Broadband Subwavelength Research Articles

Experimental Realization of On-Chip Surface Acoustic Wave Metasurfaces at Sub-GHz.

Metasurfaces, consisting of subwavelength-thickness units with different wave responses, provide an innovative possible method to manipulate elastic and acoustic waves efficiently. The application of metasurfaces to manipulate on-chip surface acoustic wave (SAW) at sub-GHz frequencies requires further exploration since their wave functions are highly demanded in nanoelectromechanical systems (NEMS), sensing, communications, microfluid control and quantum processing. Here, the experimental realization of on-chip SAW metasurfaces is reported, consisting of gradient submicron niobium (Nb) rectangular pillars positioned on a 128°Y-cut lithium niobate (LiNbO3) substrate that operate at hundreds of megahertz. The proposed SAW metasurfaces are able to manipulate transmitted SAW wavefront functions by designing on-demand pillar's profile distributions. Broadband subwavelength focusing effects as the typical functions of SAW metasurfaces are experimentally demonstrated. This study opens a door for realizing on-chip SAW metasurfaces for diverse potential applications at micro- and nanoscale.

Deep subwavelength sound absorption by a buckling plate resonator

Conventional sound-absorbing materials face great difficulties in achieving deep subwavelength broadband sound absorption due to the causal constraint on the minimum thickness. A buckling plate resonator is proposed to alleviate the causal constraint on the thickness of sound-absorbing materials. The resonator consists of a back cavity sealed by an elastic circular thin plate. When the plate is subjected to a uniform in-plane compressive force up to its critical load, it buckles and behaves as a negative stiffness spring. The positive stiffness of the back cavity is counteracted by the negative stiffness of the buckling plate, resulting in resonance absorption at the deep subwavelength scale. The sound absorption performance of the buckling plate resonator under different in-plane loads was measured in an impedance tube. Quasi-perfect sound absorption was observed in a buckling plate resonator with a thickness less than 1% of the wavelength corresponding to the resonance frequency. This work provides a solution for the design of ultra-thin broadband sound absorbers.

Optimal design of subwavelength broadband acoustic porous composite metasurface based genetic algorithm

A 3D composite acoustic metasurface (CAM) is proposed to achieve broadband sound absorption in subwavelength thickness. The designed structure consists of a parallel configuration multi-component resonant structure (MRS) for the low frequency band and a metaporous layer (ML) with periodic array components for the high frequency band. Genetic algorithm is adopted to construct an optimal design method to obtain the appropriate parameters of the CAM. The optimized result is got accurately and quickly in this way and the acoustic properties satisfy the target design. In the low frequency range, the parallel configuration of different microperforated panel systems (MPPSs) broadens the resonant frequency band. The ML based on polyurethane (PU) foam consists of four subunits with a linear phase gradient in one period, which improves the absorption of uniform porous foam with the same thickness by converting the reflected wave into the surface wave. The sound energy in high frequency range is mostly dissipated in this way. The acoustic performance of CAM is predicted theoretically and demonstrated by numerical simulation and experiment. The sample with 50 mm achieves remarkable absorption, over 80 %, in the overall frequency range from 500 Hz to 3000 Hz. Even though the thickness of the sample is reduced to 30 mm, it still presents a better sound absorption than the PU foam with same thickness in the range from 500 Hz to 3000 Hz. No matter what the azimuthal angle of incident wave is, the 3D CAM shows quasi-perfect sound absorption which the PU foam cannot provide at the design frequency 2000 Hz in this study. This work provides a method to design broadband sound absorbers efficiently, which has good application value in cabin noise control.

An ultralow crosstalk and broadband subwavelength grating-assisted chiral mode converter by encircling exceptional points

