The dynamic tuning of metamaterials is one of the most attractive research branches in recent years. The functionalities in tunable metamaterials can be controlled under the external stimulus (e.g. electricity, light, magnetism, heat, etc.) and an individual meta-device is empowered with multiple functionalities. The common tunning methods can be divided into two groups, where one is to modulate the electromagnetic responses by manipulating the active constituent elements or materials of metamaterials under the various external conditions, and the other is to utilize the mechanical strategies to change the geometric shapes of metamaterials to tune the responses. The former methods have been widely discussed and investigated by researchers and applied in the dynamic manipulations of amplitudes, polarization states, wavefronts, beam deflection, etc. For morphologically reconfigurable metamaterials, the current research concentrates on the deformation of flexible substrates or kirigami/origami platforms to tune the spectral responses such as chirality, absorption, etc. Also, a few researchers have utilized deformable units to design metalens, in which the focusing length can be enlarged or reduced by changing the periodicity of units during the stretching or the releasing process. Nevertheless, the existing mechanical deformation patterns of reconfigurable metamaterials are still relatively single and the available tuning functionalities are also limited. Therefore, developing novel methods of mechanical deformation can provide possibilities for extending the functionalities reconfigurable metamaterials furtherly.
In recent years, 3D-buckling mechanical deformation strategy has been constantly investigated to play roles in the field of flexible electronics, biology, medical science, etc. In this method, the planar structures and their array patterns can be transformed into 3D states, which can be controlled by the flexible substrate. To investigate the applications of this method in the metamaterials can provide novel ideas for reconfiguring the metamaterials and modulating their responses. A research group led by Prof. Liuyang Zhang from Xi'an Jiaotong University proposed a dynamic modulation method of electromagnetic metamaterials based on 3D-buckling deformation principle. By this method, the in-plane asymmetric units are deformed in three dimensions to break the out-of-plane symmetries to enhance the chiral responses of metamaterials. Furtherly, the transmittance difference of the opposite enantiomer structures is adopted to design a double-foci flexible metalens with tunable focusing length and focusing intensity. This work introduces a 3D mechanical deformation method into the dynamic modulation of metamaterials. The relevant research results were published in Photonics Research, Volume 11, No. 10, 2023 (Donghai Han, Wenkang Li, Tao Sun, Min Liu, Xiaoming Chen, Hongyu Shi, Zhengjie Fan, Fanqi Meng, Liuyang Zhang, Xuefeng Chen. 2D-to-3D buckling transformability enabled reconfigurable metamaterials for tunable chirality and focusing effect[J]. Photonics Research, 2023, 11(10): 1770).
As shown in Fig. 1(a), 3D-buckling reconfiguration principle refers to the out-of-plane deformation of the non-bonded regions of the metal resonant structures selectively bonded on a pre-stretched flexible substrate after releasing the substrate strain. In this work, the S-shaped structures with in-plane symmetries are selected as the units and the 3D-buckling deformation can promote the enhancement of the chiral responses. As shown in Fig. 1(b), the co- and cross-polarized transmittance difference are exhibited under circularly polarized incidences with opposite spins, and the illustrations display the localized structures of the opposite chiral 3D metamaterial samples. To furtherly extend the application scenarios of this 3D reconfiguration method in the dynamic modulations of metamaterials, the opposite chiral units are interleaved to design a transverse double-foci metalens based on the geometric-phase principle. The intensities of the two foci are identical in the planar metalens. When the metalens is exhibited with 3D deformation after releasing the substrate, the focusing intensity difference of the two foci occurs, which is induced by unequal cross-polarized transmittance amplitudes of the opposite chiral units.
Fig.1 (a) Schematic of the experimental setup. (b) Measured vector field in the x–z plane between two parallel plate electrodes. (c) Noise equivalent force and equivalent electric intensity in vacuum.
Prof. Liuyang Zhang from Xi'an Jiaotong University said, "The metamaterials fabricated by 3D buckling assembly method are empowered with stretchability and flexibility, and even though the 3D unit structures are full of stresses, they still possess great structural stability. In the conventional kirigami/origami 3D reconfigurable metamaterials, the periodic defects or creases of the substrates are often utilized to promote the rotation of the metallic resonant structures of the units in the space, while the resonant structures are not deformed. Differentially, by our proposed reconfiguration method, the resonant structures are compressed to undergo the buckling deformation that facilitates the enhancement of the chiral responses, which can also be extended toward phase-gradient metasurfaces. Owing to the flexibility and stretchability of the silicone substrate material, the entire device in the free state can be adaptive with the curved surfaces, while the 3D metallic units still maintain stable. Also, the structures can undertake the recycling loading to achieve the repeatable modulations of metamaterials.
The metalens fabricated by 3D-buckling in this work operates at microwave frequency band. In the future, the group will continue to extend such a strategy toward the applications in the aspect of dynamic hologram, adaptive imaging, etc., meanwhile to improve operating frequencies of reconfigurable metamaterials based on this method.
