Fabrication Of Metamaterials Research Articles

Design and Fabrication of Bioinspired Hierarchical Dissipative Elastic Metamaterials

Hierarchical structures with constituents over multiple length scales are found in various natural materials like bones, shells, spider silk and others, all of which display enhanced quasi-static mechanical properties, such as high specific strength, stiffness and toughness. At the same time, the role of hierarchy on the dynamic behaviour of metamaterials remains largely unexplored. This study assesses the effect of bio-inspired hierarchical organization as well as of viscoelasticity on the wave attenuation properties of continuous mechanical metamaterials. We consider single-phase metamaterials formed by self-similar unit cells with different hierarchical levels and types of hierarchy. Results highlight a number of advantages through the introduction of structural hierarchy. Band gaps relative to the corresponding non-hierarchical structures are mostly preserved, while additional "hierarchically-induced" band gaps appear. Additionally, the hierarchical configuration allows the tuning of the band gap frequencies of regular metamaterial to lower frequencies, with a simultaneous significant reduction of the global structural weight. We show that even small viscoelastic effects, not treated in the current literature, are essential in determining this behaviour. The approach we propose allows the addition of hierarchical elements to existing metamaterial configurations, with the corresponding improvement of the wave damping properties, thus providing indications for the design of structures for practical applications.

Microarchitected Stretching‐Dominated Mechanical Metamaterials with Minimal Surface Topologies

Historically, the creation of lightweight, yet mechanically robust, materials have been the most sought‐after engineering pursuit. For that purpose, research efforts are dedicated to finding pathways to emulate and mimic the resilience offered by natural biological systems (i.e., bone and wood). These natural systems evolved over time to provide the most attainable structural efficiency through their architectural characteristics that can span over multiple length scales. Nature‐inspired man‐made cellular metamaterials have effective properties that depend largely on their topology rather than composition and are hence remarkable candidates for a wide range of application. Despite their geometrical complexity, the fabrication of such metamaterials is made possible by the emergence of advanced fabrication techniques that permit the fabrication of complex architectures down to the nanometer scale. In this work, we report the fabrication and mechanical testing of nature‐inspired, mathematically created, micro‐architected, cellular metamaterials with topologies based on triply periodic minimal surfaces (TPMS) with cubic symmetries fabricated through direct laser writing two‐photon lithography. These TPMS‐based microlattices are sheet/shell‐ and strut‐based metamaterials with high geometrical complexity. Interestingly, results show that TPMS sheet‐based microlattices follow a stretching‐dominated mode of deformation, and further illustrate their mechanical superiority over the traditional and well‐known strut‐based microlattices and microlattice composites. The TPMS sheet‐based polymeric microlattices exhibited mechanical properties superior to other micrloattices comprising metal‐ and ceramic‐coated polymeric substrates and, interestingly, are less affected by the change in density, which opens the door for fabricating ultralightweight materials without much sacrificing mechanical properties.

Optical properties of single crystal silver thin films on mica for high performance plasmonic devices

We systematically investigate the optical properties of silver films to clear up the inconsistency in the published values of the dielectric function of silver. The silver films were deposited on mica by using a facing target sputtering system, which yielded a large area single crystal of silver suitable for the fabrication of high-finesse plasmonic devices and metamaterials. We confirmed that wide variations in the optical properties of silver were associated with the overall quality of the silver films including crystal structure, thickness, and surface roughness. The quality factor of the surface plasmon polaritons calculated for the obtained single crystal is 5 × 103, which is about five times higher than that for polycrystalline films.

Subwavelength nickel-copper multilayers as an alternative plasmonic material

Surface plasmon polaritons (SPP) are electromagnetic waves bound by the collective oscillations of free carrier plasma to the surface of a conductor surrounded by dielectric. These waves can be localized, concentrated or manipulated simply by processing the conductor surface. The choice of a convenient conductor is quite limited by the availability of natural materials and strong absorption losses associated with free electron plasma. For this reason new alternative plasmonic materials are actively being researched and developed. Most common approaches to this problem include doping of nonmetallic materials and fabrication of metal-dielectric nanocomposite metamaterials. In this contribution we perform an analysis of the suitability of the use of the heterometallic multilayers consisting of copper and nickel. Copper is an excellent plasmonic material, but the problem is formation of natural copper oxides on the surface. Therefore for this purpose a layer of nickel is used as a protection against oxidation of copper. Laminate composite structures of alternating nanocrystalline nickel and copper films on a cold-rolled polycrystalline copper foils were fabricated by electrochemical deposition technique. We simulated the electromagnetic properties of subwavelength Cu/Ni multilayers by the 2D finite element method using realistic material parameters to assess different electromagnetic modes. Our results show that the pair Cu/Ni can be viewed as an alternative tailorable plasmonic material. It has also been shown that it is possible to fabricate plasmonic structures without applying any complex lithographic processes.

