Anisotropic Metamaterials Research Articles

Topology optimization of extreme mechanical metamaterials considering the anisotropy of additive manufactured parts

Abstract Additive manufacturing (AM) has the advantage of fabricating complex geometries designed by topology optimization. However, the layer-by-layer stacking of AM causes the anisotropic property of the manufactured parts, which is rarely considered in the topology optimization of metamaterials. Hence, this paper presents a new method for the topology optimization of metamaterials with anisotropic properties. First, the elastic moduli in different directions of anisotropic materials are introduced into the solid isotropic material with penalization interpolation function. Then, the effective elastic properties of anisotropic metamaterials are evaluated using the energy-based homogenization method, and the formulation of the topology optimization of anisotropic metamaterials is constructed, and it is iteratively solved by the method of moving asymptotes algorithm. Finally, several two-dimensional and three-dimensional numerical examples and a set of mechanical experiment are conducted to demonstrate the effectiveness of the proposed method.

Constitutive modelling and wave propagation through a class of anisotropic elastic metamaterials with local rotation

Elastic metamaterials are typically periodic materials possessing unit cells endowed with engineered architecture much smaller than the typical phenomenological length scale. The development of continuum models capable of accurately representing the effects of this aforementioned architecture is extremely challenging, and hence a sparsely developed area. This paper develops a novel 2-D continuum model capable of capturing the dynamic behaviour of a class of anisotropic elastic metamaterials with local rotational elements in the long wavelength limit. A constitutive relation incorporating these local rotational effects is proposed, and ratified using a representative discrete model using linear Hookean springs and identical rigid disks. The new continuum model is used to generate a dispersion relation for harmonic plane waves propagating in an arbitrary direction, which is subsequently compared to the dispersion behaviour of the original discrete model. The general behaviour of this continuum when subjected to 2-D planar harmonic wave propagation in the anisotropic medium is then analyzed, with specific attention given to the effect of material anisotropy and wave propagation direction. This work is the first of its kind to create a new continuum model of a class of anisotropic elastic metamaterials with local rotational effects.

Dielectric metamaterials with effective self-duality and full-polarization omnidirectional brewster effect

Conventional dielectric solid materials, both natural and artificial, lack electromagnetic self-duality and thus require additional coatings to achieve impedance matching with free space. Here, we present a class of dielectric metamaterials that are effectively self-dual and vacuum-like, thereby exhibiting full-polarization omnidirectional impedance matching as an unusual Brewster effect extended across all incident angles and polarizations. With both birefringence and reflection eliminated regardless of wavefront and polarization, such anisotropic metamaterials could establish the electromagnetic equivalence with “stretched free space” in transformation optics, as substantiated through full-wave simulations and microwave experiments. Our findings open a practical pathway for realizing unprecedented polarization-independence and omnidirectional impedance-matching characteristics in pure dielectric solids.

Topology-optimized thermal metamaterials based on inhomogeneous and anisotropic unit array

In this paper, inhomogeneous and anisotropic thermal metamaterials are designed to achieve the manipulation of heat flow (MHF) at multiple points. The thermal metamaterial unit is constructed from mixtures of two solid-state materials with different thermal conductivities. A variable density topology optimization method is employed to design the filling fractions of the mixed materials within the unit. The thermal concentrator and cloak are designed through the optimization of the topological functional units (TFUs). The thermal gradients of 14 and 0.05 times the uniform thermal gradient are achieved by the concentrator and the cloak, respectively. The effectiveness of the proposed design method is validated through the excellent agreement between numerical simulations and experimental results.

