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.
A new short‐baseline VHF interferometer with 200 MHz bandwidth images lightning with significantly higher spatial resolution than our previous system with a 60 MHz maximum frequency. Needles (Hare et ...al., 2019, https://doi.org/10.1038/s41586‐019‐1086‐6), needle‐associated negative leaders, a nearby bidirectional leader, and their interactions were imaged during an intracloud lightning flash. Six recoil leaders traversing the positive leader channel definitively determined the leader path and verified that needles originate from positive channels. Two negative leaders initiated from the positive channel, either from or close to needles, when another active positive leader passed ~1 km away. While the needles develop continuously with a VHF source density decreasing from the front end to the back end, a bidirectional leader initiated nearby and had strong influence on subsequent needle dynamics. These rarely observed yet perhaps common lightning processes are analyzed comprehensively to give new insight into the origin and impact of needles on positive lightning leaders.
Key Points
Extending the bandwidth of a lightning VHF interferometer to 200 MHz enables dramatically improved imaging of fine lightning structures
First short‐baseline VHF imaging of needles, needle‐associated negative leaders, and a nearby bidirectional leader is reported
Needles move forward continuously with density decreasing backward, indicating no positive channel disconnection is responsible for needles
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, 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. 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.
Metasurfaces open up unprecedented potential for wave engineering using subwavelength sheets. However, a severe limitation of current acoustic metasurfaces is their poor reconfigurability to achieve ...distinct functions on demand. Here a programmable acoustic metasurface that contains an array of tunable subwavelength unit cells to break the limitation and realize versatile two‐dimensional wave manipulation functions is reported. Each unit cell of the metasurface is composed of a straight channel and five shunted Helmholtz resonators, whose effective mass can be tuned by a robust fluidic system. The phase and amplitude of acoustic waves transmitting through each unit cell can be modulated dynamically and continuously. Based on such mechanism, the metasurface is able to achieve versatile wave manipulation functions, by engineering the phase and amplitude of transmission waves in the subwavelength scale. Through acoustic field scanning experiments, multiple wave manipulation functions, including steering acoustic waves, engineering acoustic beams, and switching on/off acoustic energy flow by using one design of metasurface are visually demonstrated. This work extends the metasurface research and holds great potential for a wide range of applications including acoustic imaging, communication, levitation, and tweezers.
A programmable acoustic metasurface composed of fluid‐actuated tunable subwavelength unit cells is reported. All the unit cells can be dynamically and independently modulated through a robust fluidic system to achieve versatile functions, including steering acoustic waves, engineering acoustic beams, and switching on/off acoustic energy flow. The proposed metasurface offers a programmable, continuous, and efficient method for acoustic wave manipulation.
In this study, we show that robust and tunable acoustic asymmetric transmission can be achieved through gradient-index metasurfaces by harnessing judiciously tailored losses. We theoretically prove ...that the asymmetric wave behavior stems from loss-induced suppression of high order diffraction. We further experimentally demonstrate this novel phenomenon. Our findings could provide new routes to broaden applications for lossy acoustic metamaterials and metasurfaces.
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
Metasurfaces are a family of novel wavefront-shaping devices with planar profile and subwavelength thickness. Acoustic metasurfaces with ultralow profile yet extraordinary wave manipulating ...properties would be highly desirable for improving the performance of many acoustic wave-based applications. However, designing acoustic metasurfaces with similar functionality to their electromagnetic counterparts remains challenging with traditional metamaterial design approaches. Here we present a design and realization of an acoustic metasurface based on tapered labyrinthine metamaterials. The demonstrated metasurface can not only steer an acoustic beam as expected from the generalized Snell's law, but also exhibits various unique properties such as conversion from propagating wave to surface mode, extraordinary beam-steering and apparent negative refraction through higher-order diffraction. Such designer acoustic metasurfaces provide a new design methodology for acoustic signal modulation devices and may be useful for applications such as acoustic imaging, beam steering, ultrasound lens design and acoustic surface wave-based applications.
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.
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