Andreas Frölich

Tailored 3D Mechanical Metamaterials Made by Dip‐in Direct‐Laser‐Writing Optical Lithography

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 Poissons ratio and with sufficient overall size for direct mechanical characterization aim at demonstrating the new possibilities with respect to rationally designed effective materials.

Tapered gold-helix metamaterials as improved circular polarizers

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.

Spectroscopic characterization of highly doped ZnO films grown by atomic-layer deposition for three-dimensional infrared metamaterials [Invited]

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 free-electron 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 three-dimensional 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.

Titania Woodpiles with Complete Three‐Dimensional Photonic Bandgaps in the Visible

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.

Waveguides in three-dimensional photonic bandgap materials for particle-accelerator on a chip architectures

A promising concept for making high-energy particle accelerators less expensive and more compact is to make use of axially polarized optical modes supported by line defect waveguides in three-dimensional photonic bandgap materials. Following a theoretical proposal by B. M. Cowan we here present the first experimental realization of corresponding pilot samples for three-dimensional photonic bandgap particle-accelerator segments. The samples have been fabricated using a combination of direct laser writing and an improved silicon-double-inversion procedure. Regarding optical characterization we have performed transmittance measurements providing unambiguous evidence of a waveguide mode with axial polarization. These results represent an important first step towards actually putting into practice future “particle-accelerator on a chip” architectures.

Second-harmonic generation from atomic-scale ABC-type laminate optical metamaterials (Presentation Recording)

[invited] We introduce ABC laminate metamaterials composed of layers of three different dielectrics. Each layer has zero bulk second-order optical nonlinearity, yet centro-symmetry is broken locally at each inner interface. To achieve appreciable effective bulk metamaterial second-order nonlinear optical susceptibilities, we densely pack many inner surfaces to a stack of atomically thin layers grown by conformal atomic-layer deposition. For the ABC stack, centro-symmetry is also broken macroscopically. Our experimental results for excitation at around 800 nm wavelength indicate interesting application perspectives for frequency conversion or electro-optic modulation in silicon photonics.

Tapered gold helices as high-extinction-ratio, broadband circular polarizer

Tapering is shown to further extend the bandwidth of three-dimensional gold-helix metamaterials as broadband circular polarizers to about 1.5 octaves. Furthermore, the extinction ratio is improved. Theory and experiment show good agreement.

Photonic metamaterials by direct laser writing

We present a planar magnetic metamaterial fabricated using 3D direct laser writing and silver chemical vapor deposition as well as a negative-index bi-anisotropic metamaterial metallized via silver shadow evaporation. Calculations and experiments show good agreement.