We demonstrate a self-assembly strategy for fabricating three dimensional (3D) metamaterials. This strategy represents the desired 3D curving prongs of the split ring resonators (SRRs) erected by metal stress force with appropriate thin film parameters. Transmittance spectra and field patterns corresponding to each resonance modes are calculated by finite element method (FEM). The eigen-modes of the SRRs can be excited by normal illumination with polarization state parallel to the erected SRRs, which are unlike for the cases of planar SRRs. This method opens a promising fabrication process for the application of tailored 3D SRRs.

wileyonlinelibrary.com COMMUNICATIONis anisotropic, overall isotropic responses will effectively arise from the random arrangement of the elements. For instance, water in a glass acts as an isotropic material for light, although the water molecule itself has an asymmetric and anisotropic structure. The isotropic properties of water arise from the random and symmetric orientation of the water molecules. In the same manner, isotropic metamaterials can be realized by the highly symmetric arrangement of three-dimensional (3D) meta-atoms. To date, according to this concept, isotropic responses of metamaterials have been achieved at microwave frequencies by employing symmetric 3D alignment of metaatoms. However, in the optical regime, such isotropic metamaterials have been elusive due to the challenge of 3D nanostructure fabrication. [16][17][18][19] Few techniques have been reported to fabricate 3D SRRs at optical frequencies, and the fabrication processes were relatively complicated. [20][21][22][23][24][25][26][27][28] Isotropic metamaterials remain a theoretical prediction. [16][17][18][19] Here, we report the fi rst demonstration of an isotropic IR metamaterial consisting of fourfold-symmetric 3D SRRs. We have developed a metal stress-driven self-folding method and it enables us to mass-produce 3D metallic nanostructures ( Figure 1 a). [ 29 ] The greatest advantage of our technique is that the assembled 3D stereostructures can be formed directly from two-dimensional (2D) templates through a self-folding process that is spontaneously driven by pre-stressed fi lms. [30][31][32] This basic principle, which is totally different from other approaches, [ 22,23 ] enables the fabrication of electrically isolated 3D structures for isotropic responses. We adopt Ni/Au (10/60 nm) as the bilayer metals, and a series of the stress tests by varying the arm sizes is carried out to determine the corresponding dimensions for our 2D template (see Supporting Information, Figure S1). The 2D template of a SRR, which consists of two arms and a connection pad, was fi rstly fabricated using electron beam lithography, Ni/Au deposition, and lift-off techniques. The connection pad was purposefully designed to have a slightly larger width compared to that of the arms, serving as an adhesive area to the substrate in the self-folding process (see Supporting Information, Figure S2a). Note that, after the stress tests, we designed the arms with a width of 200 nm and a length of 2.5 μm to expect a 3D SRR with a diameter of 2.4 μm. The sample was then completed by CF 4 plasma dry-etching for the Si substrate, where folding of the arms was spontaneously induced by bilayer residual stress. When the arms were released from the Si substrate, the top Au fi lm revealed higher tensile stress than that of the back Ni fi lm, thereby folding up the arms away from the substrate. [ 33,34 ] Figure 1 b shows a scanning electron microscopy (SEM) image of the as-fabricated isotropic metamaterial with a total sample area of 4 × 4 mm 2 .

With the rapid development of optoelectronics, methods for manipulating light−matter interactions are urgently required. In this work, three-dimensional double split-ring resonators (3D-DSRRs) have been designed via a metal-stress-driven self-folding method. The 3D-DSRRs eliminate the electric response from the meta-atoms and exhibit a magnetic dipole moment radiation power that is 1244.6-fold greater than that of twodimensional metamaterials. By tailoring of the 3D-DSRR periodicity, the permeability is shown to vary from positive to zero, even reaching negative values under normal incidence. This strain engineering strategy provides an efficient method for designing and fabricating three-dimensional metamaterials and for increasing the variety of light−matter interactions in the mid-infrared region.

We experimentally and theoretically study the interplay between capacitive electric and inductive magnetic couplings in infrared metamaterials consisting of densely-packed three-dimensional (3D) meta-atoms. The meta-atom is made of metal-stress-driven assembled 3D split-ring resonators to exhibit strong bi-anisotropy, where electric and magnetic resonances occur simultaneously. By varying the spatial arrangement of the arrayed meta-atoms, the mutual coupling between meta-atoms dramatically modifies their mode profiles and resultant spectral responses. The corresponding numerical simulations evidently retrieved current densities and magnetic field strengths, as well as the transmittance, to reveal the important resonant behavior in the coupled meta-atom systems. We conclude that the mutual electric coupling between the neighboring meta-atoms plays a crucial role to the scattering behaviors of the bi-anisotropic metamaterials.