Fig. 4

Fabrication methods of soft optical metamaterials. a An illustration of the direct top-down patterning of an SU-8 based metamaterial with a sacrificial layer. After the patterning, SU-8-based Au soft metamaterial can be realized by releasing the sacrificial layer. b A PDMS-based flexible metamaterial fabricated by stencil lithography. First, a stencil is made and placed above the PDMS substrate with a precise control of the gap distance. Then, gold is deposited through the stencil onto the substrate, forming bowtie nanostructures. c Nanoimprint lithography for fabricating a large-area PDMS-based flexible metamaterial. First, a stamp consists of silicon wafers is ‘inked’. Then, the stamp is contacted against a target PDMS substrate. Finally, residual material on the stamp is removed to prepare for re-usage. d A protein-assisted self-assembly process to fabricate an AgNP-polystyrene metafluid. AgNPs are first functionalized with biotin terminated ligands (i), and then mixed with a high ionic solution of streptavidin-coated polystyrene NPs (ii) to form a metafluid consisting AgNP-polystyrene metamolecules (iii). Here, the highly specific chemical recognition between protein streptavidin and biotin ligands ensures the AgNPs to be closed packed around the polystyrene NPs. e Schematic illustration of DNA-programmable nanoparticle crystallization where the length of the linker strands can be tuned by increasing the value of n. DSP refers to the cyclic dithiol. a Reprinted with permission from [89], Copyright (2011) AIP Publishing LLC. b Reprinted with permission from [94], Copyright (2011) Wiley–VCH. c Reprinted with permission from [81], Copyright (2011) Nature Publishing Group. d Reprinted with permission from [65], Copyright (2013) ACS Publications. e Reprinted with permission from [143], Copyright (2013) Wiley–VCH