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Unidirectional devices that pass acoustic energy in only one direction have numerous applications and, consequently, have recently received significant attention. However, for most practical applications that require unidirectionality at audio and low frequencies, subwavelength implementations capable of the necessary time-reversal symmetry breaking remain elusive. Here we describe a design approach based on metamaterial techniques that provides highly subwavelength and strongly non-reciprocal devices. We demonstrate this approach by designing and experimentally characterizing a non-reciprocal active acoustic metamaterial unit cell composed of a single piezoelectric membrane augmented by a nonlinear electronic circuit, and sandwiched between Helmholtz cavities tuned to different frequencies. The design is thinner than a tenth of a wavelength, yet it has an isolation factor of >10 dB. The design method generates relatively broadband unidirectional devices and is a good candidate for numerous acoustic applications. Unidirectional acoustic devices only permit the flow of energy one way, but most implementations are large compared to acoustic frequencies. Popa and Cummer use a metamaterial approach to build such devices that are only a tenth of a wavelength thick but retain high acoustic isolation.

In addition to controlling the propagation of light, metamaterials have also received attention for controlling sound. Now, a device that can act as a broadband and omnidirectional acoustic cloak is experimentally demonstrated. The control of sound propagation and reflection has always been the goal of engineers involved in the design of acoustic systems. A recent design approach based on coordinate transformations, which is applicable to many physical systems 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , together with the development of a new class of engineered materials called metamaterials, has opened the road to the unconstrained control of sound. However, the ideal material parameters prescribed by this methodology are complex and challenging to obtain experimentally, even using metamaterial design approaches. Not surprisingly, experimental demonstration of devices obtained using transformation acoustics is difficult, and has been implemented only in two-dimensional configurations 10 , 15 . Here, we demonstrate the design and experimental characterization of an almost perfect three-dimensional, broadband, and, most importantly, omnidirectional acoustic device that renders a region of space three wavelengths in diameter invisible to sound.

Recent advances in gradient metasurfaces have shown that by locally controlling the bianisotropic response of the cells one can ensure full control of refraction, that is, arbitrarily redirect the waves without scattering into unwanted directions. In this work, we propose and experimentally verify the use of an acoustic cell architecture that provides enough degrees of freedom to fully control the bianisotropic response and minimizes the losses. The versatility of the approach is shown through the design of three refractive metasurfaces capable of redirecting a normally incident plane wave to 60°, 70°, and 80° on transmission. The efficiency of the bianisotropic designs is over 90%, much higher than the corresponding generalized Snell’s law based designs (81%, 58%, and 35%). The proposed strategy opens a new way of designing practical and highly efficient bianisotropic metasurfaces for different functionalities, enabling nearly ideal control over the energy flow through thin metasurfaces. Acoustic bianisotropy does not exist in natural materials but can be designed with acoustic metamaterials. Here, Li et al. utilized acoustic bianisotropy and develop a practical metamaterial with improved transmission efficiency which outperforms the Generalized Snell’s Law.

We investigate sequential processes underlying the initial development of in‐cloud lightning flashes in the form of initial breakdown pulses (IBPs) between 7.4 and 9.0 km altitudes, using a 30–250 MHz VHF interferometer. When resolved, IBPs exhibit typical stepped leader features but are notably extensive (>500 m) and infrequent (∼1 millisecond intervals). Particularly, we observed four distinct phases within an IBP stepping cycle: the emergence of VHF sources forming edge structures at previous streamer zone edges (interpreted as space stem/leader development), the fast propagation of VHF along the edge structure (interpreted as the main leader connecting the space leader), the fast extension of VHF beyond the edge structure (interpreted as fast breakdown), and a decaying corona fan. These measurements illustrate clearly the processes involved in the initial development of in‐cloud lightning flashes, evidence the conducting main leader forming, and provide insights into other processes known to occur simultaneously, such as terrestrial gamma ray flashes. Plain Language Summary The initial development of a lightning flash inside a cloud has long been a mystery. This study utilizes state‐of‐the‐art lightning imaging techniques with a 30–250 MHz VHF interferometer, providing clear images of the processes involved in the initial development of in‐cloud lightning flashes. New radio features suggest distinct development phases, including what we interpret as space stems, space leaders, connection between the main leader and the space leader, fast breakdown, and corona fan development within an initial breakdown pulse stepping cycle. This provides evidence of the conducting main leader in the initial breakdown stage. These observations showcase the intricate streamer discharge phenomena during initial lightning development, and shed light on other processes known to occur simultaneously, including Terrestrial Gamma ray Flashes. Key Points We observed four distinct VHF processes in the development of 300–1,000 m long initial breakdown pulses (IBPs) in in‐cloud lightning flashes These four processes appear to map to the known processes in a conventional stepped leader, including space stem and space leader formation During an initial breakdown step, fast extension over several hundred meters indicates that fast breakdown may be an essential part of in‐cloud flash