Nanomeshes at Liquid Interfaces: From Free‐Standing Hole Arrays toward Metal–Insulator–Metal Architectures

AbstractThe superior optical properties of metamaterials often rely on hierarchical structures in the sub‐wavelength regime comprising stacks of individually patterned layers. For the fabrication of metamaterials in the visible regime, specific top‐down lithographic methods have been developed. These techniques, however, have several inherent drawbacks such the lack of scalability and low production speeds. Here, a simple and modular approach for the construction of multilayer metal–insulator–metal structures based on the transfer of metal nanohole arrays via liquid interfaces is demonstrated. Gold nanohole arrays are used as model systems that are prepared via nanosphere lithography on a zinc oxide sacrificial layer. Exploiting an interface mediated method, the prepared nanohole arrays are subsequently transferred onto arbitrary hydrophilic or hydrophobic substrates without compromising the high quality of the nanomeshes. Two approaches to stack such nanostructured layers with nanometer precision in a rapid way are demonstrated. Finally, the high mechanical stability of only 100 nm thick gold nanohole arrays allows for the preparation of free‐standing nanomeshes in the order of several hundreds of square‐micrometers.

Design, optimization, and fabrication of mechanical metamaterials for vibration control

Harsh shock and vibration environments are common in engineering applications. Mechanical metamaterials are showing significant potential as candidates for controlling wave propagation and isolating sensitive structural components, but require proper design of their complex microstructures. In this talk we will present time and frequency-domain strategies for Partial Differential Constrained (PDE)-constrained design optimization of locally resonant elastic/acoustic metamaterials. We will present a variety of notch filter resonators and split ring cylinder/sphere geometries that can be easily optimized for wave control applications, along with fabrication details involving multi-material additive manufacturing. A frequency-domain approach will be presented for band-gap or notch filter materials, and a time-domain strategy for transient shock environments. As the metamaterial structures typically involve concentrated masses and elastic connections, an optimization strategy involving 3D simple shapes will be presented. [Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration. With main facilities in Albuquerque, N.M., and Livermore, C.A., Sandia has major R&D responsibilities in national security, energy and environmental technologies, and economic competitiveness.]

Buckling‐Induced Assembly of Three‐Dimensional Tunable Metamaterials

Conventional metamaterials are usually confined to planar settings due to the fabrication challenges at the nanoscale. Here, we explore the shape transformation of metasurfaces into three‐dimensional metamaterials to generate some on‐demand plasmon modes which would be otherwise unachievable in two dimensions. This method is inspired by the buckling‐induced folding of a layer of metal sheet bonded to a stretchable compliant substrate in some specified regions. Two prototypes of three‐dimensional metamaterials are obtained, demonstrating the enhancement of quality factors as well as the tunability. We find the occurrence of sharp Fano resonance and the resonance mode with toroidal dipole attributed to the structural asymmetry in depth. This work provides an effective approach to practical fabrication of nanoscale metamaterials in an extra dimension that would enable a wide range of applications in the optical spectrum.

Fabrication and electrodynamic properties of all-carbon terahertz planar metamaterials by laser direct-write

A new approach to THz metamaterial structures is proposed and experimentally realized. It is based on metal-less conductive subwavelength structures on diamond surfaces generated by laser direct-write. 200 nm thick graphitized layers with DC conductivity of 730 Ω−1 cm−1 are formed on a chemical vapour deposited polycrystalline diamond surface after irradiation with an excimer KrF laser (τl = 20 ns, λ = 248 nm). The optical properties of such layers are determined and simulated according to the Drude model. A polarizer with a graphitized subwavelength grating is fabricated and tested in the THz range (0.9–1.2 THz), and shows different transmission losses for orthogonal polarizations.

