Deep-subwavelength Thickness Research Articles

Sandwich meta-panel based on grooved corrugation for low-frequency sound absorption

In this work, a novel sandwich meta-panel (SMP) with grooved corrugations is proposed to efficiently attenuate low-frequency sound waves under deep subwavelength thickness (e.g., at 133 Hz). Most innovatively, SMP possesses tremendous mechanical characteristics (e.g., high bending stiffness) simultaneously, stemming from its corrugation core configuration. A theoretical prediction for sound absorption with an explicit expression of effective length is built, as well as a direct numerical simulation model. The theoretical and numerical results coincide well with each other, and demonstrate the SMPs' great capacity of manipulating low-frequency sound waves. Further, the SMPs can be tuned flexibly through altering perforation diameter, channel length and folding number, and multiple diverse unit cells can be coupled to get wide high-absorption bands (e.g., from 203 Hz to 249 Hz). The SMPs pave a new way for coiled-up space metastructures to engineering applications, with simple layout and perspicuous designing guidance.

Sound-absorption performance of a coupled square-neck embedded Helmholtz resonator

We propose a coupled square-neck embedded Helmholtz resonator (S-NEHR) with a deep-subwavelength thickness. By precisely designing each cell peak, the average sound–absorption coefficient of the nine coupled S-NEHR structure is greater than 0.9 at 300–410 Hz, and the thickness is only of the working wavelength. Adding a microperforated panel (MPP) in front of the coupled structure enables a smaller size and larger bandwidth (500–1000 Hz). The simulation and experimental results demonstrate that the S-NEHR structure has great application prospects in engineering noise control due to its extraordinary sound–absorption properties and compact structure.

Mechanically‐Reconfigurable Edge States in an Ultrathin Valley‐Hall Topological Metamaterial

AbstractBroadband topological metamaterials hold the key for designing the next generation of integrated photonic platforms and microwave devices given their protected back‐scattering‐free and unidirectional edge states, among other exotic properties. However, synthesizing such metamaterial has proven challenging. Here, a broadband bandgap (relative bandwidth of more than 43%) Valley‐Hall topological metamaterial with deep subwavelength thickness is proposed. The present topological metamaterial is composed of three layers printed circuit boards whose total thickness is 1.524 mm ≈ λ/100. The topological phase transition is achieved by introducing an asymmetry parameter δr. Three mechanically reconfigurable edge states can be obtained by varying interlayer displacement. Their robust transmission is demonstrated through two kinds of waveguide domain walls with cavities and disorders. Exploiting the proposed topological metamaterial, a six‐way power divider is constructed and measured as a proof‐of‐concept of the potential of the proposed technology for future electromagnetic devices.

Perfect acoustic absorption of Helmholtz resonators via tapered necks

A neck tapered Helmholtz resonator possessing brilliant sound absorption in low-frequency is designed and demonstrated in this work. The strategy offers an extra geometrical degree of freedom, which will benefit perfect absorption. The structure can not only achieve perfect-absorption at 132 Hz across a deep subwavelength thickness of λ/50, but also realize adjustable perfect-absorption from 132 to 236 Hz with the same external shape. The effects of acoustic impedance and structural parameters have been comprehensively discussed for illustrating the structural advantages of sound absorption in low-frequency. Our results can pave a way for designing high-efficiency acoustic devices for sound absorption and impedance engineering.

Scalable-Manufactured Metamaterials for Simultaneous Visible Transmission, Infrared Reflection, and Microwave Absorption.

Scalable manufacturing of metamaterials with multispectral manipulation capabilities remains highly challenging, which was generally circumvented by integrating several single-spectral metamaterials, potentially leading to complex processes, large thicknesses, and limited fabrication size. We experimentally demonstrate a standalone and scalable-manufactured multispectral metamaterial featuring simultaneous visible transmission, infrared reflection, and microwave absorption. The prepared multispectral metamaterial with an area of 255 cm2 exhibits a visible transmittance of 74.5% at wavelengths of 400-700 nm (the highest 80.2% at 510 nm), a thermal emissivity of 0.08 at the infrared (IR) wavelengths of 2.5-20 μm (the lowest 0.03 at 19.5 μm), and a microwave absorptance of 63.4% at frequencies of 8.2-12.4 GHz (the near-perfect 97.4% at 11.5 GHz) on average with a deep-subwavelength thickness of λ/47. The deep-subwavelength multispectral metamaterial consists of a submillimeter-thick polyethylene terephthalate dielectric spacer sandwiched by a patterned ultrathin metal and a metal mesh back-reflector with ultralow sheet resistances. Unlike the conventional optically transparent microwave absorbers made from indium tin oxides, the surface plasmonic modes can be excited within the submillimeter-thick multispectral metamaterial, bringing about the gap plasmon polaritons-induced microwave attenuation, together with the excellent visible transparency and high IR reflection/low IR emissivity. This work may inspire the designs and practical production of standalone multispectral metamaterials and benefit the protection against ubiquitous IR and microwave reconnaissance without impeding visual observation.