The valley degree of freedom in crystals offers great potential for manipulating classical waves, however, few studies have investigated valley states with complex wavenumbers, valley states in graded systems, or dispersion tuning for valley states. Here, we present tunable valley phononic crystals (PCs) composed of hybrid channel-cavity cells with three tunable parameters. Our PCs support valley states and Dirac cones with complex wavenumbers. They can be configured to form chirped valley PCs in which edge modes are slowed to zero group velocity states, where the energy at different frequencies accumulates at different designated locations. They enable multiple functionalities, including tuning of dispersion relations for valley states, robust routing of surface acoustic waves, and spatial modulation of group velocities. This work may spark future investigations of topological states with complex wavenumbers in other classical systems, further study of topological states in graded materials, and the development of acoustic devices. The valley degree of freedom gives additional flexibility to tunable phononic and photonic crystals. Here, the authors realise a honeycomb phononic structure where both the size of the cavities and of the air channel can be actively tuned, allowing several functionalities in a broad frequency range.

Phase gradient metagratings (PGMs) have provided unprecedented opportunities for wavefront manipulation. However, this approach suffers from fundamental limits on conversion efficiency; in some cases, higher order diffraction caused by the periodicity can be observed distinctly, while the working mechanism still is not fully understood, especially in refractive-type metagratings. Here we show, analytically and experimentally, a refractive-type metagrating which can enable anomalous reflection and refraction with almost unity efficiency over a wide incident range. A simple physical picture is presented to reveal the underlying diffraction mechanism. Interestingly, it is found that the anomalous transmission and reflection through higher order diffraction can be completely reversed by changing the integer parity of the PGM design, and such phenomenon is very robust. Two refractive acoustic metagratings are designed and fabricated based on this principle and the experimental results verify the theory. Phase gradient metagratings suffer from limits on conversion efficiency. Here, the authors show a refractive-type metagrating which can enable anomalous reflection and refraction with almost unity efficiency over a wide incident range and uncover how integer parity plays a role in higher order diffraction.

It is now well established that both thunderclouds and lightning routinely emit x-rays and gamma-rays. These emissions appear over wide timescales, ranging from sub-microsecond bursts of x-rays associated with lightning leaders, to sub-millisecond bursts of gamma-rays seen in space called terrestrial gamma-ray flashes, to minute long glows from thunderclouds seen on the ground and in or near the cloud by aircraft and balloons. In particular, terrestrial gamma-ray flashes (TGFs), which are thought to be emitted by thunderclouds, are so bright that they sometimes saturate detectors on spacecraft hundreds of kilometers away. These TGFs also generate energetic secondary electrons and positrons that are detected by spacecraft in the inner magnetosphere. It is generally believed that these x-ray and gamma-ray emissions are generated, via bremsstrahlung, by energetic runaway electrons that are accelerated by electric fields in the atmosphere. In this paper, we review this newly emerging field of High-Energy Atmospheric Physics, including the production of runaway electrons, the production and propagation of energetic radiation, and the effects of both on atmospheric electrodynamics.