Assessing the manufacturing tolerances and uniformity of CMOS compatible metamaterial fabrication

The manufacturing tolerances of a stencil-lithography variant, membrane projection lithography, were investigated. In the first part of this work, electron beam lithography was used to create stencils with a range of linewidths. These patterns were transferred into the stencil membrane and used to pattern metallic lines on vertical silicon faces. Only the largest lines, with a nominal width of 84 nm, were resolved, resulting in 45 ± 10 nm (average ± standard deviation) as deposited with 135-nm spacing. Although written in the e-beam write software file as 84-nm in width, the lines exhibited linewidth bias. This can largely be attributed to nonvertical sidewalls inherent to dry etching techniques that cause proportionally larger impact with decreasing feature size. The line edge roughness can be significantly attributed to the grain structure of the aluminum nitride stencil membrane. In the second part of this work, the spatial uniformity of optically defined (as opposed to e-beam written) metamaterial structures over large areas was assessed. A Fourier transform infrared spectrometer microscope was used to collect the reflection spectra of samples with optically defined vertical split ring from 25 spatially resolved 300 × 300 μm regions in a 1-cm2 area. The technique is shown to provide a qualitative measure of the uniformity of the inclusions.

Enhancement radiative cooling performance of nanoparticle crystal via oxidation

Nanoparticle-crystal is a promising candidate for large scale metamaterial fabrication. However, in radiative cooling application, the maximum blackbody radiation wavelength locates far from metal's plasmon wavelength. In this paper, it will be shown if the metallic nanoparticle crystal can be properly oxidized, the absorption performance within room temperature blackbody radiation spectrum can be improved. Magnetic polariton and surface plasmon polariton have been explained for the mechanism of absorption improvement. Three different oxidation patterns have been investigated in this paper, and the results show they share a similar enhancing mechanism.

Shape-Controlled 3D Periodic Metal Nanostructures Fabricated via Nanowelding.

A novel method for fabricating 3D metallic nanostructures to be used in polarized color filters based on nanoimprint lithography, electron-beam evaporation, and nanowelding is proposed. The shape of the nanostructures can be controlled by adjusting the temperature for the nanowelding process. Ag nanowires deposited on polymer patterns are accumulated by the nanowelding process to build up diverse 3D nanostructures. The morphologies of the fabricated 3D nanostructures are analyzed using scanning electron microscopy, atomic force microscopy, and focused ion beam; the heating temperature is varied from 90 to 130 °C in steps of 10 °C. In order to analyze the recrystallization phenomenon after welding, transmission electron microscopy is utilized. The 3D metallic nanostructure has different morphologies and optical properties corresponding to welding temperature conditions and accumulated layer thicknesses. Based on preliminary experimental results, the process parameters are optimized and a polarized color filter is fabricated. Optical characteristics of the filter are evaluated using polarizer and spectrometer. Through this work, it is shown that the proposed method is an effective way to realize various 3D metallic nanostructures for special optical properties, therefore the method based on nanowelding can be utilized in fabrication of functional metamaterials, optical filters, biosensors, and others.

Metamaterials based on the phase transition of VO2

In this article, we present a comprehensive review on recent research progress in design and fabrication of active tunable metamaterials and devices based on phase transition of VO2. Firstly, we introduce mechanisms of the metal-to-insulator phase transition (MIPT) in VO2 investigated by ultrafast THz spectroscopies. By analyzing the THz spectra, the evolutions of MIPT in VO2 induced by different external excitations are described. The superiorities of using VO2 as building blocks to construct highly tunable metamaterials are discussed. Subsequently, the recently demonstrated metamaterial devices based on VO2 are reviewed. These metamaterials devices are summarized and described in the categories of working frequency. In each working frequency range, representative metamaterials based on VO2 with different architectures and functionalities are reviewed and the contributions of the MIPT of VO2 are emphasized. Finally, we conclude the recent reports and provide a prospect on the strategies of developing future tunable metamaterials based on VO2.

Radiopaque Resists for Two-Photon Lithography To Enable Submicron 3D Imaging of Polymer Parts via X-ray Computed Tomography.

