Elastic metamaterials for tuning circular polarization of electromagnetic waves

www.nature.com/scientificreports OPEN received: 29 March 2016 accepted: 01 June 2016 Published: 20 June 2016 Elastic metamaterials for tuning circular polarization of electromagnetic waves Yair Zárate1, Sahab Babaee2, Sung H. Kang2,3, Dragomir N. Neshev1, Ilya V. Shadrivov1, Katia Bertoldi2 & David A. Powell1 Electromagnetic resonators are integrated with advanced elastic material to develop a new type of tunable metamaterial. An electromagnetic-elastic metamaterial able to switch on and off its electromagnetic chiral response is experimentally demonstrated. Such tunability is attained by harnessing the unique buckling properties of auxetic elastic materials (buckliballs) with embedded electromagnetic resonators. In these structures, simple uniaxial compression results in a complex but controlled pattern of deformation, resulting in a shift of its electromagnetic resonance, and in the structure transforming to a chiral state. The concept can be extended to the tuning of threedimensional materials constructed from the meta-molecules, since all the components twist and deform into the same chiral configuration when compressed. Recent advances in fabrication at micro- and macro-scales have enabled the design of metamaterials with electromagnetic properties that are distinct from their constituents, and exhibit extreme parameter values previously thought impossible1,2. Of particular interest are chiral metamaterials, which exhibit polarisation rotation and circular dichroism orders of magnitude larger than natural chiral media3. Chiral metamaterials have been demonstrated in microwave4, terahertz5 and near infra-red6 spectral ranges. Beside their ability to rotate the plane of linearly polarised waves and selectively filter by circular polarisation, their reduced symmetry makes them of great interest for the enhancement of nonlinear processes7. While the first generation of electromagnetic metamaterials was characterized by properties fixed during the assembly process8, recently there has been considerable interest to expand their functionality by designing systems capable of changing properties reversibly through external stimuli. Techniques for achieving such dynamic tunability which are applicable to chiral media include varactor diodes9, liquid crystals10 and micro-electromechanical systems (MEMS)11. Reconfiguration of the internal geometry of meta-atoms has proven to be a highly effective mechanism for tuning the transmission response and controlling chirality. Notable examples of electromagnetic functionality controlled by elastic deformation include stretchable antennas using microfluidics or silver nanowires12,13. Moreover, tunable flexible metamaterials promise a new wave of device designs and functionalities14. In another approach, metamaterial deformation is induced using electromagnetic forces, yielding to many interesting nonlinear phenomena that range from bistability, spontaneous symmetry breaking and self-oscillations15,16. However, deformation by electromagnetic forces requires systems with an extremely soft elastic response, which often exhibit unwanted mechanical degrees of freedom. This makes them unsuitable for polarisation manipulation applications outside a controlled laboratory, such as in microwave communication, sensing or imaging, since vibrations would completely degrade performance. Therefore, an important problem in the field of tunable metamaterials is the development of an electromagnetic material with carefully designed modes of mechanical deformation. In this communication, we propose and demonstrate the combination of advanced elastomeric metamaterials with resonant electromagnetic structures to achieve prescribed mechanical tunability of the electromagnetic response. Particularly, we exploit the unique geometric conformation that an elastic structure undergoes under compression to fabricate a composite electromagnetic-elastic meta-molecule with dynamic tunability. 1 Nonlinear Physics Centre and Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS), Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601 Australia. 2John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. 3 Department of Mechanical Engineering and Hopkins Extreme Materials Institute, Johns Hopkins University, Baltimore, Maryland 21218, USA. Correspondence and requests for materials should be addressed to Y.Z. (email: ) Scientific Reports | 6:28273 | DOI: 10.1038/srep28273 1 www.nature.com/scientificreports/ Figure 1. Experimental and numerical images of the buckliball, with inner diameter di = 19.8 mm and wall thickness t = 7.1 mm, at different levels of applied strains, ε = ΔL/L0. Our approach utilises an external force to deform the structure of the meta-molecule (the building block of a metamaterial), which provides precise control over its mechanical response, and therefore over its electromagnetic properties. Most importantly, the proposed mechanism of tunability is robust and can be easily scaled to three-dimensional metamaterials by using our engineered meta-molecule as a basic constituent. Our engineered electromagnetic meta-molecule can be reversibly tuned between achiral and highly chiral states by an external force. While recently there have been a number of studies demonstrating achiral to chiral transition induced by elastic deformation in 2D structures17–19, to control circular polization of electromagnetic waves we need fully 3D structures exhibiting strong coupling with electromagnetic waves. Here this is achieved by embedding metallic elements into an engineered elastic structure, a patterned spherical shell with six circular holes known as a buckliball (see Figs 1 and 2), for which significant geometric rearrangements have been observed as a result of elastic instability20. This building block has previously been used as a constituent element to create tunable 3D soft negative Poisson’s ratio (auxetic) metamaterials21 and phononic crystals22. Here we exploit its unique deformation induced by buckling under uniaxial compression to design an electromagnetic-elastic meta-molecule able to reversibly tune the circular dichroism of electromagnetic waves. This is enabled by its ability to switch between achiral and chiral configurations. Since the differential absorption of left- and right-handed light is highly affected by the chirality of the medium in which the electromagnetic waves propagate7, it is easy to see that the design of a meta-molecule with reversibly tunable circular dichroism requires a 3D structure with variable structural chirality. We start by investigating the effect of uniaxial compression along the diagonal direction on the configuration of the elastic structure. In particular, we focus on a spherical shell (inner diameter di = 19.8 mm and wall thickness t = 7.1 mm) that is patterned with a regular array of 6 slight (...truncated)