We investigated propagation of light through a uniaxial photonic metamaterial composed of three-dimensional gold helices arranged on a two-dimensional square lattice. These nanostructures are fabricated via an approach based on direct laser writing into a positive-tone photoresist followed by electrochemical deposition of gold. For propagation of light along the helix axis, the structure blocks the circular polarization with the same handedness as the helices, whereas it transmits the other, for a frequency range exceeding one octave. The structure is scalable to other frequency ranges and can be used as a compact broadband circular polarizer.

Metamaterials are artificial materials that-unlike natural substances-enable magnetism to be achieved at optical frequencies 1-3 . The vast majority of photonic metamaterials 4,5 has been fabricated by electron-beam lithography and evaporation of metal films, both of which are well-established twodimensional (2D) technologies. Although stacking of three 6 or four 7 functional layers made using these methods has been reported, a truly 3D fabrication approach would be preferable for 3D photonic metamaterials. Here, we report first steps in this direction by using a combination of direct laser writing 8,9 and silver chemical vapour deposition 10,11 -the 3D analogues of electron-beam lithography and evaporation, respectively. The optical characterization of a planar test structure composed of elongated split-ring resonators is in good agreement with theory. Retrieval of the effective optical parameters reveals the importance of bi-anisotropy. Once suitable theoretical blueprints are available, our fabrication approach will enable rapid prototyping of truly 3D photonic metamaterials.The emerging field of photonic metamaterials is driven by the fascinating visions of perfect lenses 12 , optical cloaking 13,14 or quantum levitation 15 . Realizing these visions clearly ultimately requires large-scale three-dimensional (3D) metamaterials operating at optical frequencies. One of the major challenges concerns the (realistic) design and the nanofabrication of such structures. The latter is related to the fabrication of 3D (dielectric) photonic crystals, for which extensive literature is available (see, for example, the references in a recent review 16 ). It has become clear that polymeric templates made using holography and/or direct laser writing (DLW) can subsequently be converted into, for example, silicon structures using chemical vapour deposition (CVD) and/or atomic-layer deposition (ALD). In the context of metamaterials, however, metals are crucial; silver leads to the lowest losses at optical frequencies 17 . Yet, it has been unclear whether structures made using DLW and silver CVD would ever have sufficient quality and whether achievable feature sizes would result in metamaterials operating at optical frequencies. Fortunately, we report here that the answer is positive in both regards. Combined with suitable theoretical blueprints, which still need to be developed, this result means that one of the major hurdles in the field of photonic metamaterials is starting to disappear.Lithography of 3D polymeric templates by DLW with lateral feature sizes in the 100 nm range has become routine 8,9,18 and is even available commercially 19 . Our vision is to take such a template and coat it with a silver film of a few tens of nanometres in thickness, for example leading to the 3D arrangement of magnetic split-ring resonators (SRR) shown in Fig. 1a. Several questions arise immediately. (1) Silver films made using these methods are obviously always inherently connected-in sharp contrast to those made by electron-beam lithography...

Four different types of bi‐chiral photonic crystals are experimentally realized and characterized (see figure). In these 3D bi‐chiral photonic crystals one type of handedness stems from the motif (the spirals in our case) and the other one from the corner, that is, from the fictitious skeleton onto which the motif is arranged. In nature only two of these types of bi‐chiral photonic crystals exist. We realized all four bi‐chiral combinations for the first time.

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We present the blueprint for a novel negative-index metamaterial. This structure is fabricated via three-dimensional two-photon direct laser writing and silver shadow evaporation. The comparison of measured linear optical spectra with theory shows good agreement and reveals a negative real part of the refractive index at around 3.85 μm wavelength -despite the fact that the metamaterial structure is bi-anisotropic due to the lack of inversion symmetry along its surface normal.OCIS codes: 160.3918, 220.4000, 110.6895. Metamaterials are tailored artificial materials composed of sub-wavelength metallic building blocks ("photonic atoms") that are densely packed into an effective material [1]. In this fashion, for example, an effective negative index of refraction ("optical antimatter") can be achieved [2,3]. Most metamaterials operating at optical frequencies have been fabricated using planar lithography techniques (e.g., electron-beam lithography or microcontact printing) and, hence, the 1