We present our research into the fabrication of fully three-dimensional metallic nanostructures using diffusion-assisted direct laser writing, a technique which employs quencher diffusion to fabricate structures with resolution beyond the diffraction limit. We have made dielectric 3D nanostructures by multiphoton polymerization using a metal-binding organic-inorganic hybrid material, and we covered them with silver using selective electroless plating. We have used this method to make spirals and woodpiles with 600？nm intralayer periodicity. The resulting photonic nanostructures have a smooth metallic surface and exhibit well-defined diffraction spectra, indicating good fabrication quality and internal periodicity. In addition, we have made dielectric woodpile structures decorated with gold nanoparticles. Our results show that diffusion-assisted direct laser writing and selective electroless plating can be combined to form a viable route for the fabrication of 3D dielectric and metallic photonic nanostructures. 1. Introduction Direct fs laser writing is a technique that allows the construction of three-dimensional micro-and nanostructures [1]. It is based on the phenomenon of multiphoton absorption and subsequent polymerization; the beam of an ultrafast laser is tightly focused into the volume of a photosensitive material, initiating multiphoton polymerization within the focused beam voxel. By moving the beam three-dimensionally, arbitrary 3D, high-resolution structures can be written. By simply immersing the sample in an appropriate solvent, the unscanned, unpolymerized area can be removed, allowing the 3D structure to reveal. A variety of applications have been proposed including microfluidics [2], micro-optics [3, 4], scaffolds for biomolecules and cells [5–7], and photonics and metamaterials [8–10]. There has been a lot of research efforts to improve the resolution of DLW technology, which for a long time has been in the range of 100？nm. The method which most successfully and substantially has increased the resolution not only of single lines but also of 3D structures is DLW inspired by stimulated-emission-depletion (STED) fluorescence microscopy [11, 12]. In STED-DLW, two laser beams are used; one is used to generate the radicals, and the second beam to deactivate them. Several schemes have been proposed including single-photon (rather than multiphoton) excitation [13], a one-color scheme [14] and multiphoton two-color scheme [15, 16]. Structures with very high resolution and very small intralayer distances have been fabricated using this approach.
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