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.

visible-to microwave-frequencies, [3][4][5][6] however, with rather small operation bandwidths. Likewise, 3D chiral dielectric photonic crystals [7][8][9][10][11] also show fairly small operation bandwidths.In sharp contrast, metallic helical metamaterials exhibit circular dichroism over an unmatched bandwidth of well above one octave. Starting with square arrays of single gold helices that function as broadband circular polarizers with high extinction ratio in the mid infrared, [ 12,13 ] recently research has led to more complex unit cells. For example, by tapering the helix radius along the helix axis, the extinction ratio and the bandwidth can be enhanced simultaneously. [ 14 ] Double-helical unit cells have also been proposed to lead to higher extinction ratios and enlarged bandwidths. [15][16][17][18] Circular polarization conversions are often unwanted for polarizer applications. They can be eliminated by recovering either threefold or fourfold rotational symmetry by intertwining three or four helices, respectively, within one unit cell of a lattice with the same rotational symmetry. [19][20][21][22][23] Applications beyond circular polarizers such as broadband absorbers [ 24 ] or chiral near-fi eld sources [ 25 ] have also been proposed.While for circular polarizers a large difference of the diagonal elements of the Jones transmission matrix is desired, chiral metamaterials can also be designed to exhibit strong circular polarization conversions corresponding to the off-diagonal elements. Even though converting one circular polarization to the other can be achieved by half-wave plates at visible frequencies, these wave plates are often limited in bandwidth and not readily available at many other frequency ranges.Furthermore, a large difference in these off-diagonal elements can be of interest, both in linear and circular polarization basis. This effect is often referred to as "asymmetric transmission" due to the symmetry of the Jones transmission matrix. Applications like diode-like devices exploiting asymmetric Jones matrices have been proposed. [ 26 ] Metamaterial designs with highly asymmetric polarization conversions have been realized for linearly polarized light, both at infrared-and terahertz-frequencies. [ 27,28 ] More recently, many more designs for linearly polarized light have been proposed with both high extinction ratios and large bandwidths. [29][30][31][32][33] Asymmetric polarization conversions for circular polarization, on the other hand, has only been reported recently, however, with generally much Metallic helical metamaterials give rise to broadband and scalable chirooptical effects orders of magnitude higher than found in nature. While arrays of gold helices have been suggested as compact circular polarizers, where a large difference of the diagonal elements of the Jones transmission matrix is desired, chiral metamaterials can also be designed to exhibit strong circularpolarization conversions. Here, a novel helical metamaterial design, exhibiting asymmetric, broadband circular-polari...

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The present paper describes the fabrication sequence of a LIGA mold insert by electroforming after the patterning steps of the overall process. These tools are applied for large scale fabrication of microcomponents made by molding and embossing processes. The application of an intermediate layer system leads to optimized process performance and to a better surface quality of the mold insert. The plating processes are described and the materials properties, e.g. hardness, are used for the characterization of the recrystallization behavior of the electroformed nickel which yields the high temperature application limit of the tool.

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