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International audience; AbstractThe fast growth of railway transport pushed by its sustainability requires all railway stakeholders to work together to minimise any possible drawbacks that might slow down this positive trend. Among these, the induced vibrations generated at the wheel–rail interaction during train passage are the most significant, especially in urban areas where in the form of ground-borne vibrations and/or noise negatively affects the daily life of the residents. In this direction, the innovative concept of seismic metamaterial can enhance the vibration attenuation levels of existing mitigation measures and, at the same time, ensure a straightforward application in the railway environment. This paper uses a combined vehicle/track/soil prediction dynamic model of the T2000 LRV (Light Rail Vehicle) tram operating in Brussels, based on a two-step approach already validated in the past, to analyse the effects that the inclusion of seismic metamaterials within the transmission path has in terms of vibration attenuation. In particular, a four-by-four group of piles is examined with different material proprieties for the inclusions, considering a range of velocities and distances from the track in order to quantify the achievable Insertion Loss (IL) for each configuration depending on the distances and the tram speed, attaining ILs up to 10 [dB]. The study is then extended to other possible arrangements of the group of piles, providing a direct relationship between the repetitiveness of each row of the primitive unit cell of the metamaterial and the gained levels of attenuation, in order to give a base for the costs of this type of mitigation measure. Furthermore, the influence on the attenuation levels of the soil characteristics, is evaluated by considering an ideal uniform underground, extending the IL to 18 [dB] in the case of steel inclusion. Finally, the paper gives insights into the use of metamaterials as mitigation measures for railway-induced vibration in order to understand and evaluate the potential of this novel concept and its benefits.

International audience; We provide theoretical and experimental evidence for the existence of topological Tamm states at the interface between two stubbed photonic crystals (PCs) as a function of the period and length of the stubs. Several works have addressed these states in the well-known Su-Schrieffer-Heeger model, a dimerized chain based on two resonators per unit cell where the opening of a gap at a Dirac cone results in a symmetry inversion of bulk bands between two topologically different crystals. Here, we give a detailed theoretical analysis of a mechanism based on band-edge symmetry inversion around a flat band, i.e., when the width of the pass band vanishes, while using only one resonator (stub) per unit cell. Then, we propose a simple versatile experimental platform to observe such interface states, which is based on coaxial cables operating in the radio-frequency domain. The investigation of these states was performed by using different approaches: (i) the topology of the bands based on the Zak phase and the symmetry of the band-edge modes, (ii) the sign of the reflection phase between each PC and a waveguide, and (iii) the dips or peaks in the reflection and transmission spectra when two finite photonic crystals are connected together either horizontally or vertically along a waveguide. Furthermore, we give a general rule about the existence of interface states when two PCs exhibit two common gaps with a flat band in their middle and different bulk-edge symmetries. Also, we provide closed-form expressions of the geometrical parameters and the frequency for which the interface state becomes bound state in the continuum (BIC). We show that these topological BIC states are stationary states of the cavity between the two PCs, and are very robust to any perturbation on both sides of the cavity. Finally, we show the impossibility of existence of interface states between two PCs with identical periods and different stubs. The theoretical and experimental results are discussed for both Neumann and Dirichlet boundary conditions at the end of the stubs.

With the growing interest in the field of artificial materials, more advanced and sophisticated functionalities are required from phononic crystals and acoustic metamaterials. This implies a high computational effort and cost, and still the efficiency of the designs may be not sufficient. With the help of third-wave artificial intelligence technologies, the design schemes of these materials are undergoing a new revolution. As an important branch of artificial intelligence, machine learning paves the way to new technological innovations by stimulating the exploration of structural design. Machine learning provides a powerful means of achieving an efficient and accurate design process by exploring nonlinear physical patterns in high-dimensional space, based on data sets of candidate structures. Many advanced machine learning algorithms, such as deep neural networks, unsupervised manifold clustering, reinforcement learning and so forth, have been widely and deeply investigated for structural design. In this review, we summarize the recent works on the combination of phononic metamaterials and machine learning. We provide an overview of machine learning on structural design. Then discuss machine learning driven on-demand design of phononic metamaterials for acoustic and elastic waves functions, topological phases and atomic-scale phonon properties. Finally, we summarize the current state of the art and provide a prospective of the future development directions.