Two-photon lithography (TPL) is a high-resolution additive manufacturing (AM) technique capable of producing arbitrarily complex three-dimensional (3D) microstructures with features 2-3 orders of magnitude finer than human hair. This process finds numerous applications as a direct route toward the fabrication of novel optical and mechanical metamaterials, miniaturized optics, microfluidics, biological scaffolds, and various other intricate 3D parts. As TPL matures, metrology and inspection become a crucial step in the manufacturing process to ensure that the geometric form of the end product meets design specifications. X-ray-based computed tomography (CT) is a nondestructive technique that can provide this inspection capability for the evaluation of complex internal 3D structure. However, polymeric photoresists commonly used for TPL, as well as other forms of stereolithography, poorly attenuate X-rays due to the low atomic number (Z) of their constituent elements and therefore appear relatively transparent during imaging. Here, we present the development of optically clear yet radiopaque photoresists for enhanced contrast under X-ray CT. We have synthesized iodinated acrylate monomers to formulate high-Z photoresist materials that are capable of forming 3D microstructures with sub-150 nm features. In addition, we have developed a formulation protocol to match the refractive index of the photoresists to the immersion medium of the objective lens so as to enable dip-in laser lithography, a direct laser writing technique for producing millimeter-tall structures. Our radiopaque photopolymer resists increase X-ray attenuation by a factor of more than 10 times without sacrificing the sub-150 nm feature resolution or the millimeter-scale part height. Thus, our resists can successfully replace existing photopolymers to generate AM parts that are suitable for inspection via X-ray CT. By providing the "feedstock" for radiopaque AM parts, our resist formulation is expected to play a critical role in enabling fabrication of functional polymer parts to tight design tolerances.

Microscale 3D Printing of Nanotwinned Copper.

Nanotwinned (nt)-metals exhibit superior mechanical and electrical properties compared to their coarse-grained and nanograined counterparts. nt-metals in film and bulk forms are obtained using physical and chemical processes including pulsed electrodeposition (PED), plastic deformation, recrystallization, phase transformation, and sputter deposition. However, currently, there is no process for 3D printing (additive manufacturing) of nt-metals. Microscale 3D printing of nt-Cu is demonstrated with high density of coherent twin boundaries using a new room temperature process based on localized PED (L-PED). The 3D printed nt-Cu is fully dense, with low to none impurities, and low microstructural defects, and without obvious interface between printed layers, which overall result in good mechanical and electrical properties, without any postprocessing steps. The L-PED process enables direct 3D printing of layer-by-layer and complex 3D microscale nt-Cu structures, which may find applications for fabrication of metamaterials, sensors, plasmonics, and micro/nanoelectromechanical systems.

A New Compact Octagonal Shape Perfect Metamaterial Absorber for Microwave Applications

A new compact octagonal shape perfect metamaterial absorber (PMA) design, numerical simulation, fabrication, and investigational verification of unit cell that is based on a simple structure are presented in this paper. The suggested structure comprised of three layers, in which interact to produce the plasmonic resonances. The finite-integration technique (FIT) based Computer Simulation Technology (CST) microwave electromagnetic simulator was utilized to examine the design parameters and conduct absorption analysis. The design structure exhibited peak absorption values as 99.64% and 99.95% at frequencies 8.08 GHz and 11.41 GHz, respectively. The absorption characteristics were analysed using the polarization angle of the structure, layer thickness, PMA with resistive load, and number of rings. An N5227A vector network analyser was used for the measurement. The measured results of the fabricated prototype were in good agreement with the simulation results. The suggested perfect absorber structure enables innumerable application aimed at X-band for applications like, defence, security, and stealth technology.

Additively-manufactured lightweight Metamaterials for energy absorption

Recent developments in architectural design and additive manufacturing (AM) techniques have led to advances in the design and fabrication of metamaterials. We demonstrate that by precisely designing the architecture of a lattice structure and implementing the accurate and cost-effective capabilities of polymer-based AM, it is possible to create lightweight and strong structures that are highly energy absorbing, able to recover their shape after extreme deformation, and have a high strength-over-weight ratio. Largescale AM lattices with various densities of unit cells were fabricated from two different polymers. The densities of the structures ranged from 66kg/m3 to 192kg/m3. Compression experiments established the scaling of their strength and stiffness with their relative density and revealed that the structures were able to recover their original shape after a compression up to 70% strain. The energy absorption efficiency of the structures was 11% higher than Duocel® aluminum foam and comparable to that of aluminum honeycomb, expanded polystyrene (EPS) foams, and Skydex twin hemispheres. These unconventional mechanical properties substantiate the potential of architectural design along with additive manufacturing in various applications, not only at laboratory scale, but also at industrial level.