Exceptional points (EPs) are degeneracies of two or more eigenstates and eigenvalues in non-Hermitian systems, promising applications in optoelectronics. In particular, chiral state conversion can be achieved by dynamic encircling an EP to enable backward-scattering light isolation and asymmetric mode switching. However, critical bottlenecks have plagued most mainstream EP-based chiral mode converters, since they mainly use the traditional dual-coupled waveguide systems for parametric tuning as the essential part of the chiral mode converter, which induce mode mismatch, and bandwidth-limited EP encircling path and, therefore, cause deficiencies in crosstalk and bandwidth. To overcome this challenge, we propose a chiral mode converter adding customized subwavelength gratings (SWGs) in dual-coupled waveguide systems to enhance parametric tuning. Indeed, the SWG structure decreases crosstalk and enhances bandwidth by using its refractive index control characteristics to mitigate mode mismatch and weaken the wavelength correlation of the EP encircling path. The designed device has expanded the available working band, demonstrating favorable performance in both the optical communication band (1.26–1.675 μm) and 2 μm (1.85–2.05 μm) band. At the same time, the crosstalk reduces to below −20 and −13 dB, respectively, superior to most of the previously reported devices. Furthermore, the transmission efficiency remains above 90% in the full operating bands, which is at the advanced level as the reported optimal performance of chiral mode converters. This study paves the way for developing efficient chiral transmission devices (such as optical switches, isolators, and logic gates), inspiring fascinating opportunities in future optical communication and topological quantum computing technologies.

Optical N-plasmon: topological hydrodynamic excitations in graphene from repulsive Hall viscosity

Edge states occurring in Chern and quantum spin-Hall phases are signatures of the topological electronic band structure in two-dimensional (2D) materials. Recently, a new topological electromagnetic phase of graphene characterized by the optical N-invariant was proposed. Optical N-invariant arises from repulsive Hall viscosity in hydrodynamic many-body electron systems, distinct from the Chern and Z 2 invariants. In this paper, we introduce the topologically protected edge excitation—optical N-plasmon of interacting many-body electron systems in the topological optical N-phase. These optical N-plasmons are signatures of the topological plasmonic band structure in 2D materials. We demonstrate that optical N-plasmons exhibit unique dispersion relations, stability against various boundary conditions, and edge profiles when compared with the topologically trivial edge magneto plasmons. Based on the optical N-plasmon, we design an ultra sub-wavelength broadband topological hydrodynamic circulator, which is a chiral quantum radio-frequency circuit component crucial for information routing and interfacing quantum–classical computing systems. Furthermore, we reveal that optical N-plasmons can be effectively tuned by the neighboring dielectric environment without breaking the topological properties. Our work provides a smoking gun signature of topological electromagnetic phases occurring in 2D materials arising from repulsive Hall viscosity.

Broadband subwavelength imaging of flexural elastic waves in flat phononic crystal lenses

Subwavelength imaging of elastic/acoustic waves using phononic crystals (PCs) is limited to a narrow frequency range via the two existing mechanisms that utilize either the intense Bragg scattering in the first phonon band or negative effective properties (left-handed material) in the second (or higher) phonon band. In the first phonon band, the imaging phenomenon can only exist at frequencies closer to the first Bragg band gap where the equal frequency contours (EFCs) are convex. Whereas, for the left-handed materials, the subwavelength imaging is restricted to a narrow frequency region where wave vectors in PC and background material are close to each other, which is essential for single-point image formation. In this work, we propose a PC lens for broadband subwavelength imaging of flexural waves in plates exploiting the second phonon band and the anisotropy of a PC lattice for the first time. Using a square lattice design with square-shaped EFCs, we enable the group velocity vector to always be perpendicular to the lens interface irrespective of the frequency and incidence angle; thus, resulting in a broadband imaging capability. We numerically and experimentally demonstrate subwavelength imaging using this concept over a significantly broadband frequency range.

Diffraction-limit focusing using a 60-nm-thick spiral slit.

We demonstrate a technique for diffraction-limit focusing, on the basis of a spatial truncation of incident light using spirally structured slit motifs. The spiral pattern leads to a global phase domain where the diffractive wave vectors are distributed in phase. We fabricate such a spiral pattern on a 60-nm-thick metallic film, capable of converting an orbital-angular-momentum beam to a non-helical high-resolution diffractive focusing beam, resulting in a high numerical aperture of 0.89 in air, and of up to 1.07 in an oil-immersion scenario. The topological complementarity between the incident beam and the slit motifs generates broadband subwavelength focusing. The idea can be extended to large-scale scenarios with larger constituents. The presented technique is more accessible to low-cost fabrications as compared with metasurface-based focusing elements.