This broad review summarizes recent advances and "hot"research topics in nanophononics and elastic, acoustic, and mechanical metamaterials based on results presented by the authors at the EUROMECH 610 Colloquium held on April 25-27, 2022 in Benicássim, Spain. The key goal of the colloquium was to highlight important developments in these areas, particularly new results that emerged during the last two years. This work thus presents a "snapshot"of the state-of-the-art of different nanophononics- and metamaterial-related topics rather than a historical view on these subjects, in contrast to a conventional review article. The introduction of basic definitions for each topic is followed by an outline of design strategies for the media under consideration, recently developed analysis and implementation techniques, and discussions of current challenges and promising applications. This review, while not comprehensive, will be helpful especially for early-career researchers, among others, as it offers a broad view of the current state-of-the-art and highlights some unique and flourishing research in the mentioned fields, providing insight into multiple exciting research directions., Nanophotonics, 12 (4), ISSN:2192-8614

International audience; The Fano resonance, as a phenomenon of wave scattering, is based on the interaction between a “bright mode” and a “dark mode” giving rise to an asymmetric and ultrasharp spectral peak. However, the Fano resonant frequency is sensitive to structural imperfections such as defects or disorders, which will shift the resonant peak, or even damage the Fano line shape. Here, we harness the coupling between the first-order and the higher-order topological insulators to overcome this shortcoming. We construct a first-order topological edge state to serve as a bright mode, and a second-order topological corner state to be the dark mode using the same base configuration. As a result, a topological Fano resonance is expected to occur around the resonant frequency of the dark mode. The robustness of the Fano resonance is verified by deliberately introducing various types of imperfections into the system. Our findings may further enhance confidence in using the resonance such as acoustic switching, refractive index sensing, high quality factor filters, and accurate interferometers.

Experimental and computational study of optomechanical interactions in plasmonic nanorod - polymer nanocomposites. Plasmonic coupling between adjacent metallic nanoparticles can be exploited for acousto-plasmonics, single-molecule sensing, and photochemistry. Light absorption or electron probes can be used to study plasmons and their interactions, but their use is challenging for disordered systems and colloids dispersed in insulating matrices. Here, we investigate the effect of plasmonic coupling on optomechanics with Brillouin light spectroscopy (BLS) in a prototypical metal–polymer nanocomposite, gold nanorods (Au NRs) in polyvinyl alcohol. The intensity of the light inelastically scattered on thermal phonons captured by BLS is strongly affected by the wavelength of the probing light. When light is resonant with the transverse plasmons, BLS reveals mostly the normal vibrational modes of single NRs. For lower energy off-resonant light, BLS is dominated by coupled bending modes of NR dimers. The experimental results, supported by optomechanical calculations, document plasmonically enhanced BLS and reveal energy-dependent confinement of coupled plasmons close to the tips of NR dimers, generating BLS hot-spots. Our work establishes BLS as an optomechanical probe of plasmons and promotes nanorod–soft matter nanocomposites for acousto-plasmonic applications. This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Action Grant Agreement 101003436 – PLASMMONS. G.F. acknowledges the support by ERC AdG SmartPhon (Grant 694977). T.V. acknowledges the financial support from the Polish National Science Centre (No. UMO-2021/43/D/ST3/02526). B.G. acknowledges the financial support from the Polish National Science Centre (No. UMO-2018/31/D/ST3/03882). B.D.R. and A.N. acknowledge the computer facilities provided by the group ephoni at Institut d’Electronique, Microélectronique and Nanotechnologie, University of Lille. S.Y. acknowledges the support by the National Science Foundation (NSF), DMR/Polymer program, No. DMR-210484.

International audience; ABSTRACTIn this work we study the localization of flexural waves in highly symmetric clusters of scatterers. It is shown that when the scatterers are placed regularly in the perimeter of a circumference, the quality factor of the resonances strongly increases with the number of scatterers in the cluster. It is also found that in the continuous limit, that is to say, when the number of scatterers tends to infinite, the quality factor is infinite so that the modes belong to the class of the so-called bound states in the continuum or BICs, and an analytical expression for the resonant frequency is found. These modes have different multipolar symmetries, and we show that for high multipolar orders the modes tend to localize at the border of the circumference, therefore forming a whispering gallery mode with an extraordinarily high quality factor. Numerical experiments are performed to check the robustness of these modes under different types of disorder and also to study their excitation from the far field. Although we have focused our study on flexural waves, the methodology presented in this work can be applied to other classical waves, like electromagnetic or acoustic waves, being a promising approach for the design of high-quality resonators based on finite clusters of scatterers.

