Deep-subwavelength Thickness Research Articles

Design of a locally resonant system to reduce noise inside the payload fairing of a launcher during the lift-off

The high sound pressure exerted on the payload during the launch of space vehicles can jeopardize its structural integrity. Given space and weight restrictions, designing fairing noise protection systems is not easy and the number of alternatives is limited, especially for small launchers. This work proposes the design of an acoustic protection based on an simultaneous increase of insulation and absorption in the system tailored to the payload fairing of a space launcher. To this end, the use of a panel made of Helmholtz resonators is investigated. The panel presents a deep subwavelength thickness, as well as a highly efficient acoustic protection (90% of absorption and 13 dB of Transmission Loss) over the frequency range of interest. The panel is designed by considering a reciprocal and non mirror symmetric transmission problem and the acoustic incidence from both sides of the panel. A high Transmission Loss in the frequency range of interest is then obtained when considering an incidence coming from the outside of the payload fairing, whereas the quasi-perfect absorption of acoustic waves is observed in the case where the incidence comes from the inside of the fairing. The panels are subsequently prototyped and their performance is experimentally evaluated. Measurements are correlated and discussed in view of the theoretical and numerical predictions. This mitigation approach sets a new trajectory for innovative noise reduction in small-scale space launchers.

Suspended nanomembrane silicon photonic integrated circuits

Leveraging the low linear and nonlinear absorption loss of silicon at mid-infrared (mid-IR) wavelengths, silicon photonic integrated circuits (PICs) have attracted significant attention for mid-IR applications including optical sensing, spectroscopy, and nonlinear optics. However, mid-IR silicon PICs typically show moderate performance compared to state-of-the-art silicon photonic devices operating in the telecommunication band. Here, we proposed and demonstrated suspended nanomembrane silicon (SNS) PICs with light guiding within deep-subwavelength waveguide thickness for operation in the short wavelength mid-IR region. We demonstrated key building components, namely, grating couplers, waveguide arrays, micro-resonators, etc., exhibiting excellent performances in bandwidths, back reflections, quality factors, and fabrication tolerance. Moreover, we show that the proposed SNS PICs have high compatibility with the multi-project wafer foundry services. Our study provides an unprecedented platform for mid-IR integrated photonics and applications.

Highly confined epsilon-near-zero and surface phonon polaritons in SrTiO3 membranes

Recent theoretical studies have suggested that transition metal perovskite oxide membranes can enable surface phonon polaritons in the infrared range with low loss and much stronger subwavelength confinement than bulk crystals. Such modes, however, have not been experimentally observed so far. Here, using a combination of far-field Fourier-transform infrared (FTIR) spectroscopy and near-field synchrotron infrared nanospectroscopy (SINS) imaging, we study the phonon polaritons in a 100 nm thick freestanding crystalline membrane of SrTiO3 transferred on metallic and dielectric substrates. We observe a symmetric-antisymmetric mode splitting giving rise to epsilon-near-zero and Berreman modes as well as highly confined (by a factor of 10) propagating phonon polaritons, both of which result from the deep-subwavelength thickness of the membranes. Theoretical modeling based on the analytical finite-dipole model and numerical finite-difference methods fully corroborate the experimental results. Our work reveals the potential of oxide membranes as a promising platform for infrared photonics and polaritonics.

Low-frequency sound attenuation by coiled-up meta-liner with nonuniform cross sections under grazing flow

We report a kind of coiled-up meta-liners with nonuniform cross sections (CMNC), which can efficiently attenuate low-frequency sound waves under grazing flow with a deep subwavelength thickness (e.g., ∼λ/17 at 500 Hz). At a grazing flow Mach number of 0.26, the average transmission loss of the meta-liner is 12.6 dB at 500–1000 Hz, which is twice as much as that of a double-degree-of-freedom acoustic liner of the same size. Physically, the nonuniform cross-sectional distribution and significant cross-sectional area ratio enhances vortex shedding, thus resulting in severe acoustic energy dissipation. The excellent low-frequency acoustic attenuation performance of CMNC is investigated thoroughly with experimental, theoretical, and numerical methods. This work provides an avenue for low-frequency noise reduction in grazing flow scenarios (e.g., in a high bypass ratio turbofan engine).

