Dip-in direct-laser-writing (DLW) optical lithography allows fabricating complex three-dimensional microstructures without the height restrictions of regular DLW. Bow-tie elements assembled into mechanical metamaterials with positive/zero/negative Poisson's ratio and with sufficient overall size for direct mechanical characterization aim at demonstrating the new possibilities with respect to rationally designed effective materials.

We have previously shown that square arrays of three-dimensional gold helices can serve as compact broadband circular polarizers. Here, we show by heuristic reasoning supported by numerical calculations that the bandwidth of the device can realistically be increased to 1.5 octaves by tapering the gold-helix radius. The tapering also improves the extinction ratio. Depending on the side from which light impinges onto the tapered helices, the polarization conversions are different. Therefore, the structure is either optimal as polarizer or as analyzer. Corresponding structures for the infrared spectral range are fabricated by direct laser writing and gold electroplating.

Titania woodpile photonic crystals are fabricated by a combination of stimulated-emission depletion direct laser writing and a novel titania double-inversion procedure. The procedure relies on atomic-layer deposition, which is also used to fine-tune the template geometry to maximize the gapsize. Angle- and polarization-resolved transmittance spectroscopy and a comparison with theory provide evidence for the first complete photonic bandgap in the visible.

A simple protocol for the fabrication of three-dimensional (3D) photonic crystals in silicon is presented. Surface structuring by nanosphere lithography is merged with a novel silicon etching method to fabricate ordered 3D architectures. The SPRIE method, sequential passivation reactive ion etching, is a one-step processing protocol relying on sequential passivation and reactive ion etching reactions using C 4 F 8 and SF 6 plasma chemistries. The diffusion of fresh reactants and etch product species inside the etched channels is found to play an important role affecting the structural uniformity of the designed structures and the etch rate drift is corrected by adjusting the reaction times. High quality photonic crystals are thus obtained by adding the third dimension to the two-dimensional (2D) colloidal crystal assemblies through SPRIE. Careful adjustments of both mask design and lateral etch extent balance allow the implementation of even more complex functionalities including photonic crystal slabs and precise defect engineering. 3D photonic crystal lattices exhibiting optical stop-bands in the infrared spectral region are demonstrated, proving the potential of SPRIE for fast, simple, and large-scale fabrication of photonic structures.

We systematically study the optical spectra of ZnO grown by atomic-layer deposition as a function of Al (and Ti) doping concentration. The spectra measured on films are well described by fits using a Drude freeelectron model. The derived plasma frequencies are consistent with the expected amount of doping and can be continuously and controllably tuned from small values to about 400 THz. The losses (damping) are also quantified. In addition, we achieve smooth conformal coatings of threedimensional polymer templates made by direct laser writing. Altogether, Al:ZnO appears as an attractive "tunable metal" for three-dimensional infrared metamaterials or transformation-optics architectures.

The quest for less costly and more compact high-energy particle accelerators makes research on alternative acceleration mechanisms an important enterprise. From the multitude of suggested concepts, the photonic accelerator design by B. M. Cowan [Phys. Rev. ST Accel. Beams 11, 011301 (2008)] stands out by its distinct potential of creating an accelerator on a chip [Proposal E-163, SLAC (2001)]. Herein, electrons are accelerated by the axial electric field of a strongly confined optical mode of an air waveguide within a silicon-based three-dimensional photonic band-gap material. Using a combination of direct laser writing and silicon double inversion, we here present the first experimental realization of this complex structure. Optical spectroscopy provides unambiguous evidence for the existence of an accelerating waveguide mode with axial polarization.

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