International audience; Improving radiative heating performance of textiles is becoming one of the most current research topics to reduce the energy consumption used to control the indoor areas temperature. In this work, the properties of a textile-based asymmetric design for radiative heating are studied both theoretically and experimentally, and its remarkable efficiency over a wide range of temperature is demonstrated. By sandwiching a thin metallic layer characterized by a high reflectivity in the mid-infrared (MIR) between two polyethylene (PE) membranes of same thickness, it is proposed to control the MIR emissivity of the structure by the introduction of SiO2 nanoparticles (NPs) in one of the two PE membranes. By reversing the fabric side to side, a wide comfort zone can be achieved for medium and low ambient temperatures, typically between 7.1 and 16.3 °C. Moreover, it is shown that, when transferred on a regular textile, the fabric performs the same functionalities. The experimental demonstration is done by considering an equivalent asymmetric structure with thicker membranes, without NPs. The fabrication and characterization processes are reported and a good agreement is obtained between the Fourier transform infrared spectra and the numerical data.

International audience; The concept of bound states in the continuum (BICs) in a simple cavity attracts much interest in recent works in wave physics. The BICs are perfectly confined modes with an infinite lifetime that reside inside the continuous spectrum of radiative modes, but they remain totally decoupled from it. There exist several types of BICs based on their physical origin: one of the most interesting types is Friedrich-Wintgen (FW) BICs which result from the destructive interference of two resonant modes belonging to the same cavity. Here, we investigate theoretically and experimentally the existence of FW BICs in a side-coupled loop. The cavity is made of a loop of length 2d=d2+d3 connected to a stub of length d4. The whole cavity is attached vertically to two semi-infinite waveguides by a wire of length d1. We demonstrate that the BICs can be induced either by the loop-stub system or by the two arms of lengths d2 and d3 of the loop for specific geometrical parameters. When a perturbation in the system produces a deviation from the BIC condition, the latter transforms to either electromagnetically induced transparency (EIT) or reflection (EIR) or Autler-Townes splitting (ATS) resonances. Both EIT and ATS exhibit similar features in the transmission spectrum, namely, a transparency window; however, they have different physical origins. Therefore, EIT and ATS resonances are fitted with corresponding analytical model expressions, revealing good agreements. The Akaike's information criterion is then used to quantitatively discern EIT from ATS and the transition from ATS to EIT is also carried out. Our theoretical results are obtained by means of the Green's function method which enables us to obtain the transmission and reflection coefficients, dispersion relations, as well as density of states and scattering matrix. An experimental validation of all these results is performed in the radio-frequency domain using coaxial cables.

International audience; The design and study of structures exhibiting bound states in the continuum (BICs) are the object of continuous works in wave physics. These long-lived states which are localized in some parts of the system without interacting with the background have found several potential applications due to their high sensitivities to weak perturbations, in particular in filtering and sensing. In this paper, we present a theoretical demonstration of BICs in an asymmetric loop composed of two arms of lengths d1 and d2 with both an experimental validation in the radio-frequency (RF) domain using coaxial cables and a numerical validation in the infrared (IR) domain using plasmonic metal-insulator-metal nanometric waveguides. The analytical study is performed by means of the Green’s function method, whereas the numerical calculation is obtained using finite element method. The BICs correspond to localized resonances of infinite lifetime inside the loop, without any leakage into the surrounding waveguides. We demonstrate that the condition for the existence of the BICs is to make the lengths of the two arms (d1 and d2) commensurate with each other. At the corresponding frequencies, one of the two degenerate modes of the isolated loop (associated with the clockwise and anti-clockwise propagations) couples to the waveguides while the other remains unaffected. When the lengths are slightly shifted from the BICs, the latter transform to Fano resonances exhibiting dips in the transmission spectra and sharp peaks in the density of states (DOS). As an application of our design, we show the efficiency of the Fano resonances in designing an efficient gaz-sensor with a high sensitivity and factor of merit in the IR domain. In addition, we derive an exact formula about the proportionality between DOS and the derivative of the argument of the determinant of the scattering matrix (Friedel phase) for a lossless structure; then, we discuss the validity and deviation from this rule when the loss is increased.

