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The generalized Snell's law of reflection and anomalous reflections. (a) Schematics for the derivation of the angle of reflection. ϕ and ϕ + dϕ are the phases at the two cross points separated by dy along the y direction. θr represents the anomalous reflection angle induced by the discrete phase shifts. (b) Pressure field pattern for the gradient phase profile of . The black arrows refer to the theoretical value of the reflected angle.
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The introduction of metasurfaces has renewed the Snell's law and opened up new degrees of freedom to tailor the optical wavefront at will. Here, we theoretically demonstrate that the generalized Snell's law can be achieved for reflected acoustic waves based on ultrathin planar acoustic metasurfaces. The metasurfaces are constructed with eight units...
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... Space-coiling acoustic metamaterial is a material that utilizes a curved structure to propagate sound waves, causing the sound waves' propagation path to fold and elongate, resulting in a greater phase delay, as shown in figure 7. Because of its high refractive index characteristics and bent and folded structure, it can efficiently absorb low-frequency sound waves [105,106]. Additionally, space-coiling acoustic metamaterial can act as a local resonator, achieving the ability for multi-band sound absorption and isolation, and existing research results have confirmed its potential in sound absorption and bandgap frequency aspects. ...
Noise pollution is an important problem affecting people’s lives and work quality. In the current noise reduction materials, the porous sound absorption materials usually only haveagood sound absorption effect for medium and high -frequency sound waves, and the sound absorption effect for low -frequency sound waves is relatively weak. However, in recent years, the research on acoustic metamaterials has made a breakthrough which can effectively absorb or isolate low-frequency sound waves. Therefore, researchers propose to combine porous sound-absorbing materials with acoustic metamaterials to form a composite structure, that broadens the frequency range of noise reduction, so as to achieve the goal of full-frequency domain noise reduction. This paper first introduces the research progress of porous materials and acoustic metamaterials, and then introduces the research progress of composite structures that are made of porous materials and acoustic metamaterials. Finally, the application prospect of the composite field of porous sound-absorbing materials and acoustic metamaterials are summarized.
... The spatial convolute geometry is derived from the vortex generated by water flow and has been introduced into acoustics to extend the propagation path of sound wave, exciting important applications in sound manipulation. [14][15][16][17] Ongoing developments of acoustic metasurfaces have opened up new avenues to realize sound anomalous modulation. Metasurfaces with specially designed artificial microstructures can remarkably modulate sound wave. ...
Natural creatures and ancient cultures are full of potential sources to provide inspiration for applied sciences. Inspired by the fractal geometry in nature and the fretwork frame in ancient culture, here we design the acoustic metasurface to realize sound anomalous modulation, which manifests itself as an incident-dependent propagation behavior: sound wave propagating in the forward direction is allowed to transmit with high efficiency while in the backward direction is obviously suppressed. We quantitatively investigate the dependences of asymmetric transmission on the propagation direction, incident angle and operating frequency by calculating sound transmittance and energy contrast. This compact fractal fretwork metasurface for acoustic anomalous modulation would promote the development of integrated acoustic devices and expand versatile applications in acoustic communication and information encryption.
... So far, researchers have achieved some sound field modulation works through the use of various structural designs such as Helmholtz resonators, coiled channels, mazes, and cavities [7][8][9][10][11][12][13]. Also, the researchers have utilized combinations of different materials, such as a combination of water and silicone rubber or polyurethane composites, to achieve the goal of reflective sound field modulation [14,15], other researchers use bottom-up inversion optimization algorithms to design metasurfaces [16][17][18][19]. While these phase mutation methods can achieve acoustic field modulation [20][21][22][23][24][25], it is noteworthy that all of the aforementioned methods have been studied based on the forward model of the sound field and finite element simulation. This research process requires a substantial amount of time from the researchers. ...
The use of tunable metasurface technology to realize the underwater tracking function of submarines, which is one of the hotspots and difficulties in submarine design. The structure-to-sound-field metasurface design approach is a highly iterative process based on trial and error. The process is cumbersome and inefficient. Therefore, an inverse design method was proposed based on parallel deep neural networks. The method took the global and local target sound field feature information as input and the metasurface physical structure parameters as output. The deep neural network was trained using a kernel loss function based on a radial basis kernel function, which established an inverse mapping relationship between the desired sound field to the metasurface physical structure parameters. Finally, the sound field intensity modulation at a localized target range was achieved. The results indicated that within the regulated target range, this method achieved an average prediction error of less than 5 dB for 92.9% of the sample data.