Hybrid ultrathin metasurface for broadband sound absorption

To this day, achieving broadband low-frequency sound absorption remains a challenge even with the possibilities promised by the advent of metamaterials and metasurfaces, especially when size and structural restrictions exist. Solving this engineering challenge relies on stringent impedance matching and coupling of the multiple independent local resonators in metasurface absorbers. In this Letter, we present an innovative design approach to broaden the sound absorption bandwidth at low-frequency regime. A hybrid metasurface design is proposed where four coupled planar coiled resonators are also coupled to a well-designed thin planar cavity. This hybrid metasurface creates a broad sound absorption band (130–200 Hz) that is twice as wide as that of the traditional single layer metasurface utilizing four coiled cavities at a deep subwavelength thickness (∼λ/51). This design strategy opens routes toward engineering a class of high-performance thin metasurfaces for ultra-broadband sound absorption, while keeping the planar size unchanged.

Tunable acoustic metamaterial for broadband low-frequency sound absorption by continuous acoustic impedance manipulation

In this paper, a continuously tunable acoustic metamaterial is proposed for broadband low-frequency sound absorption. The metamaterial can be considered as a reconfigurable Helmholtz resonator with an upper perforated panel that can be continuously rotated to adjust the neck length. A theoretical model is developed to study the tunable sound absorption performance of the metamaterial, which demonstrates that it exhibits efficient low-frequency sound absorption ([Formula: see text]) in the frequency range (from 186 Hz to 319 Hz). Furthermore, perfect sound absorption ([Formula: see text]) is achieved at 197 Hz with a deep subwavelength thickness ([Formula: see text]). To validate this model, numerical simulations and experimental measurements are performed, and the physical mechanism of tunable sound absorption is explored. Results show that the acoustic impedance manipulation is realized by rotating the perforated panel to tune the acoustic mass of the Helmholtz resonator, which is the key to the continuous tunability of the proposed metamaterial. Based on the continuous tunability of each metamaterial unit, a tunable broadband sound absorber is designed to achieve a variety of broadband sound absorption performance with a single fabricated structure. This work paves the way for the manual realization of flexible tunability of sound absorbers and helps to achieve tunable absorption of complex noise according to variable acoustic application scenarios.

Broadband sound absorption based on impedance decoupling and modulation mechanisms.

In sound absorbers, acoustic resistance and reactance are usually coupled together and affect each other, which brings difficulties to impedance matching. An impedance decoupling method is proposed to make acoustic resistance and acoustic reactance vary independently. For the same thickness and perforation rate, acoustic reactance of the perforated panel with tube bundles (PPTBs) varies with the diameter of the tube, but acoustic resistance remains constant. Theoretical and simulated results show that a PPTB absorptive unit can exhibit resonance modes with varying damping states through impedance decoupling. It is found that through well-modulation, the PPTB unit in a slightly over-damped state cannot only maintain high sound absorption coefficients, but also expand the absorption peak bandwidth. Utilizing the mechanism of impedance decoupling, a broadband absorber is designed and evaluated by comprising the PPTB and microperforated panel (MPP). Measurement results indicate that it possesses an average absorption coefficient of 85% spanning more than a 3-octave bandwidth from 160 Hz to 1400 Hz with a deep sub-wavelength thickness. The impedance decoupling method helps to implement sound absorbers with highly efficient low-frequency broadband absorption.

Superior underwater sound-absorbing metasurface based on wave mode conversion and cavity-plate coupling resonance

Underwater sound-absorbing metasurfaces are critical for applications like underwater acoustic stealth and noise control. However, achieving both broadband and low-frequency absorption remains a significant challenge. In this work, we present a novel design of underwater sound-absorbing metasurface based on two mechanisms: wave mode conversion and cavity-plate coupling resonance. The optimized design achieves broadband (0.47–10 kHz) and low-frequency (down to sub-kilohertz, i.e., 0.47 kHz) absorption with a deep subwavelength thickness (50 mm, 1/63 wavelength at 0.47 kHz). We experimentally verify the design using various viscoelastic materials, and the results are highly consistent with simulations, demonstrating the excellent absorption performance of our design. Then, we conduct parametric sweeps to assess the contribution of each design parameter, providing further validation of the two underlying mechanisms. Our findings suggest that this novel design has great potential for engineering applications, facilitating the development of underwater sound-absorbing technologies.

