Metamaterials consisting of artificially engineered periodical metal structures can control electromagnetic waves, including visible light, and they provide fascinating functions of negative refraction [1–4], invisibility cloaking [5–7], and perfect superlenses [8] that do not occur in conventional natural materials. Pendry et al. proposed a split-ring resonator (SRR), i.e., a ring with a slit, for metamaterials [9]. Micrometer- to submicrometer-scale engineering is required to fabricate metamaterials working in the wavelength region from visible to infrared [10]. Direct laser writing (DLW) with two-photon polymerization (TPP) and two-photon absorption (TPA) is a powerful tool to fabricate complexed three-dimensional (3D) microstructures using commercially available positive [11] and negative photoresists [12]. Photo reduction of metal ions is also used [13, 14].
Spiral, helical and omega microstructures are fabricated using TPA-DLW for broadband circular polarizers [15–19]. In previous studies on spiral and helical 3D metamaterials on the micrometer scale, compact, dense, intricate, and bichiral microstructures were used to avoid collapsing the fabricated microstructures [16, 17]. A twisted omega microstructure was supported by two points on the substrate [19]. In past studies, the fabrication of large-scale micrometer metamaterials on the order of mm2 has not been reported. Recently, a new type of DLW method using a phase mask of a double helix on a spatial light modulator (SLM) was proposed, in which the single-exposure femtosecond photoreduction of silver ions was used to fabricate the silver helix metamaterial [20].
In this report, we present the fabrication of more than 18,000 free-standing 3D helical structures on the micrometer scale over a total area on the mm2 scale by TPA-DLW. The key point for large area fabrication is to stabilize the fabricated free-standing helix microstructures supported by one point on the substrate. For this purpose, we introduced the foundation underneath the fabricated helix microstructure in advance, which usefully avoids collapsing the fabricated helix microstructures. Furthermore, the neighboring distance between the helix microstructures is also an important parameter to avoid collapsing the structures. Electroless silver plating [17, 19, 21] was employed in the present study. Another plating method, such as electrode gold plating [15] or chemical vapor deposition [22], can also be employed.