... There are many classic books [40,41,49,50] and reviews on these methods [23,51]. Theoretical methods related to the band gap and the response of acoustic metamaterials mainly include acoustic wave [1], crystal lattice [52,53], energy band theory [54], effective medium theory, and general Snell's law [55,56]. ...
Acoustic metamaterials (AMs) composed of periodic artificial structures have extraordinary sound wave manipulation capabilities compared with traditional acoustic materials, and they have attracted widespread research attention. The sound insulation performance of thin-walled structures commonly used in engineering applications with restricted space, for example, vehicles' body structures, and the latest studies on the sound insulation of thin-walled metamaterial structures , are comprehensively discussed in this paper. First, the definition and math law of sound insulation are introduced, alongside the primary methods of sound insulation testing of specimens. Secondly, the main sound insulation acoustic metamaterial structures are summarized and classified , including membrane-type, plate-type, and smart-material-type sound insulation metamateri-als, boundaries, and temperature effects, as well as the sound insulation research on composite structures combined with metamaterial structures. Finally, the research status, challenges, and trends of sound insulation metamaterial structures are summarized. It was found that combining the advantages of metamaterial and various composite panel structures with optimization methods considering lightweight and proper wide frequency band single evaluator has the potential to improve the sound insulation performance of composite metamaterials in the full frequency range. Relative review results provide a comprehensive reference for the sound insulation metamaterial design and application.
... Metasurfaces [1][2][3][4][5] are artificially constructed materials with subwavelength thicknesses in the direction of wave propagation, exhibiting an excellent ability to manipulate wavefronts, as well as having a more compact physical space and simplicity compared to conventional metamaterials [6][7][8][9][10]. Metasurfaces have been widely used in various fields, such as medical imaging, holographic imaging, and signal processing, following studies on their extraordinary beam modulation ability in the fields of electromagnetics [11][12][13] and acoustics [14][15][16][17][18][19]. Recently, the research on metasurfaces has been expanded to the field of elastic waves [20][21][22][23][24][25][26][27][28]. ...
An active elastic metasurface has more flexibility than a passively modulated elastic metasurface, owing to the manipulation of the phase gradient that can be realized without changing the geometrical configuration. In this study, a negative proportional feedback control system was employed to provide positive active control stiffness for adaptive unit cells, with the aim of achieving the active modulation of the phase gradient. The relationship between the control gain and the phase velocity of the flexural wave was derived, and the transfer coefficients and phase shifts of the flexural wave through the adaptive unit cells were resolved using the transfer matrix method. Finite element simulations for wave propagations in the adaptive unit cells were conducted, and they verified the analytic solutions. Based on this theoretical and numerical work, we designed active elastic metasurfaces with adaptive unit cells with sub-wavelength thicknesses according to the generalized Snell’s law. These metasurfaces show flexibility in achieving abnormal functions for transmitted waves, including negative refraction and wave focusing, and transforming guided waves at different operating frequencies by manipulating the control gain. Therefore, the proposed active metasurface has great potential in the fields of the tunable manipulation of elastic waves and the design of smart devices.
... Massive-data connectivity has driven the need for efficient, directed communications through beamforming arrays [1][2][3][4][5][6][7][8][9][10] . Delay elements are critical in any beamforming signal chain. ...
... Mobile devices, driving the expansion of ubiquitous communication, require compact, integrated beamformers. In any microwave 1-6 , acoustic 7,8 or optical 9,10 beam-steering system, there is a fundamental choice in architecture between using phased arrays 11 and timed arrays [12][13][14] . An array of phase shifters [15][16][17][18][19] occupies a small footprint and can theoretically have continuous phase tuning; however, it introduces frequency dependence to the spatial delay, limiting the desired beam pattern to a narrow bandwidth. ...
