Aurelien Merkel

The Su–Schrieffer–Heeger model becomes non-Hermitian in general when the reciprocity of wave propagation is broken in the couplings. Here, we introduce phase non-reciprocity in the coupling of an acoustic Su–Schrieffer–Heeger chain by adding direction-dependent imaginary parts in the coupling strengths while keeping the real parts independent on the direction. We theoretically and numerically investigate the effect of the phase non-reciprocity on the band structure and the topology of the chain. The hermiticity is broken in general, but there is still one special case where the model remains Hermitian. An acoustic analog of this case is build where fluid flow is introduced in the couplings. In the non-Hermitian cases, the skin effect is visible except for one specific transition case where the system remains non-Hermitian but where reciprocity is maintained.

Non-Hermiticity alone can trigger topological phase transition in physical systems. Here, we construct different unit cells in an acoustic Su Schrieffer Heeger chain with different distributions of onsite losses. We theoretically and numerically investigate the different edge modes that can occur at the domain walls of different finite chains. Three types of edge modes are identified. The first type comes from the topology of the unit cells. The second type comes from the local Parity symmetry at the interface, which are cavity modes. The third one comes from the Parity-Time symmetric domain wall. The robustness against coupling disorder is then examined, confirming the robustness of the topologically protected modes. The evolution with increasing disorder of the interface modes due to the Parity-Time symmetric domain wall is singular as they appear first as more robust than the cavity modes before diverging. These results show the ability of the onsite losses ingredient to control wavefields.

We numerically and experimentally report the acoustic analogue of the super-Klein tunneling in a heterojunction of phononic crystals formed with Willis scatterers that exhibit pseudospin-1 Dirac cones. By comparing with the pseudospin-1/2 Dirac cones, pseudospin-1 ones require in the band structure an additional flatband across the Dirac points. The conventional Klein tunneling, which is predicted in pseudospin-1/2 systems like graphene, consists of perfect transmission only under normal incidence through a potential barrier of any width. However, the super-Klein tunneling that we evidence here is defined for pseudospin-1 systems as a perfect transmission for all incidence angles at one single frequency within the energy barrier. This direct observation may have important implications in the exploration of the rich physics of pseudospin-1 quasiparticles.

Integrating different reliable functionalities in metastructures and metasurfaces has become of remarkable importance to create innovative multifunctional compact acoustic, optic or mechanical metadevices. In particular, implementing different wave manipulations in one unique material platform opens an appealing route for developing integrated metamaterials. Here, the concept of Janus acoustic metascreen is proposed and demonstrated, producing two-faced and independent wavefront manipulations for two opposite incidences. The feature of two-faced sound modulations requires nonreciprocal phase modulating elements. An acoustic resonant unit cell with rotating inner core, which produces a bias by a circulating fluid, is designed to achieve high nonreciprocity, leading to decoupled phase modulations for both forward and backward directions. In addition, the designed unit cell consisting of tunable phase modulators is reconfigurable. A series of Janus acoustic metascreens including optional combinations of extraordinary refraction, acoustic focusing, sound absorption, acoustic diffusion, and beam splitting are demonstrated through numerical simulations and experiments, showing their great potential for acoustic wavefront manipulation.

This paper examines the feasibility of cloaking an obstacle using Plate-type Acoustic Metamaterials (PAMs). We present two distinct strategies to cloak this obstacle, using either the near-zero-density regime of a periodic arrangement of plates or the acoustic doping phenomenon to achieve simultaneous zero-phase propagation and impedance matching. The strong limitations induced by viscothermal and viscoelastic losses that cannot be avoided in such a system are studied. A hiding zone is reported analytically, numerically, and experimentally. In contrast to cloaking, where zero-phase propagation must be accompanied by total transmission and zero reflection, the hiding configuration requires that the scattering properties of the hiding device must not be affected by the presence of the obstacle embedded in it. Contrary to cloaking, the hiding phenomenon is achievable even with a realistic PAM possessing unavoidable losses.

Mirror-symmetric acoustic metascreens producing perfect absorption independently of the incidence side are theoretically and experimentally reported in this work. The mirror-symmetric resonant building blocks of the metascreen support symmetric and antisymmetric resonances that can be tuned to be at the same frequency (degenerate resonances). The geometry of the building blocks is optimized to critically couple both the symmetric and the antisymmetric resonances at the same frequency, allowing perfect absorption of sound from both sides of the metascreen. A hybrid analytical model based on the transfer-matrix method and the modal decomposition of the exterior acoustic field is developed to analyze the scattering properties of the metascreen. The resulting geometry is three-dimensionally printed and experimentally tested in an impedance tube. The experimental results agree well with the theoretical predictions, proving the efficiency of these metascreens for the perfect absorption of sound in ventilation problems.

We theoretically, numerically, and experimentally investigate the feasibility of acoustic doping, i.e., changing one of the effective properties of a medium by adding an impurity, to achieve supersqueezing. This effect, characterized by perfect and zero-phase transmission, can be obtained with zero index media. In acoustics, zero-phase propagation can be achieved with a plate-type acoustic metamaterial (PAM) acting as a density-near-zero metamaterial (DNZ). We point out the possibility of modifying the compressibility of a DNZ medium by mounting a Helmholtz resonator in parallel with the PAM. We are then able to dope the system and to turn it into a density-and-compressibility-near-zero medium, thus allowing supersqueezing.

Three-dimensional (3D) Weyl and Dirac semimetals garner considerable attention in condensed matter physics due to the exploration of entirely new topological phases and related unconventional surface states. Nodal line and ring semimetals, on the other hand, can facilitate 3D band crossings characterized by nontrivial links such as coupled chains and knots that are protected by the underlying crystal symmetry. Experimental complexities and detrimental effects of the spin-orbit interaction, among others, pose great challenges for the advancement that can be overcome with other systems such as bosonic lattices. Here we demonstrate that a 3D mechanical metamaterial made of granular beads hosts multiple intersecting nodal rings in the ultrasonic regime. By unveiling these yet unseen classical topological phases, we discuss the resilience of the associated novel surface states that appear entirely unaffected to the type of crystal termination, making them a promising platform in ultrasonic devices for non-destructive testing and material characterization.