Topological negative refraction of surface acoustic waves in a Weyl phononic crystal (1808.04647v1)

Abstract: Reflection and refraction occur at interface between two different media. These two fundamental phenomena form the basis of fabricating various wave components. Specifically, refraction, dubbed positive refraction nowadays, appears in the opposite side of the interface normal with respect to the incidence. Negative refraction, emerging in the same side by contrast, has been observed in artificial materials1-5 following a prediction by Veslago6, which has stimulated many fascinating applications such as super-resolution imaging7. Here we report the first discovery of negative refraction of the topological surface arc states of Weyl crystals, realized for airborne sound in a novel woodpile phononic crystal. The interfaces are one-dimensional edges that separate different crystal facets. By tailoring the surface terminations of such a Weyl phononic crystal, open equifrequency contours of surface acoustic waves can be delicately designed to produce the negative refraction, to contrast the positive counterpart realized in the same sample. Strikingly different from the conventional interfacial phenomena, the unwanted reflection can be made forbidden by exploiting the open nature of the surface equifrequency contours, which is a topologically protected surface haLLMark of Weyl crystals8-12.

• The paper reports the first experimental observation of topologically protected negative refraction of surface acoustic waves in a Weyl phononic crystal, validating theoretical predictions.
• It utilizes a woodpile-shaped design with rotated epoxy rod layers to create open equifrequency contours and stabilize Weyl points for reflectionless wave propagation.
• Experimental measurements at 5.75 kHz confirmed distinct propagation directions for positively and negatively refracting waves, advancing acoustic control techniques.

Topological Negative Refraction of Surface Acoustic Waves in a Weyl Phononic Crystal

The paper in question investigates the novel phenomenon of topological negative refraction of surface acoustic waves (SAWs) within a Weyl phononic crystal (PC). This work introduces significant advancements in the manipulation of SAWs through sophisticated design of Weyl phononic structures, providing both theoretical and experimental frameworks for this phenomenon.

Summary of Findings

The authors present the first experimental observation of topologically protected negative refraction of SAWs within a woodpile-shaped Weyl phononic crystal. This phenomenon is realized by exploiting the unique surface states (Fermi arcs) characteristic of Weyl semimetals, which in this case are applied to a three-dimensional classical wave system. The Weyl phononic crystal structure is carefully tailored to exhibit open equifrequency contours (EFCs), allowing for the realization of negative refraction. The design employs a careful configuration of layer rotations and twist symmetries to stabilize Weyl points, characterized by their nontrivial topological charges and Berry flux.

Key Findings and Methods

• Open Equifrequency Contours and Reflectionless Interfaces: The research highlights the application of open EFCs as a crucial mechanism to bypass the typically unavoidable reflection at media interfaces, enabling novel refraction phenomena that uphold topological protection principles. This feature accentuates the utility of Weyl phononic crystals in maintaining coherent wave propagation without backscattering.
• Architectural Design of Weyl Crystals: The crystal was composed of stacked layers of square epoxy rods, each layer rotated to achieve a nontrivial topology. This geometric design was crucial in creating the Weyl points within the k-space, around which the SAWs exhibit linear and quadratic dispersions depending on their locality in the crystal structure.
• Experimental Validation: Through meticulous experiments with sound sources placed at strategic locations on the crystal surfaces, the authors mapped surface wave phenomena in both real and momentum space. The crucial differentiation between positively and negatively refracting SAWs was corroborated by observing the propagation direction relative to interface normals, verified at specific frequencies (notably at the Weyl point frequency of 5.75 kHz).

Implications and Future Directions

The implications of this paper are multifold, introducing new avenues for wave manipulation in both acoustic and photonic systems. By demonstrating zero-reflection interfaces using topologically protected surface states, applications such as super-resolution imaging and novel waveguiding methods can be envisioned.

This research potentially lays groundwork for the development of novel acoustic devices leveraging topological properties, which may include robust acoustic pathways immune to disorder and defects. Furthermore, the principles applied here could translate to other wave systems, offering insights into electromagnetic, electronic, and even material design based on topological characteristics.

Future studies could expand upon this work by exploring varying lattice symmetries or seeking tunable Weyl PCs that respond dynamically to environmental stimuli or applied external fields. The synthesis of materials with higher-order topological phases might also offer richer phenomenological features with further applications in condensed matter physics and beyond.

In summary, this paper exemplifies the intersection of topology, material science, and wave physics, presenting a solid foundation for a burgeoning field of research within topological acoustics and phononics.