Acoustic performance of architected hybrid metamaterials for sound attenuation applications

Acoustic insulators in sound attenuation applications usually have a structure with a characteristic dimensions equivalent to the operating wavelength, posing significant challenges in low-frequency sound attenuation applications. Recent advancements in material science, advanced materials called acoustic metamaterials (AMMs) have paved the way for designing efficient noise insulation passive and active devices. This paper proposes heterogeneous integrated passive metamaterials for enhanced sound insulation in broad frequency bands. Six acoustic meta-unit cells (UCs) with deep subwavelength thickness were proposed and analyzed numerically for the acoustic band structure and transmission characteristics. Out of these six UCs, three high-performing UCs were chosen for further analysis. Finite element (FE) analysis results showed that the maximum transmission loss conforms with the computed band structures. Next, we proposed several heterogeneous meta-structures consisting of different combinations of chosen meta-unit cells. The FE analysis results showed that a broader band of sound absorptions can be achieved through the hybrid design of AMMs. Our research revealed that the suggested structures may be utilized to create metamaterials with a wider absorption band, making them suitable for a variety of applications.

Ultrasparse and omnidirectional acoustic ventilated meta-barrier

Sound barriers, aiming to block the noise propagation, have attracted substantial interest in diverse scenarios such as architectural design, transportation development, and aircraft/automotive engineering. However, conventional sound-proofing barriers in the bulky configuration simultaneously impede the free airflow, which is highly required in the applications calling for ventilation. Although the advances in acoustic metamaterials have opened new routes for designing air-permeable barriers, a well-ventilated and omnidirectional sound barrier with deep-subwavelength thickness remains challenging. Here, we theoretically propose and experimentally demonstrate an ultrasparse, ultrathin, and ultralight acoustic ventilated meta-barrier that is highly prohibitive to omnidirectional sound. The acoustic meta-barrier is designed by sparsely arranging the units composed of two central-symmetric cavities with the thickness about 0.12λ (λ is the sound wavelength) and the sparsity achieving up to 80%. A sound wave from all directions impinging to the meta-barrier is efficiently blocked, and the airflow is allowed to freely transport, maintaining the wind velocity ratio measured higher than 90%. Moreover, geometric parameter analysis is investigated to optimize and customize the meta-barrier according to the target frequency spectrum. The ultrasparse and omnidirectional acoustic ventilated meta-barrier in such a compact and open configuration would hold out promising solutions to sound insulation and offer an additional scheme to design sound-proofing yet air-permeable devices in various applications.

Metasurface‐Based Hierarchical Structures: Excellent Electromagnetic Wave Absorbers with Low‐Infrared Emissivity Layer

Derived from the increasing demand for radar and infrared camouflage integration, a two‐layer hierarchical structure that combines coating and a square patch‐based metasurface is designed. Under deep subwavelength thickness, the infrared stealth layer (IRSL) can markedly reinforce microwave absorption layer (MAL) performance. More than 90% of the electromagnetic wave energy in the 3.7–5.1 GHz and 12.8–18 GHz bands can be absorbed. The hierarchical structure expands the effective absorption bandwidth of the exposed coatings from 0 to 6.6 GHz, and the maximum reflection loss increases from −8 to −40 dB. Moreover, the emissivity in the midinfrared range is maintained at an extremely low value of 0.281. The delay of magnetic resonance is found in the simulation model, indicating that strong magnetic resonance is formed in coatings after covering the IRSL. Meanwhile, the absorption peaks of the coatings are controlled by low‐pass filter metasurfaces. These two factors contribute to the enhancement effect of IRSL on MAL and break the boundary between radar and infrared stealth. Herein, an original solution for tuning multispectral electromagnetic (EM) waves is provided and it has broad application prospects in advanced compatible camouflage material.

