Reflection forbidden and refraction reversed in an artificial crystal

At the interface between two facets of an artificial crystal, sound waves can be transmitted in the opposite direction to that expected, and undergo no reflection. Such wave behaviour could have many applications.
Baile Zhang is in the Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore.

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Waves change direction when they pass from one medium to another — a phenomenon called refraction. This effect underlies most optical lenses and instruments, and is widely found in acoustics when an acoustic beam behaves like an optical beam. In general, some of the waves are reflected during the refraction process. In a paper in Nature, He et al.1 report an impressive demonstration of a previously unobserved refraction phenomenon. They show that, in a certain artificially engineered material, an acoustic beam can be refracted in the opposite direction to that seen in ordinary materials, without reflection. The authors’ findings could lead to improved control of waves in electronic and photonic systems.

When an acoustic or optical ray strikes the interface between two different media, part of its energy passes through the interface to form a refracted ray (Fig. 1a). The remaining energy reflects from the interface to produce a reflected ray. In nature, the incident and refracted rays are always on opposite sides of the normal — an imaginary line perpendicular to the interface. But, in theory, this need not be the case.

Figure 1 | Comparison of refraction phenomena. a, In conventional refraction, when an acoustic or optical ray (red) hits the interface between two different media, a reflected ray (dark blue) and a refracted ray (light blue) are produced. The incident and refracted rays exist on opposite sides of the normal — an imaginary line perpendicular to the interface. b, In negative refraction, the refracted ray emerges on the same side of the normal as the incident ray. c, He et al.1 report a previously unobserved type of refraction for acoustic rays, in which not only are the incident and refracted rays on the same side of the normal, but also there is no reflected ray. (Figure adapted from ref. 1.)

In 1968, the Russian physicist Victor Veselago considered a hypothetical material that has a negative refractive index2. A refractive index describes how waves propagate in a medium, and is positive in all conventional materials. Veselago showed that the way in which refraction usually occurs could be reversed in a negative-index material: the refracted ray could emerge on the same side of the normal as the incident ray (Fig. 1b).

Although intriguing, negative refraction did not trigger much attention, and was considered impossible for more than 30 years because it was thought that negative-index materials could not exist. The situation changed in 2000, when the British physicist John Pendry made a shocking prediction3: that negative refraction could be used to make a lens that could focus light more tightly than is normally possible. He also identified a practical way to construct negative-index materials in the lab using artificial structures. Such materials, now generally referred to as metamaterials, stimulated research into concepts such as invisibility cloaking4 that had previously existed only in science fiction.

In the years since Pendry’s work, the pursuit of negative refraction has led to developments in optics, acoustics, plasmonics (the study of how light interacts with electrons in metals) and even graphene-based electronics5. Versions of negative refraction have been realized in each of these areas. However, the phenomenon is generally accompanied by reflection, which is often undesirable. In many cases, such as in experiments involving the refraction of electrons through an interface5, reflection can even dominate negative refraction.

The property of reflection immunity is not found in natural optical materials for light. However, it does occur in exotic phases of matter known as topological quantum matter, for quantum-mechanical electronic waves. A well-studied example is the topological insulator, which is an electrical insulator in its interior, but conducts electricity on its surface through electronic waves called topological surface states. Such states are able to propagate unidirectionally — they bypass obstacles and defects, rather than being reflected.

He and colleagues’ demonstration was directly inspired by another emerging topological quantum matter: the Weyl semimetal6. The topological surface states in this material cannot propagate in all directions; propagation is confined to a certain range of directions, which connect to form what are known as Fermi arcs6. Because the limited range of propagation directions does not include the direction in which reflection would normally occur, reflection is forbidden (Fig. 1c).

In their experiment, He et al. used an artificial crystal that is an acoustic analogue of the Weyl semimetal. They found that, at the interface between two adjacent facets of the crystal, airborne acoustic waves could undergo negative refraction without reflection. The authors’ results represent the first realization of negative refraction for topological surface states.

There are a few limitations of the work. For instance, the refraction does not occur in a flat plane, contrary to the common impression of refraction. Moreover, the interface scatters some of the acoustic waves into the crystal’s interior, resulting in energy loss. Nevertheless, the demonstration opens the door to many exciting opportunities for further research.

The immediate question is whether He and colleagues’ refraction phenomenon could be realized in optical systems for light and condensed-matter systems for electrons. Another question, which will be of interest to both optical and condensed-matter physicists, is how to engineer the range of propagation directions — and, in turn, the Fermi arcs — to achieve greater control of negative refraction. In this sense, the authors’ work provides the first practical use of Fermi arcs, which are currently being enthusiastically explored in condensed-matter systems7,8 and in optical structures called photonic crystals9.

The refraction phenomenon could also find widespread use in acoustics. For example, the combination of negative refraction and zero reflection could lead to improved resolution in ultrasonic imaging and testing. Moreover, acoustic waves are used in biomedical microfluidic devices to trap, sort and deliver cells and drug particles. Reflection-free acoustic waves are strongly desirable in such applications, because reflections at the interfaces and sharp corners of microfluidic channels are currently a huge limitation to device efficiency. Topological acoustics is therefore a promising research field that not only can produce phenomena that are difficult to realize in other physical systems, but could also bring about transformative technologies.

Nature 560, 37-38 (2018)

doi: 10.1038/d41586-018-05806-6


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