A device containing a circulating fluid breaks the symmetry of acoustic waves and allows one-way transmission of sound. [Also see Report by Fleury et al. ] Structures that admit flow in only one direction are commonplace—consider one-way streets, insect traps, and the staple of the police procedural story, the one-way mirror. However, creating a device that allows waves to pass in only one direction, termed an isolator, is challenging because of the inherently symmetric physics of wave phenomena. On page 516 of this issue, Fleury et al. ( 1 ), taking inspiration from a natural electromagnetic phenomenon, designed and demonstrated an engineered structure that allows one-way transmission of sound waves.

Upward Terrestrial Gamma‐Ray Flashes (TGFs) are mainly produced during the upward propagating negative leaders inside thunderclouds. The exact source position of TGFs, which is crucial to understanding TGF source properties, is still unclear. The link between positive energetic in‐cloud pulses (+EIPs) and TGFs provides us with a potential target to aim at. In this study, the low‐frequency radio emissions of 75 +EIPs are analyzed to retrieve the source altitudes with an improved ray theory model. Furthermore, the meteorology contexts of +EIPs derived from the ground‐based weather radars and satellite‐based infrared cloud top temperature measurements are investigated. +EIPs are produced at 8.8–13.7 km, with an average of 11.3 km inside thunderclouds, and at an average of ∼2.5 km below cloud tops. These altitudes indicate that a total number of 1.7 × 1016 to 2.6 × 1018 gamma ray photons with energy greater than 1 MeV are required for an EIP‐TGF to be measured by spaceborne detectors. Plain Language Summary Terrestrial gamma‐ray flashes (TGFs) are high‐energy photon emissions generated during thunderstorms and related to the initial development of intra‐cloud discharges. How TGFs are produced inside thunderclouds is still an open question, and one crucial issue is where TGFs are produced. Till now, it has been challenging to obtain the TGF positions directly. Recently, a distinct type of high‐peak current events, which are named energetic in‐cloud pulses (EIPs), are found to be closely linked to TGFs. The radio emissions of EIPs can be measured by ground‐based radio sensors deployed hundreds of kilometers away from the source. In this study, a ray theory model is improved to retrieve the signature of very low‐frequency/low‐frequency radio signals of EIPs propagating in the Earth‐ionosphere waveguide to obtain the source altitudes of EIPs. A total of 75 EIPs were found to be produced at 8.8–13.7 km, with an average of 11.3 km inside thunderclouds, and at an average of ∼2.5 km below cloud tops. With the source position information and combining the previously reported method to estimate the total number of source electrons, we suggest a total number of about 1017 is needed for TGFs being detected by spaceborne gamma ray detectors. Key Points An improved ray theory model is developed to retrieve energetic in‐cloud pulse (EIP) positions Seventy‐five +EIPs are located at 8.8–13.7 km, with a mean altitude of 11.3 km A mean gap of ∼2.5 km between EIPs and cloud tops was estimated

Acoustic tweezers use ultrasound for contact-free manipulation of particles from millimeter to sub-micrometer scale. Particle trapping is usually associated with either radiation forces or acoustic streaming fields. Acoustic tweezers based on single-beam focused acoustic vortices have attracted considerable attention due to their selective trapping capability, but have proven difficult to use for three-dimensional (3D) trapping without a complex transducer array and significant constraints on the trapped particle properties. Here we demonstrate a 3D acoustic tweezer in fluids that uses a single transducer and combines the radiation force for trapping in two dimensions with the streaming force to provide levitation in the third dimension. The idea is demonstrated in both simulation and experiments operating at 500 kHz, and the achieved levitation force reaches three orders of magnitude larger than for previous 3D trapping. This hybrid acoustic tweezer that integrates acoustic streaming adds an additional twist to the approach and expands the range of particles that can be manipulated. Although acoustic and optical tweezers are widely used, it is challenging to create a 3D trap with a simple set-up. Here, acoustic vortex streaming is combined with radiation force to realise 3D trapping of particles in a fluid.