A compact multifunctional metastructure for Low-frequency broadband sound absorption and crash energy dissipation

Sound absorption structures, as an effective approach to reducing sound radiation, have always received extensive attention in the engineering of vibration and noise reduction. In this work, a novel compact multifunctional metastructure is proposed for both low-frequency broadband sound absorption and excellent crash energy dissipation. Cavity resonators with internally extended tubes (CRIET) with hexagonal honeycomb configuration were selected as unit components. An optimization method was proposed to against degradation from impedance coupling effect and contribute to flatter and higher sound absorption coefficient (SAC) curves. The as-designed metastructure has been demonstrated experimentally to have quasi-perfect broadband sound absorption in the target range of 600–1000 Hz with an average SAC above 0.9 and a deep-subwavelength thickness of 30 mm. Compression experiment showed that the CRIET metastructure with hexagonal honeycomb configuration had the same unique excellent load-bearing and crash energy dissipation performance as an intact honeycomb structure. This study provides inspirations and methods for the compact and multifunctional design of broadband sound absorption structures.

Topological design of lattice materials with application to underwater sound insulation

Lattice materials with low acoustic impedance can be treated as acoustic soft boundaries compared to water and have potential application to underwater sound insulation. However, limited by the intuitive design method, only a few simple topologies have been reported, leaving a huge design space and potential performance of lattice materials untapped. This paper shows that lattice materials with low acoustic impedance can be systematically designed using topology optimization combined with the homogenization method. Aside from the material occupation, the azimuth of lattice material as an additional design variable is added to the optimization formulation. By imposing different effective modulus constraints, the results show that two types of bi-mode lattice materials can achieve extremely low acoustic impedance alongside an optimal azimuth. The first type of bi-mode lattice material possesses strong anisotropy and a positive Poisson’s ratio, whereas the second has weak anisotropy and a negative Poisson’s ratio. The optimal azimuth for minimum acoustic impedance is dependent on the degree of anisotropy of bi-mode materials. The minimum value of acoustic impedance is associated not only with the degree of anisotropy but also with feature parameters related to easy-deformation mode. Based on the topology optimized lattice materials, various acoustic metascreens with deep sub-wavelength thickness are proposed to shield underwater sound in a low and broad frequency range. Finally, two samples among the designed acoustic metascreens are fabricated and tested in a water-filled tube. Despite that the thickness of both metascreens is only 17 mm, the experimental results show that the sound transmission is reduced averagely by over 17 dB from 500 Hz to 2000 Hz.

Characteristics of high-contrast gratings based spoof surface plasmon polaritons and the application in microwave all-dielectric metamaterial absorbers

High-contrast gratings (HCGs) based spoof surface plasmon polaritons (SSPPs), as a new kind of SSPPs, have been proposed recently. Here, we study their particular mechanism and characteristics by comparing them with traditional metal-type SSPPs. The different fundamental dielectric resonator modes decided by the height–length ratio R of the dielectric element are found to be the key factor of the mechanism, leading to new characteristics and providing more working modes. In low R mode, HCG-based SSPPs can work under deep-subwavelength thickness and can be easily excited by free space wave and, thus, are considered to be applied to all-dielectric metamaterial absorbers (ADMMAs). Simulations and analysis confirm the microwave absorption effect of HCG-based SSPPs as a new mechanism of ADMMAs. Based on this, a carbon black based ADMMA is designed and the simulation and experimental results verify the important role of HCG-based SSPPs in broadband absorption. It is expected that this work can give rise to further and wider studies on the characteristics and applications of HCG-based SSPPs.

Perfect low-frequency sound absorption of rough neck embedded Helmholtz resonators.

In this paper, an acoustic metamaterial, composed of rough neck embedded Helmholtz resonators, is proposed to achieve perfect sound absorption in the low-frequency range. The wall shape of the embedded neck in Helmholtz resonators can be adjusted to improve the low-frequency sound absorption performance of acoustic metamaterials. As a concern, a full-rough neck embedded Helmholtz resonator (FR-NEHR) is designed, which achieves perfect sound absorption (α>0.999) with a deep subwavelength thickness ( λ/44) at 150 Hz. A theoretical model is developed to predict the performance of the FR-NEHR, which is validated against the experimental measurement and numerical simulation. The results show that for the rough embedded neck, when the axial and circumferential roughness of the neck exist, the sound energy dissipation increases not only in the neck but also in the air cavity. As a result, the acoustic absorption peak value of the FR-NEHR increases 20.2%, and the peak position shifts 20.2% to a lower frequency. This work extends Maa's 50-year-old sound absorption theory from smooth channels to full-rough channels, further developing the traditional channel sound absorption theory. It provides useful guidance for the structural design of broadband low-frequency sound-absorbing metamaterials.