International audience; We present a non-Hermitian metabeam exhibiting an exceptional point (EP) induced by enforcing parity-time (PT) symmetry through applied external forces. The EP is formed by the hybridization of two flexural wave modes and its output displacement is enhanced by attaching two pillars on top of the beam. The introduction of a tiny mass perturbation that breaks the PT symmetry leads to a splitting of the eigenfrequencies at the EP with a square-root dependence on the perturbation mass. This effect manifests itself in a splitting of resonant peaks in the frequency response. The enhanced sensitivity of the EP paves the way to the detection of small perturbations such as tiny masses and cracks. Another property of the metabeam is the existence of skin modes whose energies are localized at one end of the beam and can be generated by implementing nonreciprocal feedback interactions between the pillars. We demonstrate that the skin modes are broadband and independent of the excitation positions. From a practical perspective, we show the great potential of skin modes in broadband energy harvesting. Our study proposes approaches to manipulate the non-Hermitian elastic wave phenomena, paving the way for the development of highly sensitive sensors, vibration control, and energy harvesting.

International audience; We present an analytical and numerical study of the possibility of existence of surface localized modes, the so-called Tamm states, in a one-dimensional (1D) comb-like phononic crystal (PnC). The structure is made out of periodic array of stubs of lengths d2grafted along a waveguide and separated from each other by a tube of length d1. We show the existence of surface modes for the semi-infinite structure. In particular, when one considers two complementary semi-infinite systems obtained by cutting the infinite one into two parts, we obtain one surface mode per gap induced by the surface of the two complementary systems. Furthermore, we demonstrate that these surface modes can be detected from the maxima and minima of the transmission spectrum, when the finite structure is grafted vertically along a homogeneous acoustic waveguide. That means that one can observe experimentally this type of modes for acoustic waves in slender tubes. These results may find many practical applications in noise control and highly sensitive PnC sensors. © 2021 Elsevier Ltd. All rights reserved.

International audience; We consider, in the frame of long-wavelength Heisenberg model, a simple magnetic device consisting of two dangling side stubs grafted at the same site along a waveguide. This simple structure is designed to obtain bound in continuum (BIC) modes, magnonic induced transparency (MIT) and magnonic induced absorption (MIA), the analogues of electromagnetic induced transparency (EIT) and absorption (EIA) resonances respectively. By detuning the lengths of the two stubs, a resonance can be squeezed between two transmission zeros induced by the two resonators. We give detailed analytical expressions for the transmission and reflection coefficients as well as the absorption coefficient of the whole structure. A sharp peak with high Q factor is found as the consequence of the constructive interference of the waves in the two stubs. For some specific detuning values, the Q factor may reach infinity giving rise to the so-called bound in continuum state. Also, we give several explicit expressions for other physical characteristics, e.g., the transmission function close to the resonance, the position, the width and the Fano parameters of the resonances as a function of the difference between the lengths of the stubs. The reported analytical results are obtained by means of a Green's function method. These results should have important consequences for designing a selecting or rejecting magnon filter.

International audience; The vibrational eigenmodes of dumbbell-shaped polystyrene nanoparticles are recorded by Brillouinlight spectroscopy (BLS), and the full experimental spectra are calculated theoretically. Different fromspheres with a degeneracy of (2l + 1), with l being the angular momentum quantum number, theeigenmodes of dumbbells are either singly or doubly degenerate owing to their axial symmetry. The BLSspectrum reveals a new, low-frequency peak, which is attributed to the out-of-phase vibration of the twolobes of the dumbbell. The quantization of acoustic modes in these molecule-shaped dumbbell particlesevolves from the primary colloidal spheres as the separation between the two lobes increases