Hybrid one-dimensional photonic crystals containing anisotropic metamaterials: Angle-driven photonic band gaps and angle-driven Tamm plasmon polaritons

Hybrid one-dimensional photonic crystals containing anisotropic metamaterials: Angle-driven photonic band gaps and angle-driven Tamm plasmon polaritons

Nanophotonics with Plasmonic Nanorod Metamaterials

AbstractTaking advantage of their unique and tuneable electromagnetic properties, plasmonic metamaterials constitute a versatile platform for a variety of applications ranging from high‐resolution imaging, sensing and spontaneous emission engineering to ultrafast nonlinear optics and light polarization control. Here, recent advances in the design and applications of plasmonic metamaterials based on aligned metallic nanorod assemblies are reviewed, and the fast‐developing trends in their exploitation in nanophotonics are outlined. The fabrication of the nanorod‐based metamaterials is introduced and their linear and nonlinear optical properties are presented from both microscopic and phenomenological considerations. Multiscale structuring allowing precise engineering of the electromagnetic modes supported by the nanorod metamaterials, as well as nonlocal spatial dispersion effects and their impact on nanophotonic performance are discussed. To illustrate the developing field of practical applications of these metamaterials, some of their main application domains in sensing, photochemistry and nonlinear optics are overviewed and novel designs of anisotropic metamaterials directly derived from the nanorod architecture are introduced.

Geometrically Engineered Heating Arrays Enabled by Electric-Field-Driven 3D Printing for On-Demand Thermal Patterning

Arbitrary and high-precision thermal patterning has long been desired in the field of thermal functional materials. However, existing thermal patterning strategies have not been widely applied, either hampered by the difficulty in fabricating anisotropic metamaterials or limited by complex thermal manipulation. We propose an on-demand thermal patterning scheme that sandwiches geometrically engineered heating arrays between a substrate and an encapsulation layer to form composite structures and control the omnidirectional transfer of the heat flux generated by the heating arrays. These heating arrays are digitally assembled from multiple heater cells of varying widths and continuously printed using electric-field-driven 3D printing. A design strategy for thermal patterning with good uniformity within individual regions and high contrast between regions is proposed. The performance of the on-demand thermal patterning is verified via high-precision thermal printing. The proposed scheme provides a general and reproducible method for designing thermal functional materials, with potential applications in thermochromics, messaging, thermal camouflage, and illusions.

Lightweight Potential of Anisotropic Plate Lattice Metamaterials

Additive manufacturing enables the production of lattice structures, which have been proven to be a superior class of lightweight mechanical metamaterials whose specific stiffness can reach the theoretical limit of the upper Hashin–Shtrikman bound for isotropic cellular materials. To achieve isotropy, complex structures are required, which can be challenging in powder bed additive manufacturing, especially with regard to subsequent powder removal. The present study focuses on the Finite Element Method simulation of 2,5D anisotropic plate lattice metamaterials and the investigation of their lightweight potential. The intentional use of anisotropic structures allows the production of a cell architecture that is easily manufacturable via Laser Powder Bed Fusion (LPBF) while also enabling straightforward optimization for specific load cases. The work demonstrates that the considered anisotropic plate lattices exhibit high weight-specific stiffnesses, superior to those of honeycomb structures, and, simultaneously, a good de-powdering capability. A significant increase in stiffness and the associated surpassing of the upper Hashin–Shtrikman bound due to anisotropy is achievable by optimizing wall thicknesses depending on specific load cases. A stability analysis reveals that, in all lattice structures, plastic deformation is initiated before linear buckling occurs. An analysis of stress concentrations indicates that the introduction of radii at the plate intersections reduces stress peaks and simultaneously increases the weight-specific stiffnesses and thus the lightweight potential. Exemplary samples illustrate the feasibility of manufacturing the analyzed metamaterials within the LPBF process.

AI-aided design and multi-scale optimization of mechanical metastructures with controllable anisotropy

Anisotropic mechanical metamaterials with controllable properties are crucial for additive manufacturing design. However, manually regulating microstructural anisotropy remains challenging. This study introduces a method for artificial intelligence-aided design (AIAD) and mechanical metastructures optimization (MMO) to achieve extensive multi-scale structural enhancements. The approach involves compiling a comprehensive database of lattice materials with anisotropic characteristics. This is achieved by manipulating the central node position and rod diameter of a cubic-BCC microstructure. Homogenization theory then determines the elastic tensor of each microstructure. A 3D convolutional neural network (3D-CNN) maps the relationship between geometric properties and mechanical performance. An inverse design model based on a backpropagation neural network (NN) and parametric design acquires microstructures with desired elastic tensor attributes. Finally, a novel optimization approach for large-scale multiscale structures applies this method to control structural anisotropy. The resulting material distribution resembles a truss, significantly improving structural performance.