Massive-data connectivity has driven the need for efficient, directed communications through beamforming arrays1–10. Delay elements are critical in any beamforming signal chain. However, these elements impose fundamental limits on size, channel capacity, power efficiency and effective isotropic radiated power¹¹. Although passive phase shifters do not consume DC power, they suffer from narrow bandwidth, poor phase resolution and low power-handling capacity. They introduce a beam squint, in which different frequency components experience different time delays, blurring signals so that they cannot be resolved. This severely limits the data rate of the wireless link, that is, its channel capacity. Although true time delay (TTD) elements¹² solve this problem and service a broad bandwidth, they comprise wavelength-scale transmission lines, making them prohibitively area-inefficient for modern semiconductor processes. Here we address this long-standing problem by introducing a quasi-true time delay (Q-TTD) that miniaturizes TTD elements and breaks fundamental channel-capacity limits of these wireless links. We demonstrate this mechanism for a microwave device implemented in a complementary metal–oxide–semiconductor (CMOS) technology. Key to shrinking the footprint is a reflective-type phase-shifting structure with 3D variable TTD reflectors within a sub-wavelength footprint. This achieves ultra-broadband phase tuning by using them to vary the length of the waveguide’s path to ground. They produce a delay-to-area ratio that yields a substantially higher on-chip channel capacity compared with existing state-of-the-art methods. This component, when integrated in arrays, enables high-resolution imaging and low-squint beamforming for wideband communication, on-chip radar and other applications.
... They clarified that transmitted light and reflected light can propagate in any direction in their respective half space, and the propagation direction depends on the direction of the metasurface phase gradient, the numerical magnitude and the refractive index of the surrounding medium. Metasurfaces have been widely used in wavefront engineering [3][4][5][6], focusing or imaging elements [7][8][9][10][11], beam shaping [12], vortex beam generation [13][14][15][16], holographic technology [17][18][19], biosensing [20], electromagnetic concealment [21], achromatic focusing [22], electromagnetic wave modulation [23] and nonlinear interactions [24,25]. Multifocal lenses have been realized based on metasurface. ...
Metasurface is a new type of micro-optical element developed in recent years. It can intelligently modulate electromagnetic waves by adjusting the geometrical parameters and arrangement of dielectric structures. In this paper, a bifocal metalens based on modulation of propagation phase was designed for the potential application in displacement measurement. The phase of the bifocal lens is designed by the optical holography-like method., which is verified by the scalar diffraction theory. We designed a square aperture lens with a side length of 200μm to realize two focal spots with focal lengths of 900μm and 1100μm. The two focal spots aren’t on one optical axis. The polarization insensitive TiO2 cylinders are chosen as structure units. Four structures with different radius are selected to achieve the four phase steps. We fabricated the designed bifocal metalens using electron beam lithography and atomic layer deposition (ALD) techniques, and measured the light intensity in the areas near the two foci in the direction of the longitudinal axis. The differential signal was calculated, from which we obtained a linear interval. It demonstrates the ability of bifocal differential measurement to be applied to displacement measurement. Because the metasurfaces production process is semiconductor compatible, the bifocal lens is easy to integrate and can be used for miniaturized displacement measurements, micro-resonators, acceleration measurements, and so on.
... Özellikle son on beş yılda araştırmacılar akustik metamalzeme [21][22][23][24] adı verilen, doğada olmayan bu malzemelerin düşük frekansta [25,26], geniş bantta [27,28] ve ayarlanabilir ses yutum [29,30], hafif [31][32][33] ve çok fonksiyonlu [34][35][36][37] ses yalıtım [38,39] ve saçıcılık [40][41][42] özelliklerinin yanı sıra olağandışı yansıma [43,44], akustik görünmezlik [45][46][47] gibi sıra dışı özelliklere sahip olabileceğini de göstermiştir. Ayrıca hava geçirgen metamalzemeler [48][49][50][51][52] ses engelleme özelliklerine ek olarak hava akışını sağlayabildikleri için geleneksel gürültü bariyerlerine güçlü bir seçenek olarak karşımıza çıkmaktadır. ...
Mineral yünü ve delikli paneller gibi geleneksel ses yutucu malzemeler düşük frekans bölgesinde zayıf akustik performans göstermektedir. Buna karşın, akustik metayüzeyler yapısal özellikleri doğrultusunda istenilen frekans aralığında dalga boyu altı kalınlığında yüksek ses yutumu sağlayabilir. Bu çalışmada ses yutucu metayüzeyin geometrik parametrelerindeki değişimin akustik performansına etkilerini sunuyoruz. Metayüzey, dış katmanında delikli yüzey ve arkasında hacimsel sarmallı kanallardan oluşmaktadır. Delik sayısı, panel kalınlığı, delik çapı ve kanal derinliği parametrelerinin ses yutum performansına etkisi irdelenmiştir. Akustik analizler COMSOL Çoklu Fizik yazılımının Basınç Akustiği modülü ile gerçekleştirilmiştir. Teorik ve sayısal sonuçların uyumlu bir şekilde örtüştüğü görülmektedir. Tasarlanan metayüzey ince kesitli yapısı ile istenilen frekans aralığında yüksek ses yutumunu sağlayabilmektedir. Bu nedenle hacim akustiği ve gürültü kontrolü alanlarında geleneksel malzemelere seçenek olması beklenmektedir.