International audience; We investigate both analytically and numerically the existence of localized surface modes, the so-called plasmonic Tamm states (PTSs), in a new and versatile platform based on a periodic array of metal-insulator-metal (MIM) stubs grafted along a MIM waveguide. By considering a semi-infinite structure in which we modify the length of the segment at the surface, we show the existence of surface states inside the bandgaps of the periodic structure and investigate the dependence of the localized modes as a function of the geometrical parameters and the boundary conditions applied at the surface. Three types of surface boundary conditions are considered, namely, two limiting cases of zero surface impedance (or perfect electric conductor), infinite surface impedance (or perfect magnetic conductor), and a third case where the structure is in contact with a real metal. In the latter case, we show that the existence of the interface state can be demonstrated based on topological arguments using the Zak phase. We also demonstrate that if a finite size comb-crystal is vertically grafted along a horizontal waveguide, the PTSs can be detected from the dips in the amplitudes of transmission and reflection coefficients as well as from the peaks in their delay times and the local density of states (LDOS). Our theoretical study is first performed analytically with the help of a Green’s function method, which allows the calculation of the dispersion relations of the bulk and surface modes and the LDOS, as well as the transmission and reflection coefficients of the plasmonic comb-like structure. Then, these results are confirmed by a numerical simulation utilizing a 2D finite element method. Besides providing a deep physical analysis of the PTSs, our work demonstrates the capability of the analytical method as a predictive approach in more complex structures. The proposed designs in this paper can be useful to realize highly sensitive plasmonic nanosensors.

International audience; Non-Hermitian systems have gained a great deal of interest in various wave problems due their ability of exhibiting unprecedented phenomena such as invisibility, cloaking, enhanced sensing, or the skin effect. The latter manifests itself by the localization of all bulk modes in a specific frequency range at a given boundary, with an unconventional bulk-boundary correspondence. In this work, we propose to realize the skin effect for flexural waves in a non-Hermitian piezoelectric phononic beam with feedback control between a sensor and an actuator in each unit cell. By implementing a non-Hermitian parameter, effective gain and loss can be achieved in the phononic beam characterized by complex eigen frequencies, and non-reciprocal pass bands are obtained. We highlight that the split point separating the gain and loss areas can occur not only at the edges of the Brillouin zones but also inside the same Brillouin zone. We further analyze the influence of the geometric and non-Hermitian parameters on the complex dispersions and the split point. The topology of the complex bands is characterized by the winding number, which supports the skin effect together with the non-reciprocity. The localization degree of the skin mode manifested by the enhanced beam's vibration energy at one boundary is related to the strength of the non-reciprocity, and the skin mode can be always excited regardless of the source position. Our results provide a potential platform to introduce non-Hermiticity into phononic or metamaterial systems with novel functions for elastic waves such as topological insulators, vibration attenuation or amplification, and energy harvesting.

Piezoelectric energy harvesting has attracted tremendous interest for designing sustainable self-powered devices/systems targeted to special environment such as wireless or wearable applications. The traditional cavity (e.g., phononic cavity mode) excitation is highly applicable in terms of sufficient power generation, nevertheless, has to endure the drawback of extremely poor robustness intrinsic to the trivial cavity modes. We propose to use phononic thin plate systems for robust energy harvesting application relying on zero-dimensional cavities confined by the Kekule distorted topological vortices. The harvesting power induced by topological cavities is about 30 times that of the bare plate. Further studies on the effects of deliberately introduced defects on the output power show that the proposed energy harvesting system is highly robust against symmetry-preserving defects, and is less influenced even for symmetry-breaking defects at moderate perturbation level. Beyond the reported energy harvesting application, we foresee that our work may open avenues for robust operations in the realm of wireless sensing and structural health monitoring.

Ponencia presentada en Fifteenth International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials), 20-24 Sept. 2021, New York, USA In this work, we discuss the interaction between Helmholtz resonators, using the finite element method. We show that the coupling between two spheres gives rise to two resonant modes at low frequencies. One is symmetric and corresponds to a mono-polar breathing mode, as currently observed. The second one is anti-symmetric and behaves as a dipole. We show that the first one generates an isotropic wave in the far pressure field while the second one gives rise to a directional propagative wave. The two modes are discussed as a function of the geometrical and physical parameters. When considering the thermo-viscous properties of air for a specific set of parameters, one finds that the behavior of the dipolar mode can be changed from a diffuser to an absorber. By managing double Helmholtz resonators, we aim to create a multifunctional meta-surface operating at two different frequencies.