Underwater metagratings for sub-kilohertz low frequency and broadband sound absorption

The anechoic coating capable of absorbing sound energy in low frequency and broadband is essential to conceal underwater vehicles. In this work, metagrating has been demonstrated as a tunable quasi-perfect absorber in low frequency, which is constructed with the single layer viscoelastic medium embedded with the periodic cylindrical cavities and the steel plate. Upon effective medium approximation and genetic algorithm optimization, such tunable absorbers could be put forward via deep subwavelength thicknesses. Broadband underwater absorption would be further approached in the multi-layer metagratings while the longitudinal waves convert into shear waves efficiently via the local resonance coupling and multiple scattering effects. Distinct from previous studies that limit the configuration of anechoic coatings to a given law and inefficiently alter numerous geometric parameters for optimal acoustic performance, the metagrating comprising multi-layer voids with random periods is adopted to precisely modulate the surface impedance in the target spectrum, leading to quasi-perfect absorption performance in broadband. Our results will contribute to designing lightweight underwater absorbers to improve the stealth performance of vehicles.

Broadband absorption and asymmetric reflection of flexural wave by deep-subwavelength lossy elastic metasurface

The elastic metasurface with phase discontinuities has received much attention for realizing diverse wave steering applications. To date, the elastic metasurfaces capable of realizing the multi-reflection-enhanced absorption of flexural waves at a deep subwavelength scale have rarely been reported. Here, we propose a novel concept of deep-subwavelength lossy elastic metasurface (DLEM) composed of gradient lossy force-moment resonators with a small amount of loss. Based on the diffraction theorem, we provide an accurate approach for predicting diffracted flexural wave behaviors considering the coupling of flexural and longitudinal diffraction modes in a plate and DLEM. The underlying mechanism of high-efficiency broadband absorption and asymmetric reflection of flexural wave is theoretically revealed, which stems from multi-reflection-enhanced absorption of the n=0 and n=-1 order diffractions. For the composite subunit of the DLEM implemented by a lead block atop damping silicone rubber block, a simplified two degree-of-freedom (2DOF) mass-spring-damping model and the corresponding accurate model are both theoretically derived to reveal the deep-subwavelength mechanism of controlling the reflected wave phase. The experiments of the broadband wave absorption and asymmetric reflection are further carried out to validate the performance of the designed DLEMs. The proposed methodology opens a new perspective for broadband vibration suppression and wave manipulation of lossy elastic metasurface with a deep-subwavelength thickness.

Investigation of Giant Nonlinearity in a Plasmonic Metasurface with Epsilon-Near-Zero Film

Plasmonic metamaterials can exhibit a variety of physical optical properties that offer extraordinary nonlinear conversion efficiency for ultra-compact nanodevice applications. Furthermore, the optical-rectification effect from the plasmonic nonlinear metasurfaces (NLMSs) can be used as a compact source of deep-subwavelength thickness to radiate broadband terahertz (THz) signals. Meanwhile, a novel dual-mode metasurface consisting of a split-ring resonator (SRR) array and an epsilon-near-zero (ENZ) layer was presented to boost the THz conversion efficiency further. In this paper, to explore the mechanism of THz generation from plasmonic NLMSs, the Maxwell-hydrodynamic multiphysics model is adopted to investigate complex linear and intrinsic nonlinear dynamics in plasmonics. We solve the multiphysics model using the finite-difference time-domain (FDTD) method, and the numerical results demonstrate the physical mechanism of the THz generation processes which cannot be observed in our previous experiments directly. The proposed method reveals a new approach for developing new types of high-conversion-efficiency nonlinear nanodevices.

Ultrathin acoustic metamaterial as super absorber for broadband low-frequency underwater sound

In this work, an ultrathin acoustic metamaterial formed by space-coiled water channels with a rubber coating is proposed for underwater sound absorption. The proposed metamaterial achieves perfect sound absorption (alpha > 0.99) at 181 Hz, which has a deep subwavelength thickness (lambda {/}162). The theoretical prediction is consistent with the numerical simulation, which demonstrate the broadband low-frequency sound absorption performance of the proposed super absorber. The introduction of rubber coating leads to a significant decrease of the effective sound speed in the water channel, resulting in the phenomenon of slow-sound propagation. From the perspective of numerical simulations and acoustic impedance analysis, it is proved that the rubber coating on the channel boundary causes slow-sound propagation with inherent dissipation, which is the key to meet the impedance matching condition and achieve perfect low-frequency sound absorption. Parametric studies are also carried out to investigate the effect of specific structural and material parameters on sound absorption. By tailoring key geometric parameters, an ultra-broadband underwater sound absorber is constructed, with a perfect absorption range of 365–900 Hz and a deep subwavelength thickness of 33 mm. This work paves a new way for designing underwater acoustic metamaterials and controlling underwater acoustic waves.