... To the aim of the present work, it is worth mentioning a specific class of acoustic metamaterials, the so-called phase-gradient-based metamaterials. While most acoustic metamaterials exploit local resonances to produce their effects, the phase-gradient approach is based on structures, or cells, decorating some domain boundary, that can induce several peculiar behaviors through the introduction of a tailored distribution of phase delays in the reflected or refracted acoustic field [18,19]. When a metamaterial device is built with a thickness significantly below the working wavelength, it is usually called a metasurface. ...
... There is, hence, a great interest in the research and development of metasurfaces. One of the most interesting designs that can be found in the literature presents metasurfaces engineered to show peculiar behaviors like arbitrarily-shaped virtual surfaces to modify the local reflection angle [18,20,21], or to obtain anomalous transmission for lens-like behaviors (see, e.g., Xia et al. [22], where a dual-layer metasurface is designed to achieve a one-way controlled transmission), or even to transform the propagation pattern from spherical to plane or surface waves [23,24]. Recent advances have shown the possibility of exploiting the Willis coupling phenomenon to gain full control over acoustic refraction and avoid scattering toward undesired directions [14,25]. ...
... However, only a piecewise approximation of the continuous Δϕ profile is achievable by combining the elementary unit cells in the building set, since the cells have a finite width and a quantization error is unavoidable. By appropriately combining the elementary units, the designer would be able to build a metasurface introducing the desired phase-delay distribution in the reflected acoustic field and hence obtain the corresponding local extraordinary reflection angle [18,20]. Figure 2 summarizes schematically the above-mentioned steps for the design of a phase-gradient-based metasurface. ...
Metamaterials and metasurfaces disclosed new degrees of freedom in controlling the acoustic field. Exploiting the generalized Snell law and the generalized law of reflection, the assembly of subwavelength unit cells is able to achieve extraordinary refraction and reflection by means of a controlled phase delay introduced in the field by the treated boundaries. The space-coiling design is one of the most powerful for cells in this metadevice class, providing effective low-thickness metasurfaces. However, space-coiling suffers from a narrow frequency operating range due to the intrinsic connection between the design operating wavelength and the characteristic dimensions of the metasurface. This work defines a procedure based on numerical optimization for designing space-coiling cells for modular acoustic metasurfaces, extending the frequency range in which the metasurface is effective. The set comprises eight different unit cells, each introducing a tailored phase shift in the reflected field that can be arranged to produce the desired acoustic effect. The broadband design is obtained by minimizing the dependency on the operating frequency of phase delay introduced by the cells, keeping the overall thickness below a quarter of the design wavelength. Results are shown for the benchmark problem of a metasurface modifying the reflection angle from a boundary.
... Efficient wave manipulation can now be achieved passively at the sub-wavelength scale by precisely processing a microstructure 3 . Innovative functions such as abnormal reflection 4 and refraction 5 , stealth 6 , focusing 7 , and acoustic holography 8 have been ingeniously brought into reality. So far, various methods for metasurface design, including the generalized Snell's law 9 , impedance matching techniques [10][11][12] , and diffractive metagrating theory 13,14 , have been reported and widely implemented. ...
Impedance theory has become a favorite method for metasurface design as it allows perfect control of wave properties. However, its functionality is strongly limited by the condition of strict continuity of normal power flow. In this paper, it is shown that acoustic impedance theory can be generalized under the integral equivalence principle without imposing the continuity of power flow. Equivalent non-local power flow transmission is instead realized through local design of metasurface unit cells that are characterized by a passive, asymmetric impedance matrix. Based on this strategy, a beam splitter loosely respecting local power flow is designed and demonstrated experimentally. It is concluded that arbitrary wave fields can be connected through arbitrarily shaped boundaries, i.e. transformed into one another. Generalized impedance metasurface theory is expected to extend the possible design of metasurfaces and the manipulation of acoustic waves.
