Negative Refraction Without Reflection

$Negative Refraction Without Reflection$

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ACOUSTICS 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). Negative refraction In their experiment, He et al. used an artificial crystal that is an acoustic analogue of the Weyl semimetal. They found that, at the without reflection interface between two adjacent facets of the crystal, airborne acoustic waves could undergo At the interface between two facets of an artificial crystal, sound waves can be negative refraction without reflection. The transmitted in the opposite direction to that expected, and undergo no reflection. authors’ results represent the first realization Such wave behaviour could have many applications. See Letter p.61 of negative refraction for topological surface states. There are a few limitations of the work. For BAILE ZHANG cloaking4 that had previously existed only in instance, the refraction does not occur in a flat science fiction. plane, contrary to the common impression of aves change direction when they In the years since Pendry’s work, the pursuit refraction. Moreover, the interface scatters pass from one medium to another of negative refraction has led to developments some of the acoustic waves into the crystal’s — a phenomenon called refraction. in optics, acoustics, plasmonics (the study of interior, resulting in energy loss. Nevertheless, WThis effect underlies most optical lenses and how light interacts with electrons in metals) the demonstration opens the door to many instruments, and is widely found in acoustics and even graphene-based electronics5. Ver exciting opportunities for further research. when an acoustic beam behaves like an opti sions of negative refraction have been realized The immediate question is whether He and cal beam. In general, some of the waves are in each of these areas. However, the phenom colleagues’ refraction phenomenon could be reflected during the refraction process. On enon is generally accompanied by reflection, realized in optical systems for light and con page 61, He et al.1 report an impressive dem which is often undesirable. In many cases, such densed-matter systems for electrons. Another onstration of a previously unobserved refrac as in experiments involving the refraction of question, which will be of interest to both opti tion phenomenon. They show that, in a certain electrons through an interface5, reflection can cal and condensed-matter physicists, is how to artificially engineered material, an acoustic even dominate negative refraction. engineer the range of propagation directions beam can be refracted in the opposite direc The property of reflection immunity is not — and, in turn, the Fermi arcs — to achieve tion to that seen in ordinary materials, without found in natural optical materials for light. greater control of negative refraction. In this reflection. The authors’ findings could lead to However, it does occur in exotic phases of sense, the authors’ work provides the first improved control of waves in electronic and matter known as topological quantum matter, practical use of Fermi arcs, which are currently photonic systems. for quantum-mechanical electronic waves. A being enthusiastically explored in condensed- When an acoustic or optical ray strikes the well-studied example is the topological insu matter systems7,8 and in optical structures interface between two different media, part lator, which is an electrical insulator in its called photonic crystals9. of its energy passes through the interface to interior, but conducts electricity on its surface The refraction phenomenon could also find form a refracted ray (Fig. 1a). The remaining through electronic waves called topological widespread use in acoustics. For example, the energy reflects from the interface to produce surface states. Such states are able to propagate combination of negative refraction and zero a reflected ray. In nature, the incident and unidirectionally — they bypass obstacles and reflection could lead to improved resolution refracted rays are always on opposite sides of defects, rather than being reflected. in ultrasonic imaging and testing. Moreover, the normal — an imaginary line perpendicular He and colleagues’ demonstration was acoustic waves are used in biomedical micro to the interface. But, in theory, this need not directly inspired by another emerging topo fluidic devices to trap, sort and deliver cells be the case. logical quantum matter: the Weyl semimetal6. and drug particles. Reflection-free acoustic In 1968, the Russian physicist Victor The topological surface states in this material waves are strongly desirable in such applica Veselago considered a hypothetical material cannot propagate in all directions; propaga tions, because reflections at the interfaces and that has a negative refractive index2. A refrac tion is confined to a certain range of direc sharp corners of microfluidic channels are cur tive index describes how waves propagate in tions, which connect to form what are known rently a huge limitation to device efficiency. a medium, and is positive in all conventional materials. Veselago showed that the way in which refraction usually occurs could be a b c

reversed in a negative-index material: the Interface Normal 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 consid ered impossible for more than 30 years because it was thought that negative-index materials Medium 2 could not exist. The situation changed in 2000, Medium 1 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. 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 He also identified a practical way to construct ray (light blue) are produced. The incident and refracted rays exist on opposite sides of the normal — an negative-index materials in the lab using arti imaginary line perpendicular to the interface. b, In negative refraction, the refracted ray emerges on the ficial structures. Such materials, now gener same side of the normal as the incident ray. c, He et al.1 report a previously unobserved type of refraction ally referred to as metamaterials, stimulated for acoustic rays, in which not only are the incident and refracted rays on the same side of the normal, but research into concepts such as invisibility also there is no reflected ray. (Figure adapted from ref. 1.)