Design and optimization of Anisotropy–Inspired AlSi10Mg metamaterials with tailored mechanics and mass transport properties in tissue engineering

Metamaterials are widely used for achieving tailored multi–physical properties. However, it is challenging to satisfy multiple property requirements simultaneously. For example, the mechanical and mass transport properties of bionic scaffolds have to be compromised. In this work, 3D–printed metamaterials with adjustable topological features are proposed, and a topological optimization strategy for coordinating multiple properties is developed. Tradeoffs between the mechanics, energy absorption capacity, and mass transport properties are accomplished under given a porosity. Anisotropic metamaterials with permeabilities of 2.34 ∼ 3.44 × 10-7 m2 and elastic moduli of 1.37 ∼ 3.14 GPa were obtained, and the gradient scaffolds were further designed to realize adjustable local characteristics. The optimized design technique described here can serve as an effective way to design more complex multilayered scaffolds with structural characteristics and biomechanical properties similar to those of natural tissue, so as to achieve an unprecedented level of tailoring multi–physics properties in tissue engineering, especially in the design of gradient scaffolds. Our study also represents advances in property decoupling, individual customization and collaborative design of metamaterials, which can be generalized to fluid and heat transport fields.

Temporal interfaces in complex electromagnetic materials: an overview [Invited

Time-varying metamaterials are currently at the forefront of research, offering immense possibilities for intriguing wave manipulations. Temporal modulations of metamaterials have paved the way for unconventional realizations of magnetless nonreciprocity, wave amplification, frequency conversion, pulse shaping, and much more. Here, we overview the fundamentals and recent advancements of temporal interfaces in isotropic, anisotropic, and bianisotropic materials and metamaterials. Delving into the fundamentals of temporal scattering in media of different material classes, we draw insightful comparisons with phenomena observed at spatial interfaces. We specifically emphasize the potential of time-switched anisotropic and bianisotropic metamaterials in unlocking extraordinary temporal scattering phenomena. Furthermore, an overview of possible platforms to realize time-varying bianisotropic metamaterials is provided. Concluding with a glimpse into the future, we make a research outlook for time-varying anisotropic and bianisotropic metamaterials, highlighting their potential in obtaining exotic photonic time crystals and other dynamic electromagnetic structures.

Ultra-broadband TM-pass polarizer based on anisotropic metamaterials in lithium niobate on an insulator.

An ultra-broadband TM-pass polarizer is designed, fabricated, and experimentally demonstrated based on subwavelength grating (SWG) metamaterials in a lithium niobate on an insulator (LNOI) platform. According to our simulation, the designed device is predicted to work at a 220 nm wavelength range from 1460 to 1680 nm, covering the S-, C-, L-, U-bands of optical fiber communication. By depositing and subsequently etching a silicon nitride thin film atop the LNOI chip, the SWG structures are formed successfully by using complementary metal-oxide semiconductor (CMOS)-compatible fabrication processes. The measured results show a high polarization extinction ratio larger than 20 dB and a relatively low insertion loss below 2.5 dB over a 130 nm wavelength range from 1500 to 1630 nm, mainly limited by the operation bandwidth of our laser source.

Eliminating the Scattering of Thin Film Structures.

Rendering invisibility in the wide application scenarios has seen a surge in interest in recent years. Though various approaches have been proposed to realize concealments under different conditions, achieving polarization-independent invisibility for large objects remains a big challenge. Here, we propose to attain invisibility of a large dielectric slab with polarization constraints being totally lifted. This is accomplished by employing an antiscattering coating made of anisotropic metamaterials. We show that by tailoring the electric resonance of a triangular mushroom structure, antiphase electric dipole moment can be induced, resulting in an antipolarization response of the whole metamaterial coatings. By putting the proposed coatings on both sides of a large dielectric slab, a neutralization effect of the total polarization is observed, leading to the peculiar phenomenon of full-polarization invisibility. Our results are validated through full-wave simulations and experimental measurements. Remarkably, the intrinsic null-polarization property of the coating-slab-coating structure guarantees the invisibility feature of a large-scale bulk made by simply stacking the sandwiched composites, which facilitates the application of invisibility in practical scenarios such as the invisibility cloaks and the reflectionless antenna radomes.