Perfect sound absorption of Helmholtz resonators with embedded channels in petal shape

The acoustic metamaterial in the form of a petal-shaped channel embedded Helmholtz resonator (P-CEHR) is proposed for perfect sound absorption. According to theoretical predictions, numerical simulations, and experiments, the P-CEHR achieves perfect low-frequency (e.g., 200 Hz) sound absorption across a deep subwavelength thickness (e.g., 1/34 of the corresponding acoustic wavelength). Compared with the circular-shaped channel embedded Helmholtz resonator, the sound absorption peak and bandwidth of P-CEHR are significantly improved (e.g., increased by 20.9% and 60.0%, respectively) under fixed overall dimensions. Physically, the introduction of the petal shape changes the fluid dynamic characteristics of the channel, resulting in the periodic distribution of particle velocity along the circumferential direction and the expansion of the area of the viscous boundary layer. By adjusting the morphology of the embedded channel, the tortuosity ratio and the relative static flow resistance of the channel can be regulated appropriately, so that the resonator can meet the acoustic impedance matching condition and achieve excellent sound absorption performance. This work provides a method for improving the performance of acoustic absorption metamaterials with built-in air channels and has guiding significance for the control of low-frequency noise.

All-optical polarization and amplitude modulation of second-harmonic generation in atomically thin semiconductors

Second-harmonic generation is of paramount importance in several fields of science and technology, including frequency conversion, self-referencing of frequency combs, nonlinear spectroscopy and pulse characterization. Advanced functionalities are enabled by modulation of the harmonic generation efficiency, which can be achieved with electrical or all-optical triggers. Electrical control of the harmonic generation efficiency offers large modulation depth at the cost of low switching speed, by contrast to all-optical nonlinear devices, which provide high speed and low modulation depth. Here we demonstrate all-optical modulation of second-harmonic generation in MoS2 with a modulation depth of close to 100% and speed limited only by the fundamental pulse duration. This result arises from a combination of D3h crystal symmetry and the deep subwavelength thickness of the sample, it can therefore be extended to the whole family of transition metal dichalcogenides to provide great flexibility in the design of advanced nonlinear optical devices such as high-speed integrated frequency converters, broadband autocorrelators for ultrashort pulse characterization, and tunable nanoscale holograms.

Broadband low-frequency sound absorbing metastructures based on impedance matching coiled-up cavity

We report a kind of sound absorbing metastructure based on micro-perforated panel (MPP) and impedance matching coiled-up cavity (IMCC), which can broaden the low-frequency working bandwidth tremendously using single unit (e.g., 3.13 times compared to conventional designs) within a deep sub-wavelength thickness (e.g., ∼λ/20 at 322 Hz). Physically, impedance matching stemming from the dramatic cross-sectional area contrast of coiled-up channels reduces the acoustic reactance variation with respect to frequency, leading to broader low-frequency bandwidth. Theoretical predictions of sound absorption coefficients of the metastructure coincide well with the numerical and experimental results. The band broadening characteristics of the proposed IMCC are confirmed in relatively wide range of MPP parameters and partition panel positions. This work contributes to expanding the designing degrees of freedom for coiled-up metastructures to manipulate low-frequency waves in broadband.

Ultrathin High Quality‐Factor Planar Absorbers/Emitters Based on Uniaxial/Biaxial Anisotropic van der Waals Polar Crystals

AbstractTailoring the absorption/emission through nanostructures from broadband to narrowband has attracted increasing attention due to the merits of high efficiency and compactness. However, most of the reported narrowband emitters require either complex fabrication process or thick coating of the suitable materials (~mm). In this paper, by exciting Berreman mode through a combination of Au mirrors and ultra‐thin layers of low‐loss uniaxial/biaxial anisotropic 2D van der Waals polar crystal hBN/α‐MoO3, a narrowband absorption/emission peak is realized in the mid‐infrared region. The measured quality‐factor of Berreman mode for hBN/Au and α‐MoO3/Au structure can reach up to 134 and 164, respectively. The deep subwavelength thickness (~nm) and bilayer architecture simplify the fabrication process. Furthermore, taking advantage of the in‐plane birefringence originating from α‐MoO3, the polarization of the absorbed/emitted radiation can be controlled through this planar configuration. Such ultrathin narrowband absorbers/emitters provide new opportunities for the advancements in thermal sources, infrared sensors, and thermophotovoltaic power generation systems.

