineered square lattice system. Thus far, symmetry protected topological modes in hexagonal systems have primarily been studied in electromagnetism and acoustics, i.e., dispersionless media. Herein, we show experimentally how crucial geometrical properties of square structures allow for topological transport that is ordinarily forbidden within conventional hexagonal structures. We perform numerical simulations that take into account the inherent dispersion within water waves and devise a topological insulator that supports symmetry-protected transport along the domain walls. Our measur…
Cloaking In-Plane Elastic Waves with Swiss Rolls
We propose a design of cylindrical cloak for coupled in-plane shear waves consisting of concentric layers of sub-wavelength resonant stress-free inclusions shaped as Swiss rolls. The scaling factor between inclusions&rsquo
Dispersion engineering for photonic crystal based nanophotonic devices
International audience
Optical force rectifiers based on PT-symmetric metasurfaces
We introduce here the concept of optical force rectifier based on parity-time symmetric metasurfaces. Directly linked to the properties of non-Hermitian systems engineered by balanced loss and gain constituents, we show that light can exert asymmetric pulling or pushing forces on metasurfaces depending on the direction of the impinging light. This generates a complete force rectification in the vicinity of the exceptional point. Our findings have the potential to spark the design of applications in optical manipulation where the forces, strictly speaking, act unidirectionally. R.A. and B.G. would like to acknowledge financial support from the Max Planck Society. J.C. acknowledges the suppor…
Optically assisted trapping with high-permittivity dielectric rings: Towards optical aerosol filtration
Controlling the transport, trapping, and filtering of nanoparticles is important for many applications. By virtue of their weak response to gravity and their thermal motion, various physical mechanisms can be exploited for such operations on nanoparticles. However, the manipulation based on optical forces is potentially most appealing since it constitutes a highly deterministic approach. Plasmonic nanostructures have been suggested for this purpose, but they possess the disadvantages of locally generating heat and trapping the nanoparticles directly on surface. Here, we propose the use of dielectric rings made of high permittivity materials for trapping nanoparticles. Thanks to their abilit…
New Twists of 3D Chiral Metamaterials
Rationally designed artificial materials, called metamaterials, allow for tailoring effective material properties beyond ("meta") the properties of their bulk ingredient materials. This statement is especially true for chiral metamaterials, as unlocking certain degrees of freedom necessarily requires broken centrosymmetry. While the field of chiral electromagnetic/optical metamaterials has become rather mature, the field of elastic/mechanical metamaterials is just emerging and wide open. This research news reviews recent theoretical and experimental progress concerning 3D chiral mechanical and optical metamaterials, with special emphasis on work performed at KIT.