Crosstalk prohibition at the deep-subwavelength scale by epsilon-near-zero claddings

Abstract To prevent the crosstalk between adjacent waveguides in photonic integrated circuits, the minimum thickness of the cladding layers is around half a wavelength, which imposes a fundamental limitation to further integration and miniaturization of photonic circuits. Here, we reveal that epsilon-near-zero claddings, either isotropic or anisotropic, can break the above bottleneck by prohibiting the crosstalk for the modes with magnetic field polarized in the z direction at a deep-subwavelength thickness (e.g., λ 0/30, λ 0 is the free-space wavelength), therefore bestowing ultra-compact waveguide systems. The physical origin of this remarkable effect attributes to the divergent impedance of epsilon-near-zero materials far beyond those of dielectric or epsilon-negative claddings. Through full-wave simulations and microwave experiments, we have verified the effectiveness of the ultrathin epsilon-near-zero cladding in crosstalk prohibition. Our finding reveals the significant impact of impedance difference in waveguide designs and opens a promising route toward ultra-compact photonic chips.

Fabrication of core-shell Ni@C@PANI nanocomposite-based bionic coating with multi-bands EWM adaptability inspired by porous structure of pachliopta aristolochiae wings

Due to the Planck-Rozanov limit, stealth materials are prevented from realizing ultra-wideband absorption at deep sub-wavelength thickness. Herein, inspired by the absorption model of Pachliopta aristolochiae wings porous structure, a core-shell Ni@C@PANI nanometer powder-based bionic coating with multi-bands EWM adaptability was designed. The synergistic effect of excellent impedance matching, significant attenuation system, conduction loss, dipole polarization, interface polarization and good magnetic loss endow the hierarchical core-shell Ni@C@PANI nanocomposite with excellent EMW absorption properties. Additionally, the absorption of EMW in the low frequency bands mainly originates from the multiple reflection of EMW by the bionic porous structure. At high frequencies, in addition to resonance and reflection, the hole edges convert the linearly polarized electric field vector into a vortex form, thus dissipating the magnetic field energy. Consequently, a minimum reflection loss (RL) of −50.83 dB with a matching frequency of 16.5 GHz is achieved, and the EBA is as wide as 14.75 GHz (3.25–18 GHz), covering all C, X, Ku bands and part of S band. This discovery provides a way for using natural model to break the Plank-Rozanov limit to develop new-type bionic coating with multi-bands EMW adaptability.

A Compact Tunable Broadband Acoustic Metastructure with Continuous Gradient Spiral Channels

Sound‐absorbing structures, an effective measure to reduce vibration and noise, have always been the desirable research hotspot in scientific and engineering communities. However, the mutual restriction between ultrathin thickness and broadband sound absorption is the key issue that hinders its development. Herein, a compact tunable broadband acoustic metastructure with continuous gradient spiral channels is presented, which has the ultrathin thickness and can efficiently realize low‐frequency broadband sound absorption. Coiled‐up individual absorbers are introduced to constitute the acoustic metastructure. The continuous gradient spiral channels are formed by successively decreasing the effective spiral length of labyrinthine cavity to achieve efficient broadband sound absorption. It is showed in the results that the sound‐absorption metastructure coupled with eight individual absorbers maintains the excellent broadband sound‐absorption effect over 0.8 in the frequency range of 487–930 Hz and a deep subwavelength thickness of 37 mm. In addition, the highly efficient low‐frequency broadband sound absorption can be implemented in the target range of 480–990 Hz through optimizing the adjustable parameters such as the effective spiral length, cross‐section height, orifice radius, and the coupling number of single absorbers. Herein, inspirations and threads are provided for the compact tunable broadband sound‐absorption design of acoustic metastructures.

Deep subwavelength hybrid metamaterial for low-frequency underwater sound absorption by quasi-Helmholtz resonance

We proposed an acoustic metamaterial with deep subwavelength thickness for low-frequency underwater sound absorption. The proposed hybrid metamaterial has a perforated facesheet, a fluid-filled square honeycomb core with inside rubber coating, and a fixed backsheet. A theoretical model is established to predict the sound absorption performance of this perforated honeycomb hybrid metamaterial based on the sound absorption theory of the micro-perforated panel and electro-acoustic analogy. The theoretical model agrees well with our finite element simulation. Results suggest that perfect sound absorption (99.9%) of the metamaterial occurs at 375 Hz, at which the thickness of the metamaterial is only 1/80 of the underwater sound wavelength. According to the simulation, most of the sound energy is consumed by the rubber coating. It can be analyzed that the rubber coating replaces the fluid in the square honeycomb resonant cavity improving the acoustic capacitance and acoustic resistance and triggering a quasi-Helmholtz resonance. This acoustic metamaterial also exhibits a broadband underwater sound absorption performance by parallel design with different perforations, which has a promising potential in engineering applications.