Broadband subwavelength tunable valley edge states induced by fluid filling acoustic metastructure

Topological acoustic insulators demonstrate unusual characteristics in manipulating sound wave, which attract much attention from researchers. However, most of the recent researches are based on passive system, hampering their dispersion tunability. In this paper, a broadband subwavelength tunable fluid filling acoustic topological metastructure is studied. It is composed of perforated cells with tunable water height in the hole, which enables the dispersion of the edge state to be tuned. The inversion symmetry is broken by expanding and shrinking the adjacent holes in the unit cell. Thus, the valley Hall states with opposite Chern number form at the K point in the Brillouin zone. The edge states emerge at the boundary of the different valley Hall phases. The robustness of the edge states is verified by the straight and Z-shaped waveguide. Furthermore, the dispersion of the edge state can be altered continuously by raising and reducing the water height, giving rise to broadband variable topological states, which greatly expands the bandwidth from 40 Hz to 1033 Hz. This work offers a new method to control the topological states and shows great potential for practical application.

On the broadband vibration isolation performance of nonlocal total-internal-reflection metasurfaces

The concept of a nonlocal elastic metasurface has been recently proposed and experimentally demonstrated in Zhu et al. (2020). When implemented in the form of a total-internal-reflection (TIR) interface, the metasurface can act as an elastic wave barrier that is impenetrable to deep subwavelength waves over an exceptionally wide frequency band. The underlying physical mechanism capable of delivering this broadband subwavelength performance relies on an intentionally nonlocal design that leverages long-range connections between the units forming the fundamental supercell. This paper explores the design and application of a nonlocal TIR metasurface to achieve broadband passive vibration isolation in a structural assembly made of multiple dissimilar elastic waveguides. The specific structural system comprises shell, plate, and beam waveguides, and can be seen as a prototypical structure emulating mechanical assemblies of practical interest for many engineering applications. The study also reports the results of an experimental investigation that confirms the significant vibration isolation capabilities afforded by the embedded nonlocal TIR metasurface. These results are particularly remarkable because they show that the performance of the nonlocal metasurface is preserved when applied to a complex structural assembly and under non-ideal incidence conditions of the incoming wave, hence significantly extending the validity of the results presented in Zhu et al. (2020). Results also confirm that, under proper conditions, the original concept of a planar metasurface can be morphed into a curved interface while still preserving full wave control capabilities.

Customized broadband pentamode metamaterials by topology optimization

Pentamode metamaterials (PMs), a kind of metafluids composed of complex solid medium, have shown enormous potential for both elastic wave and underwater acoustic wave manipulation. However, due to the lack of thorough understanding of the formation mechanism, most reported artificial and empirical PMs share very similar topological features, thus depriving the possibility of obtaining rigorous combination of wave parameters that are required to deliver desirable and prescribed properties and functionalities. To tackle this challenge, with the assumption of C2v, C4v and C6v symmetries in both square and triangle lattices, we propose a unified inverse strategy to systematically design and explore a series of novel isotropic or anisotropic PM microstructures through bottom-up topology optimization. Optimized PM microstructures are designed to provide customized effective mass density, elastic modulus, anisotropy degree and pentamode features on demand. We demonstrate that most optimized microstructures possess broadband single-mode range of exclusive longitudinal waves; some even feature record-breaking relative single-mode bandwidths exceeding 150%. Upon shielding lights on the beneficial topological features of the broadband PMs, we extract the main topological features to form simplified PM configurations, i.e., multiple symmetric solid blocks with slender rods, which can induce the multiform multiple-order rotational vibrations or the integration of the low-order rotational vibrations and anisotropic local resonances for the broadband single-mode nature. At a higher design level, we establish a dedicated inverse-design strategy, under the function-macrostructure-microstructure paradigm, to conceive a novel broadband subwavelength underwater pentamode shielding device, which enables the conversion of propagating acoustic wave to the evanescent surface wave mode within the frequency range [1000 Hz, 4000 Hz]. Our study offers new possibilities for the practical realization of broadband PMs and underwater pentamode devices with rigorously tailored effective parameters, thus bring the PM-based technology within reach for practical applications.