Ultra-thin and broadband low-frequency underwater acoustic meta-absorber

Acoustic metamaterials with deep-subwavelength thickness have aroused increasing interests for potential applications in low-frequency sound and vibration control. Most reported metamaterials, however, are for airborne sound, with fewer for low-frequency waterborne sound absorption because of water's much longer wavelength, weaker dissipation and closer impedance to solids. Current underwater sound absorption (SA) approaches merely work at broadband high frequencies (typically above 2 kHz) or narrowband low frequencies (by introducing discrete narrowband spring-mass local resonators (LRs)). Herein, an ultra-thin meta-absorber is proposed to achieve broadband low-frequency underwater SA via inserting thin and thickness-graded circular-elastic-plate scatterers (CPSs) into an elastomer matrix. Capitalizing on the thickness gradient among the CPSs and a backing plate behind the elastomer, the proposed design entails continuous broadband LRs, enriches the content of both local and coupled resonance modes inside the meta-absorber unit and enhances the coupling among them, thus enabling high and quasi-perfect SA at multiple frequencies and broad low-frequency range with a deep subwavelength thickness. Notably, quasi-perfect SA (>0.97) is realized at 415 Hz with an absorber whose thickness is 1.7% of the sound wavelength. An optimized design yields excellent sound absorption (>0.9, 0.952 on average) in the low frequency range from 500 to 2000 Hz. Such broadband low-frequency SA is confirmed by experiments. This research offers a novel and effective solution to achieve broadband low-frequency underwater SA, which may open up a new avenue to broadband low-frequency sound control using sub-wavelength structures.

Sound absorption metasurface with symmetrical coiled spaces and micro slit of variable depth

An acoustic metasurface (AMS) is proposed comprising a panel containing a micro slit of variable depth coupled to a thin cavity of symmetrical coiled spaces. Theoretical and numerical analyses are used to demonstrate that this metasurface has an effective low frequency sound absorption capability (i.e., between 100 Hz and 600 Hz) with a deep-subwavelength thickness. The mechanism underlying the sound absorption is explored by the different ways in which energy is dissipated in the absorber. It was observed that the dissipation of viscous energy takes place in the narrow region of the micro slit and represents 99.7% of the total energy dissipated while the thermal dissipation is 0.3% and occurs in the inner coiled spaces. Furthermore sound absorption of the AMS was investigated experimentally in an impedance tube for normal incidence. The results of the experimental work show that, by varying the depth of micro slit simultaneously with increasing the number of symmetrical coiled spaces, the peak frequency of the absorption can be shifted to a lower frequency without changing the total absorber thickness. A control of 93% of the sound energy, with a relative bandwidth of 50.4% in the frequency range of interest, was acquired with the proposed absorber presenting a thickness of λ/48. Furthermore, a coupled system was also evaluated experimentally which revealed the broadband absorption to be consistent with the theoretical model.

Ultrathin Composite Metasurface for Absorbing Subkilohertz Low-Frequency Underwater Sound

The crucial ability to suppress low-frequency waterborne sound efficiently has far-reaching implications for underwater noise-control engineering. Here, we propose an ultrathin composite metasurface in deep subwavelength thickness based on pentamode metamaterials (PMs) backed by rubber-metal resonators (RMRs). As a demonstration, average absorptance $87.8\mathrm{%}$ at subkilohertz frequencies ranging from 760 to 920 Hz (wavelength $\ensuremath{\lambda}$ from 30.9 to 25.5 times of absorber thickness) is achieved. In the composite system, PM layer functions as an energy converter, which converts incident waterborne sound into mechanical vibrations, suggesting that sound waves could be absorbed only if the converted vibrations are eliminated by the backing RMRs. Beyond as a converter, PM itself also exhibits an additional stringlike resonant mode originating from the fixed-boundary constraints, which gives rise to intense vibrations of RMRs that facilitate energy dissipation. Further investigation demonstrates the unique property of robust high-efficiency absorption for extended frequency region and wide oblique incidence angles. The proposed methodology paves the way for a class of low-frequency underwater absorber design platforms, allowing devices for versatile applications to be envisioned.

Structural designs, principles, and applications of thin-walled membrane and plate-type acoustic/elastic metamaterials

Many advanced physical properties can be realized by using well-designed acoustic metamaterial (AM) structures, which have significant application value in engineering. In particular, thin-walled membrane, plate, and shell-type structures with deep subwavelength thicknesses that can meet light weight requirements have attracted the attention of many researchers and engineers from various specialized fields. This Tutorial systematically introduced the structural design methods, acoustic/elastic wave attenuation and regulation principles, and engineering applications of thin-walled AMs for low-frequency sound insulation, sound absorption, and vibration reduction. In particular, the design methods and sound insulation/absorption properties of thin-walled AMs for realizing narrow-band and broadband sound attenuation were explored. Furthermore, the local resonance bandgap characteristics, quantitative extraction method for the bending wave bandgap, vibration suppression properties, and the design method for local resonance vibration dampers for elastic wave regulation by thin-walled elastic metamaterials were summarized successively. Moreover, other thin-walled AM applications, such as the wavefront steering performance of thin-walled acoustic/elastic metasurfaces, and the active thin-walled AMs, were introduced as well.