Smart controllable wave dispersion in acoustic metamaterials using magnetorheological elastomers

The success in the flexible design of smart acoustic metamaterials is crucially contingent upon the degree of control over the parameters that uniquely define the spectrum of band structure and morphology of dispersion surfaces. In this work, we have studied the driving physical mechanisms, the control of which makes possible operating, in real-time, on the set of band gaps formed in 3D metamaterials based on magneto elastomers. In such acoustic structures, the stiffness of the medium in which unit cells are immersed, as well as the stiffness of the shells surrounding multi-particle cores depend on the induction, B(t), of an external magnetic field. The results obtained are systematized through the qualitative analysis of diversе scenarios for the evolution of the frequency characteristics of the metamaterial and summarized in the following complete physical picture of their dynamics. Variation of the stiffness of the medium/shells changes the wavelength of the shell's surface waves at characteristic frequencies of the core vibrations and, as a result, the level of coupling between the vibration modes of the flexible shells and cores. Consequently, with an increase/decrease in the stiffnesses of the shells/medium, the dispersion surfaces of the entire acoustic system shift up/down along the frequency axis with noticeably different ‘mobilities’ that reversibly lead either to the formation of the band gaps in initially dense frequency spectrum or to the transformation of the band gaps formed into pass bands. The tuning of the set of dispersion surfaces depending on the range of changes in the magnetic field induction can be carried out in dynamic, quasi-stationary, and over-critical regimes when some of the dispersion surfaces degenerate into planes. In anisotropic metamaterials, simultaneously with creating full band gaps, it is possible to create tunable directional band gaps with adjustable frequency and angular widths. The results obtained, within the framework of the idealized discrete mass-spring model, are in good agreement with the data presented in the available experimental works and can be useful in the design of acoustic systems with desirable properties.

Ultra-wideband chiroptical response by tri-layer anisotropic plasmonic metamaterial

The use of plasmonic chiral metamaterials for the control of circular polarization has the potential to replace conventional optical equipment for polarization-related applications. The ultra-broadband chiroptic response using plasmonic constituents is delivered by elaborate three-dimensional (3D) helical structures, nevertheless, their implementation is complicated, time-consuming, and poses a significant scaling difficulty at the nano level. Ultra-broadband response from planar constituents is particularly necessary as a means to circumvent the challenges of 3D metamaterials. Here we present a planar plasmonic structure composed of tri-layer anisotropic arrays constituting nanowires and cut-wires to generate dual overlapped chiral bands. Based on this tri-layer approach, we numerically realized ultra-broadband planar plasmonic metamaterials to function in the near- and mid-infrared regions with a bandwidth range of 1.38–3.07 µm and 4.00–8.10 µm, and maximum circular dichroism performance of 0.90 and 0.92 respectively. The structures are ultracompact, misalignments tolerant, and can be extended to additional spectral regions through structural engineering. The proposed metamaterial has the potential to be used in the creation of ultra-compact, high-performance devices for a wide variety of uses, such as those in the fields of optical communication, biological diagnosis, high-contrast polarization imaging, high-accuracy polarimetric measurements, and spectroscopy.

Thermal control of polarization of light with nonlocal plasmonic anisotropic metamaterials

Plasmonic nanostructures have demonstrated significant potential for engineering the intensity and polarization state of light with further opportunities to actively manipulate them by external stimulation using nonlinear effects. Plasmonic metamaterials composed of arrays of vertically oriented metallic nanorods have shown a dynamically tunable optical response based on the change of the electron temperature. The modulation of the optical properties is particularly pronounced in the epsilon-near-zero regime and can be further enhanced by nonlocal effects. Here, we experimentally study the optical properties of gold nanorod metamaterials with a strong nonlocal response under optically-induced heating, exploiting temperature dependence of the metal permittivity. Recovering the Stokes parameters of light transmitted through the metamaterial, we demonstrate the change in the polarization of the transmitted light by more than 20% for temperature changes under hundred degrees. Combined with a numerical analysis, this shows the possibility of controlling transmission and polarization state of light by using metamaterial-assisted thermal modulation.