Subwavelength Broadband Photonic Spin Hall Devices via Optical Slot Antennas

AbstractThe photonic spin Hall effect, wherein left‐ and right‐handed circularly polarized components can be separated during transportation after reflection or refraction by an optical interface through spin–orbit interaction, has gained increasing attention from the scientific and technological fields. Here, broadband subwavelength photonic spin Hall devices (PSHDs) are demonstrated on the basis of L‐shaped optical slot antennas. A basic PSHD comprises two L‐shaped slot antennas with a footprint size of 300 nm × 700 nm. The angle between the two radiation directions of the incident lights with different spins varies from 56° to 31° in the wavelength range of 750 to 850 nm. An array of antenna pairs is used to control radiation directions and improve the performance of the PSHDs. The proposed PSHDs are highly useful in integrated nanophotonic devices given their ultrasmall size, broadband response, and flexible design method.

Folded metaporous material for sub-wavelength and broadband perfect sound absorption

This Letter reports a folded metaporous surface optimized to achieve sub-wavelength and broadband perfect absorption. Its unit cell is composed of four different helicoidal cavities filled by porous media, which are structured and quasi-isotropic micro-lattices with a variable lattice constant. The effective thickness and intrinsic losses of each helicoidal cavity can be adjusted independently by varying their macro- and micro-structures, namely, the number of revolution of the folded structure and the lattice constant of the micro-lattice. An analytical model predicting the physical properties of this metaporous surface is developed. The macro- and micro-structures are then jointly optimized for sub-wavelength broadband perfect absorption. Finally, the system is 3D printed and experimentally tested. The experimental results are found to be in good agreement with the theory and show an almost perfect absorption over a frequency range out of reach for the homogeneous constitutive porous medium and the only helicoidal cavities.

Broadband subwavelength sensitivity to symmetry defects of disordered slabs

We observe a broadband and subwavelength sensitivity of the recently reported enhanced transmission through symmetric diffusive slabs [Ch\'eron et al., Phys. Rev. Lett. 122, 125501 (2019)]. From a perfectly symmetric distribution of scatterers on both sides of an opaque barrier within a waveguide, a shift as small as $\ensuremath{\lambda}/200$ significantly alters the multiple interferences giving rise to the transmission enhancement, and this subwavelength sensitivity appears on a very broad frequency range and without averaging. Analyses, from either full wave numerics or random matrix theory, show how the minimum shifting distance to suppress the transmission enhancement scales with the wavelength and the strength of the barrier.

Subwavelength broadband sound absorber based on a composite metasurface

Suppressing broadband low-frequency sound has great scientific and engineering significance. However, normal porous acoustic materials backed by a rigid wall cannot really play its deserved role on low-frequency sound absorption. Here, we demonstrate that an ultrathin sponge coating can achieve high-efficiency absorptions if backed by a metasurface with moderate surface impedance. Such a metasurface is constructed in a wide frequency range by integrating three types of coiled space resonators. By coupling an ultrathin sponge coating with the designed metasurface, a deep-subwavelength broadband absorber with high absorptivity ({>}80%) exceeding one octave from 185 Hz to 385 Hz (with wavelength lambda from 17.7 to 8.5 times of thickness of the absorber) has been demonstrated theoretically and experimentally. The construction mechanism is analyzed via coupled mode theory. The study provides a practical way in constructing broadband low-frequency sound absorber.

Design of subwavelength broadband hybrid sound absorption structure based on micro-perforated plate and coiled channels

In this paper, we propose a hybrid subwavelength broadband sound absorber based on micro perforated plate and multiple coiled channels. And the mechanism of low frequency broadband sound absorption of the hybrid sound absorber is analyzed in detail. Based on this, the theoretical analysis model and the finite element numerical analysis model are established, and the mutual verification of theoretical and numerical solutions is completed. The structure can theoretically achieve the low-frequency and high-efficiency sound absorption with an average absorption coefficient of 0.8 in a frequency band of 200–500 Hz when the overall thickness of the sound absorbing structure is 60 mm. At the same time when the overall thickness is 90 mm, quasi-perfect sound absorption with peaks up to 0.95 in a frequency range of 180–350 Hz is realized theoretically. The composite sound absorption structure has a certain application prospect in engineering low frequency noise in future.

