Nano‐ and Micro‐Auxetic Plasmonic Materials

Abstract

and epoxy-resin casting.[ 19 ] Soft lithography, [ 12 ] femtosecond laser ablation, [ 13 ] and digital micromirror device projection printing [ 14 ] have allowed the fabrication of negative Poisson's ratio structures with characteristic sizes of hundreds of micrometers. Smaller dielectric auxetics with unit cells on the order of 100 µm have been made by laser micromachining, [ 15 ] while dielectric auxetics with a characteristic size of 10 µm have been realized by direct laser writing. [ 16 ] However, negative Poisson's ratio materials with nanoscale lattice para meters cannot be fabricated using such techniques due to their limited resolution and the complexity of auxetic designs. Nevertheless, nanoscale auxetics would be particularly interesting as optical materials, considering that the combination of transformation optics concepts [ 20 ] with dielectric properties of auxetic meta materials [ 19,[21][22][23][24][25] already promises better electromagnetic cloaking devices for microwaves. [ 26,27 ] Here, we realize micro-and nanoscale metamaterial structures and demonstrate negative Poisson's ratios by mechanical actuation ( Figure 1 ). Our nano-auxetic metamaterials are based on the re-entrant honeycomb design, which is known for auxetic properties for a wide range of beam thicknesses, sizes, and angles. [ 11 ] Inspired by recent work in the fi eld of nanomechanical photonic devices, [ 28 ] nanomembrane technology was used to fabricate auxetics with lattice parameters in the range from a few micrometers to hundreds of nanometers. Nanomembrane technology has already provided simple and effi cient solutions for reconfi guration of nanoscale structures that are not auxetic, enabling nanomechanical devices and sensors. In particular, engineered resonant optical properties and anisotropic light modulation with up to 50% optical contrast has been demonstrated with reconfi gurable metamaterials actuated by thermal, [ 29 ] electric, [ 30 ] and magnetic [ 31 ] signals, and fabricated from plasmonic-thin-fi lm-coated nanomembranes.The auxetic materials are shown by Figure 2 and were fabricated by thermally evaporating a 60 nm-thick gold plasmonic layer on a commercially available silicon nitride membrane of 50 nm thickness and then milling the re-entrant honeycomb structure through both layers using a gallium-focused ion-beam system (Helios 600 NanoLab by FEI). All structures were milled with an ion-beam energy of 30 keV. Infrared auxetic metamaterial microstructures were fabricated with 70 pA ion-beam current and a spot size of 21 nm, while nano-auxetic structures were milled with a reduced ion current of 9 pA and a smaller spot size of 12 nm. The honeycomb pattern has a rhombic unit cell and wallpaper symmetry group cmm . [ 32 ] For simplicity, we specify the smallest rectangular cell, p x × p y , that describes each structure, where the periodicities p x and p y along x and y correspond to the diagonals of the elementary rhombic unit cell, see Auxetics, materials with a negative Poisson's ratio, are rare in nature. ...

Stiff auxetics: Hierarchy as a route to stiff, strong lattice based auxetic meta-materials

“…The significance of this work to a wide variety of technological application is enhanced by the recent fabrication of mechanical meta-materials on micro/nano scale through use of nanomembrane technologies 9 and digital printing techniques 55 – 57 . The use of nanomembrane technologies restricts the fabrication such lattices to plate-like structures that, under compression, will experience out of plane elastic instabilities; such structures could be artificially restrained to remain planar, in which case all the results of the present work will hold, or under tension, without restraint, the lattice is expected to follow the results obtained in section 2.1.…”

“…When a material deviates from this behavour (that is to say, it has a negative Poisson’s ratio) it is termed auxetic 1 . Auxetic materials are of broad interest across the sciences 2 – 6 due to their wide range of potential applications including novel fasteners 7 , biomedical use 8 , particle filters 9 , scaffolds for reconfigurable metasurfaces 10 , and energy damping/transmitting devices 11 .…”

Bioinspired Hierarchical Designs for Stiff, Strong Interfaces between Materials of Differing Stiffness

“…Furthermore, we demonstrate that the ratio of maximum stresses in the intermediate material to those in the lamellae can be manipulated over orders of magnitude through alterations in the geometry of the structure. These findings are of particular relevance given the recent growth of digital manufacturing techniques that allow the fabrication of two-dimensional structures with features on the nano or micro length scale [51]. In many areas of application, the materials to be joined may be specified by the functionality of the desired structure, and thus η is set by the application.…”

Metamaterials at the University of Southampton and beyond

“…We have identified that nanomembranes are the ideal platform for such nano-optomechanical reconfigurable metamaterials [65]. Metamaterials on structured membranes can be reconfigured thermoelastically [66], mechanically [67], electrically [68] and magnetically [69]. They can also be reconfigured by the light-induced forces that occur between elements of illuminated metamolecules.…”