Elastic metamaterials for tuning circular polarization of electromagnetic waves

Abstract

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 three-dimensional materials constructed from the meta-molecules, since all the components twist and deform into the same chiral configuration when compressed.

Figure 1. Experimental and numerical images of the buckliball, with inner diameter d_i = 19.8 mm and wall thickness t = 7.1 mm, at different levels of applied strains, ε = ΔL/L₀.

Figure 2. Engineered meta-molecule made of a buckliball (a rubber cube with six holes) with metallic insertions.

(a) The sample is symmetric in the uncompressed state (b) however, it becomes chiral when compressed. (c) Evolution of the structural chirality index, measured in radians, as a function of applied strain. The blue shaded region represents the strain at which buckling occurs (i.e. −0.30 ≤ ε ≤ −0.12). Shown in (d,e) are the two resonance modes and the resulting surface current distribution after exciting the sample with a linearly polarized electromagnetic wave. They correspond to (d) symmetric, and (e) antisymmetric modes.

Figure 3. Electromagnetic chirality of the engineered meta-molecule.

Experimental (a) and numerical (c) circular dichroism at various strain values. (b,d) show the achieved optical activity (measured in radians) for the experiment and the simulations, correspondingly. Both quantities have been retrieved from the linear scattering parameter through Eq. (2).

Figure 4. Comparison of the experimental (⚬) and numerical (◊) maximum circular dichroism.

It can be seen that for low levels of compression the electromagnetic behavior (qualitatively) matches with the structural buckling (cf. Fig. 2c), however they differ for high levels of compression where the V-shaped antennas become flat.

Figure 5. Circular dichroism (top) and optical activity (bottom) of the 3D auxtic metamaterial for different levels of strain obtained from numerical simulations.

The inset shows the unit cell, a central meta-molecule surrounded by eighths of meta-molecules on each of its corners, of the three-dimensional metamaterial (BCC crystal) for the strain of ε = −0.27.