Programmable Supra‐Assembly of a DNA Surface Adapter for Tunable Chiral Directional Self‐Assembly of Gold Nanorods

AbstractAn important challenge in molecular assembly and hierarchical molecular engineering is to control and program the directional self‐assembly into chiral structures. Here, we present a versatile DNA surface adapter that can programmably self‐assemble into various chiral supramolecular architectures, thereby regulating the chiral directional “bonding” of gold nanorods decorated by the surface adapter. Distinct optical chirality relevant to the ensemble conformation is demonstrated from the assembled novel stair‐like and coil‐like gold nanorod chiral metastructures, which is strongly affected by the spatial arrangement of neighboring nanorod pair. Our strategy provides new avenues for fabrication of tunable optical metamaterials by manipulating the directional self‐assembly of nanoparticles using programmable surface adapters.

Concepts, Working Principles, and Applications of Coding and Programmable Metamaterials

AbstractAs a digital version of metamaterials, coding and programmable metamaterials have experienced rapid development since they were initially proposed in 2014. Unlike conventional metamaterials that are characterized by the sophisticated effective medium theory, coding metamaterials are described in a much simpler manner with binary codes, which builds up a bridge between the physical world and the digital world. In this article, the development of coding and programmable metamaterials in the past three years is reviewed, focusing primarily on the basic concept, working principle, design method, fabrication, and experimental validation. First, reflection‐type and refraction‐type coding metamaterials, along with two bifunctional coding metamaterials, are presented in the microwave, terahertz, and acoustic regimes. Second, the digital convolution theorem and information entropy of coding metamaterials are introduced to demonstrate the strong connection between metamaterials and information science. Then, recent progresses on engineering realization of field‐programmable metamaterials are demonstrated, including the compensation technique of plane waves under point source illumination, and applications in single‐sensor single‐frequency imaging systems. Finally, future directions and potential applications are summarized, followed by discussions on major challenges encountered in the design and fabrication of programmable metamaterials at higher frequencies.

Mesoscale evolution of non-graphitizing pyrolytic carbon in aligned carbon nanotube carbon matrix nanocomposites

Polymer-derived pyrolytic carbons (PyCs) are highly desirable building blocks for high-strength low-density ceramic meta-materials, and reinforcement with nanofibers is of interest to address brittleness and tailor multi-functional properties. The properties of carbon nanotubes (CNTs) make them leading candidates for nanocomposite reinforcement, but how CNT confinement influences the structural evolution of the PyC matrix is unknown. Here, the influence of aligned CNT proximity interactions on nano- and mesoscale structural evolution of phenol-formaldehyde-derived PyCs is established as a function of pyrolysis temperature ( $$T_{\mathrm {p}}$$ ) using X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. Aligned CNT PyC matrix nanocomposites are found to evolve faster at the mesoscale by plateauing in crystallite size at $$T_{\mathrm {p}}$$ $$\sim$$ 800 $$^{\circ }\hbox {C}$$ , which is more than $$200\,\,^{\circ }\hbox {C}$$ below that of unconfined PyCs. Since the aligned CNTs used here exhibit $$\sim$$ 80 nm average separations and $$\sim$$ 8 nm diameters, confinement effects are surprisingly not found to influence PyC structure on the atomic-scale at $$T_{\mathrm {p}}$$ $$\le $$ 1400 $$^{\circ }\hbox {C}$$ . Since CNT confinement could lead to anisotropic crystallite growth in PyCs synthesized below $$\sim$$ 1000 $$^{\circ }\hbox {C}$$ , and recent modeling indicates that more slender crystallites increase PyC hardness, these results inform fabrication of PyC-based meta-materials with unrivaled specific mechanical properties.

Tunable metamaterials fabricated by fiber drawing

We demonstrate a practical scalable approach to the fabrication of tunable metamaterials. Designed for THz wavelengths, the metamaterial is comprised of polyurethane filled with an array of indium wires using the well-established fiber drawing technique. Modification of the dimensions of the metamaterial provides tunability: by compressing the metamaterial we demonstrated a 50% plasma frequency shift using THz time domain spectroscopy. Releasing the compression allowed the metamaterial to return to its original dimensions and plasma frequency, demonstrating dynamic reversible tunability.