Broadband ventilated sound insulation in a highly sparse acoustic meta-insulator array

Ventilated acoustic insulators can block sound whereas simultaneously allowing airflow, which has potential applications for noise control in ventilation structures. In this paper, we propose a highly sparse acoustic meta-insulator array consisting of multiple tunable units. The meta-insulator unit with a minimum deep subwavelength thickness of $0.064\ensuremath{\lambda}$ has openings on two sides and a joint capacity. We propose an analytical model to reveal its physical mechanism. We numerically and experimentally demonstrate the highly sparse acoustic meta-insulator array with 20 units capable of ventilated sound insulation with a broad bandwidth of over 1 octave (from approximately 880 to 1915 Hz), a high sparsity of about 70%, and sound attenuation of more than 17 dB suitable for both near-field and far-field sound sources. Our designs may offer alternative solutions to ventilation noise control issues on various practical occasions.

Broadband low-frequency sound absorbing metastructures composed of impedance matching coiled-up cavity and porous materials

In this work, we propose a sound absorbing metastructure based on impedance matching coiled-up cavity embedded with porous materials (IMCCP), which demonstrates tremendous broadband (e.g., 4.55 times compared to conventional designs) low-frequency absorption capability with deep subwavelength thickness (e.g., ∼λ/10.96 at 602 Hz), without considering multiple units coupling. By tuning the geometric parameters of the non-coupled unit, relative bandwidths ranging from 111.07 % to 142.55 % are achieved. In particular, two notable perfect absorption peaks are obtained in the low- to mid-frequency range by changing the number of coiled-up channels. Theoretical predictions, numerical simulations and experimental measurements are conducted to investigate the acoustic characteristics of IMCCPs, and the results agree well with each other. Physically, the bandwidth broadening stems from the impedance matching effect which increases the degree of smoothness of sound wave propagation inside coiled-up space with strong inherent dissipation. The bandwidth broadening effect is generally confirmed for diverse geometrical and transport parameters of IMCCPs. This work contributes to understanding the sound wave propagation inside porous coiled-up space and designing broadband porous metastructures.

Double‐Layer Structural Compatible Protective Material with Excellent Microwave Absorption and Heat Shielding Performance

AbstractIn view of the practical demands and mechanistic conflicts between microwave absorption and heat shielding compatibility, a double‐layer composite coating based on moth‐eyed hexagonal anti‐reflective structure is prepared by combining the advantages of metamaterials and coatings. The hexagonal periodic sequential configuration of the upper infrared shield layer (IRSL) can break the continuity of the electric field, induce a networked concentration of the surface current and internal electric field of the microwave absorption layer, thus optimizing the impedance matching and counteract the attenuated absorption performance due to the high dielectric IRSL. In addition, the emissivity of IRSL can be tuned artificially by changing the filler composition and particle size. The structural double‐layer composite coatings have a maximum effective absorption (reflection loss←10 dB) bandwidth of 7.4 GHz in the SKu bands at deep subwavelength thickness, while the infrared emissivity is 0.292. This work provides a novel approach for multispectral modulation in the field of advanced compatible protection systems.

Low-frequency waterborne sound insulation by an acoustic metascreen with a metal chiral structure

Low sound speed or low-density materials can be used as soft acoustic boundaries in water, potentially as low-frequency underwater sound insulation. This study uses a chiral structure to construct an acoustic metascreen with deep subwavelength thickness. The results show that the transmission coefficient of the metascreen decreases noticeably in the low-frequency range when adjusting the chiral structure. The displacement pattern and the effective acoustic impedance are used to investigate the sound insulation mechanism. Low sound speed and effective acoustic impedance are found in the anisotropic chiral structure, and an extensive range of quasi-longitudinal wave phase velocities from 116.70 m/s to 3935.48 m/s can be obtained by adjusting the structural parameters without changing the filling rate. Finally, the effect of the oblique incidence angle on the sound insulation of the metascreen is investigated.