2D-to-3D buckling transformability enabled reconfigurable metamaterials for tunable chirality and focusing effect

Recently, multifarious deformation approaches in nature have promoted dynamic manipulation for electromagnetic (EM) waves in metamaterials, and those representative strategies are mainly focused on the modulation of spectral parameters. Several works have also achieved tunable phase-gradient meta-devices. Here, to broaden the modulation freedom of mechanical deformation, we initially propose two reconfigurable metamaterials consisting of mirrored S-shaped meta-atoms selectively bonded on biaxially pre-stretched substrates. Planar meta-atoms with spin-insensitive transmittance are buckled into 3D morphologies to break residual symmetries by releasing the stress and to facilitate spin-dependent transmittance under circularly polarized incidence. Owing to the geometric anisotropy of S-shaped meta-atoms along the x and y axes, 3D chiral meta-atoms exhibit discriminate circularly cross-polarized transmittance under opposite spins. The underlying physical mechanism reveals that EM resonance originates from the excitation of electric dipoles and magnetic dipoles, and their cross coupling finally triggers the chiral effects of 3D meta-atoms. By introducing the gradient-phase design that keeps unchanged under various strains, two types of meta-atoms with specified orientations are interleaved to design a double-foci metalens, and its 2D-to-3D morphology transformation shortens the focusing length and facilitates the intensity change of two foci. Our approach in designing reconfigurable EM metamaterials with 2D-to-3D buckling transformability can be further extended toward terahertz even optical wavebands, and it may assist with deriving more applicable multi-functionalities in the aspects of imaging, sensing, and holograms.