Bifunctional switchable terahertz metamaterial in the same operating band based on VO2

A bifunctional device is designed to provide flexible control of broadband absorption and polarization conversion based on the insulator-to-metal phase transition material vanadium dioxide (VO2). When VO2 is metallic, the switchable metamaterial serves as a broadband absorber consisting of a VO2 square, the dielectric spacer, and VO2 film. According to the simulated results, the designed system can obtain more than 90% absorption from 2.62 THz to 6.65 THz with a relative bandwidth of 86.95%. The broadband absorption is unaffected by polarization attributed to the symmetry structure, and it still performs effectively up to incidence angle of 60°. When VO2 is insulating, a simple anisotropic metamaterial is used to construct a terahertz polarization converter. The numerical calculation shows a linear polarization conversion rate over 90% from 2.2 THz to 6.9 THz, with a corresponding relative bandwidth of 103.2%. The above structure features high efficiency, wide-angle and broad bandwidth advantages. In addition, the design exhibits dual function that can yield broadband absorption and polarization conversion in the same operating band, which opens up new avenues for the development of switchable devices for potential applications in stealth and communications.

Machine learning-driven optimization design of hydrogel-based negative hydration expansion metamaterials

Hydrogel-based negative hydration expansion (NHE) metamaterials are composite structures composed of responsive hydrogels and polymers, and their properties depend on their unique structures. In this paper, an optimization method based on the combination of the back-propagation neural network (BPNN) and the multi-population genetic algorithm (MPGA) is developed to rapidly design isotropic and anisotropic hydrogel-based metamaterials with specific NHE effects. In this method, several dimensionless design parameters are introduced to describe the structural characteristics of the metamaterial. The initial dataset is constructed based on the finite element method simulation results, and the mapping relationship between the design parameters and the equivalent linear strain is constructed by the BPNN, and the metamaterial with specific effect is efficiently optimized by combining the MPGA. The method is proved to have high accuracy and efficiency, and is applied to design many novel 2D and 3D metamaterials. The 3D metamaterial designed by this method has an ultra-large NHE ratio about 82 %. Compared with the topology optimization method, this method can significantly reduce the amount of computation, and can effectively avoid falling into the local optimum. The results show that the optimization method based on machine learning is an efficient means to design hydrogel-based metamaterials.

Temperature-insensitive and low-loss single-mode silicon waveguide crossing covering all optical communication bands enabled by curved anisotropic metamaterial

Abstract We propose two designs of low-loss and temperature-insensitive single-mode waveguide crossing on silicon-on-insulator (SOI) platform with 415-nm operation bandwidth covering all optical communication bands. Both designs are enabled by subwavelength grating (SWG) modeled as an anisotropic metamaterial. The initial design applies straight SWG as the lateral cladding of the waveguide crossing to minimize the refractive index contrast and reduce the insertion loss (IL), but needs a relatively long taper. An improved design is then proposed where the curved SWG is introduced to replace the straight SWG to decrease the taper length and improve the performance. The waveguide crossing with the improved design achieves a calculated maximum IL of 0.229 dB and maximum crosstalk of −35.6 dB over a 415-nm wavelength range from 1260 nm to 1675 nm. The proposed devices are fabricated and characterized. Measured results of the improved design show a maximum IL of 0.264 dB and maximum crosstalk of −30.9 dB over a 230-nm wavelength range including O-, C-, and L-bands, which accord well with the simulation. Low temperature sensitivity has also been demonstrated in both simulations and experiments.

Controlling the polarization dependence of the complex transmission spectrum using the Brewster effect in metafilms.

We propose a method for controlling the polarization dependence of the complex transmission spectrum using the Brewster effect in a two-dimensional array of meta-atoms with finite thickness, which we refer to as a metafilm. We show that the complex transmission spectra of the orthogonal linear polarization components can be controlled independently without reflection by stacking multiple metafilms that exhibit only an electric dipole resonance. As a proof-of-concept numerical demonstration, we design several broadband waveplates with high transmission efficiency based on simple design principles. The proposed method would enable us to easily design anisotropic metamaterials with various complex transmission spectra.