Dielectric nanoantennas to manipulate solid-state light emission

Thanks to their enhanced and confined optical near-fields, broadband subwavelength resonators have the ability to enhance the spontaneous emission rate and brightness of solid-state emitters at room temperature. Over the last few years, high-index dielectrics have emerged as an alternative platform to plasmonic materials in order to design nanoresonators/optical nanoantennas with low ohmic losses. In particular, the excitation of electric and magnetic multipolar modes in dielectric resonators provides numerous degrees of freedom to manipulate the directivity and radiative decay rates of electric or magnetic quantum emitters. We review recent theoretical and experimental applications of dielectric nanoantennas to enhance or control decay rates of both electric and magnetic emitters but also to manipulate their radiation pattern through the coherent excitation of electric and magnetic modes; before discussing perspectives of this emerging field.

Multipole Resonance in Arrays of Diamond Dielectric: A Metamaterial Perfect Absorber in the Visible Regime.

Excellent characteristics and promising application prospects promote the rapid development of metamaterials. We have numerically proposed and demonstrated a novel subwavelength broadband metamaterial perfect absorber (BMPA) based on diamond dielectric arrays. The proposed absorber is composed of an ultra-thin two-layer structure covering the dielectric periodic array on a metal substrate. The materials of dielectric silicon (Si) and gold (Au) substrate are discussed in detail. In addition, different dielectric and refractory materials are also applied to achieve broadband absorption, which will make the proposed absorber greatly broaden the application field. A perfect absorption window (i.e., absorption rate exceeding 90%) can be obtained from near-ultraviolet to the visible range. The average absorption rate of 93.3% is achieved in the visible range. The results of multipole decomposition show that broadband absorption is mainly caused by electromagnetic dipole resonance and lattice resonance in a periodic array of Si. The proposed absorber can be extended freely by adjusting the structural parameters. The polarization-independent and incident angle insensitivity are proved. The proposed absorber may well be used in light energy acquisition, as well as for the scalability of optoelectronic and sensing devices.

High rectification in a broadband subwavelength acoustic device using liquid crystals

Acoustic diodes can be relevant to improve the audible comfort of indoor environments or to provide better ultrasound images. However, such diodes are usually based on nonlinear materials and microstructured and nanostructured asymmetries, making it difficult to produce them. We present in this article a high rectification acoustic device based on the liquid crystal 5CB, forming an escaped radial disclination kept in a conical frustum tube. Solving the wave equation numerically for this system, we observe that the lack of spatial inversion symmetry along the device's axis produces rectifications up to 1300% for a continuous frequency range from 20 Hz to 20 kHz. We performed a study varying the wave frequency, the tube geometry, and the liquid crystal orientation to identify values that produce the maximum acoustic rectification. Because these liquid crystals have been known for a long time and have well-known manipulation techniques, our results have practical significance in designing novel liquid crystal devices, such as acoustic rectification films.

Efficient and broadband subwavelength grating coupler for 3.7 μm mid-infrared silicon photonics integration.

A grating coupler is an essential building block for compact and flexible photonics integration. In order to meet the increasing demand of mid-infrared (MIR) integrated photonics for sensitive chemical/gas sensing, we report a silicon-on-insulator (SOI) based MIR subwavelength grating coupler (SWGC) operating in the 3.7 μm wavelength range. We provide the design guidelines of a uniform and apodized SWGC, followed by numerical simulations for design verification. We experimentally demonstrate both types of SWGC. The apodized SWGC enables high coupling efficiency of -6.477 dB/facet with 3 dB bandwidth of 199 nm, whereas the uniform SWGC shows larger 3dB bandwidth of 263.5 nm but slightly lower coupling efficiency of -7.371 dB/facet.

Achromatic Flat Subwavelength Grating Lens Over Whole Visible Bandwidths

One of the major challenges for emerging planar subwavelength micro lens is the significant chromatic behavior due to phase mismatch of subwavelength phase shifters. In this letter, the achromatic micro grating lens covering the whole visible wavelength is demonstrated by utilizing relatively low index contrast gratings. Based on the unique chromatic phase shift behavior of polymer nanostructure, we have designed and fabricated a broadband subwavelength achromatic microlens that can cover 250 nm of visible bandwidths (from 435 to 685 nm) with focal shift less than 5%. Our works also represent the first time to design, fabricate, and characterize micrograting lens ( $7~\mu$ m in size) promising for compact integrated nanophotonic devices on chip. In addition, there are more advantages of using a polymer-based microlens, such as easy fabrication on flexible substrate and potential applications, including imaging, spectroscopy, lithography, laser fabrication, and future integrated wearable devices.