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Topological acoustics is therefore a promising e-mail: [email protected] research field that not only can produce phe 1. He, H. et al. Nature 560, 61–64 (2018). nomena that are difficult to realize in other 2. Veselago, V. G. Sov. Phys. Usp. 10, 509–514 (1968). physical systems, but could also bring about 3. Pendry, J. B. Phys. Rev. Lett. 85, 3966–3969 transformative technologies. ■ (2000). 4. Pendry, J. B., Schurig, D. & Smith, D. R. Science 312, 1780–1782 (2006). Baile Zhang is in the Division of Physics 5. Chen, S. et al. Science 353, 1522–1525 (2016). and Applied Physics, School of Physical 6. Wan, X., Turner, A. M., Vishwanath, A. & Savrasov, S. Y. Phys. Rev. B 83, 205101 (2011). 50 Years Ago and Mathematical Sciences, Nanyang 7. Xu, S.-Y. et al. Science 349, 613–617 (2015). Technological University, Singapore 637371, 8. Lv, B. Q. et al. Phys. Rev. X 5, 031013 (2015). The prospect of beer by the litre Singapore. 9. Lu, L. et al. Science 349, 622–624 (2015). for British drinkers came a step closer last week. Mr Anthony Wedgwood Benn, the Minister of METABOLISM Technology, announced that the United Kingdom is to adopt the metric system of measurement by 1975 — the target date already An unexpected trigger accepted by British industry for its timetable … The industrial change is going ahead fast … but the non- for calorie burning industrial sector of the economy and the general public have been The molecule succinate, which is a product of metabolism, promotes heat lagging behind … It is “imperative” production and therefore calorie burning in brown fat in mice. This discovery for the planning of the change in could have implications for combating obesity in humans. See Letter p.102 the general sectors of the economy to be put in hand; if this is not done, “the dynamism of the industrial SHENG HUI & JOSHUA D. RABINOWITZ and thereby generates most of the body’s change will be lost” … The cost- usable energy. But in brown fat, protons pass effectiveness of metrication is not … here are two ways to lose weight: eat through another protein, uncoupling pro likely to be known until it is a fact. less to reduce the number of calories tein 1 (UCP1). This transporter uncouples the So far, certainly, hunch has played a available for metabolism by the body, process of crossing the mitochondrial mem greater part than has sober analysis Tor burn more calories, for example through brane from that of ATP production, effectively of the situation. exercise. On page 102, Mills et al.1 identify wasting the proton gradient’s energy as heat From Nature 3 August 1968 a molecule produced during nutrient meta (reviewed in ref. 4). bolism that, surprisingly, induces calorie This capacity of brown fat to dissipate burning. This metabolite, succinate, activates calories as heat has attracted much attention, energy expenditure in brown fat. Remarkably, in the hope of activating the process to combat 100 Years Ago supplementing the drinking water of mice obesity5. To do this, it is necessary to know with succinate can prevent the animals from what switches on calorie burning by brown … Finally, there is the personal use gaining weight. fat. At the macroscopic level, the main answer of insecticidal preparations as aids Brown fat is different from the white is exposure to cold. At the mechanistic level, it to the primitive method of getting fat that builds up around our waistlines. has been proposed that the brain senses cold rid of [lice] — now referred to as Whereas white fat acts as an energy reserve, and sends signals to brown fat through a pro “chat”-hunting … [T]he preparation brown fat specializes in heat generation, and cess mediated by proteins called β-adrenergic should be of quick action and easy is essential for mammals to maintain their receptors2. But drugs that activate these recep of application to clothing, and body temperature in the cold2. Brown-fat tors have not been successful in curbing obe its issue should be as general and cells contain smaller lipid droplets than do sity6. Thus, there is intense interest in finding comprehensive as that of food … white-fat cells, and have many more orga new pathways that activate heat generation in [P]astes are more economical nelles called mitochondria3, which enable brown fat. and convenient than powders; brown fat to generate heat. Mills et al. began by searching for fluids are out of the question. In mitochondria, a metabolic pathway metabolites that are selectively abundant in Crude “unwhizzed” naphthalene, called the TCA cycle breaks down nutrients brown fat, and whose concentration in this produced by coke-oven plants, such as glucose, lactate and fat into carbon tissue increases during cold exposure. Their affords the most effective base, dioxide, using the energy stored in the nutri survey identified succinate, one of the meta and may be conveniently mixed ents to generate high-energy electrons. These bolic intermediates of the TCA cycle. into paste form by the addition of electrons are used to pump protons (hydro The TCA cycle is generally assumed to be a soft soap or some grease, such as gen ions, H+) out of the interior matrix of cell-intrinsic process in which most intermedi vaseline, in the proportion of 10 to the mitochondrion into the space between ates are trapped in the mitochondrial matrix. 20 per cent … When it is necessary the organelle’s inner and outer membranes, Thus, most succinate is consumed by the same to use an anti-lice preparation on a thereby converting the energy into a proton cell that produces it. Some succinate, how hair-clad surface the use of vaseline, to gradient. Normally, protons re-enter the ever, makes its way into the bloodstream. The which has been added ½ per cent. of mitochondrial matrix through a membrane- authors provide evidence that a key trigger for veratrine dissolved in 5 per cent. of spanning protein complex called the proton the release of succinate may be muscle activity, benzene, may be recommended. ATPase. This complex uses the energy stored because shivering in response to cold increased From Nature 1 August 1918 in the proton gradient to convert ADP mol blood succinate levels in mice. ecules into energy-carrying ATP molecules, To trace the fate of succinate circulating in