International audience; We present a theoretical and experimental study of photonic demultiplexers based on detuned stubs. The demultiplexers consist of Y-shaped structures with one input line and two output lines. Two different types of structures are proposed to achieve a selective transfer of a single mode in one output line without disturbing the second one. (i) In the first platform each output contains two different stubs attached at two different sites (U-shaped resonators). We derive in closed form the geometrical parameters of the stubs to achieve a selected frequency in each line while keeping the other line unaffected. The frequency selection can be made on the basis of two different mechanisms, namely a Fano or an electromagnetic induced transparency (EIT) resonance. Consequently, different demultiplexing schemes can be designed by a combination of the two mechanisms, such as Fano-Fano, Fano-EIT or EIT-EIT. In particular, the width of the Fano or EIT resonances can become zero for an appropriate choice of the stubs’ lengths, giving rise to trapped modes also called bound in continuum states (BICs) with infinite quality factors. We also show that the crosstalk between the two outputs can reach minimum values around −45 dB. (ii) In the second platform, each output line contains a photonic comb with a defect stub. The latter is appropriately designed to filter one or a few frequencies in the bandgap of the photonic comb. The analytical calculations are performed with the help of the Green’s function method which enables us to derive the transmission and reflection coefficients as well as the density of states (DOS). These results are confirmed by experimental measurements using coaxial cables in the radio frequency domain.

International audience; Broadband attenuation of low frequency acoustic waves in compact structure designs is a challenging issue, especially in the application to seismic waves. In this work, we propose a new seismic metamaterial constituted by a combination of two different attenuating structures, namely pillars above the ground and core-shell inclusions embedded in the soil. As compared to the two constituting parts, this structure enables us to broaden the bandgap while shifting it towards lower frequencies. Additionally, the analysis of the symmetry of the excited Rayleigh wave and the eigenmodes in the dispersion curves reveal that the frequency range of efficient wave attenuation in transmission is much wider than that of the corresponding bandgap. Finally, gradient combined metamaterial is designed to achieve wave attenuation in a broadband range of [4.3, 20.0] Ηz while the unit cell size keeps compact, of the order of 2 m. The results show practical and economic applications in ground surface vibration isolation to protect large infrastructures or civil engineering architectures.

International audience; The achievement of an efficient acoustic biosensor based on one dimensional solid-liquid phononic crystals supporting bound states in the continuum (BICs) and Fano resonances is reported herein. The structure used in this work is made of solid-liquid-solid triple layer embedded in water. The main results in this paper consist to study the impact of: i) the effect of albumin concentration, ii) the effect of the albumin layer thickness and iii) the influence of the incidence angle θ on albumin detection performance.

Resonant mechanical sensors are often considered as mass balance, which responds to an analyte adsorbed on or absorbed in a thin sensitive (and selective) layer deposited on the surface of the resonant device. In a more general sense, the sensor measures properties at the interface of the mechanical resonator to the medium under inspection. A phononic crystal (PnC) sensor employs mechanical resonance as well; however, the working principle is fundamentally different. The liquid medium under inspection becomes an integral part of the PnC sensor. The liquid-filled compartment acts as a mechanical resonator. Therefore, the sensor probes the entire liquid volume within this compartment. In both sensor concepts, the primary sensor value is a resonant frequency. To become an attractive new sensing concept, specifically as a bio and chemical sensor, the PnC sensor must reach an extraordinary sensitivity. We pay attention to the liquid viscosity, which is an important factor limiting sensitivity. The main part of our analysis has been performed on 1D PnC sensors, since they underlie the same material-related acoustic dissipation mechanisms as 2D and 3D PnC sensors. We show that an optimal relation of frequency shift to bandwidth and amplitude of resonance is the key to an enhanced sensitivity of the sensor-to-liquid analyte properties. We finally address additional challenges of 2D and 3D PnC sensor design concept. We conclude that the sensor should seek for a frequency resolution close to 10−6 the probing frequency, or a resolution with speed of sound approaching 1 mm s−1, taking water-based analytes as an example.