Justyna K. Gansel

Gold Helix Photonic Metamaterial as Broadband Circular Polarizer

Corkscrew Polarizer Strong optical activity, measured in terms of a materials ability to polarize light, usually requires a material several hundred wavelengths thick. Gansel et al. (p. 1513, published online 20 August; see the cover) show that the tunable electromagnetic response of metamaterials may offer a route to reduce the amount of material required for strongly optically active materials. Based on the standard metamaterials design of the splitring resonator, photolithography was used to define an array of three-dimensional gold nanocorkscrews. Just a single wavelength thickness of the corkscrew design was required for a circular polarizer operating over an octave of bandwidth. A three-dimensional array of gold nano-helices can polarize light over a wide range of wavelengths. 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.

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.

On metamaterial circular polarizers based on metal N-helices

Polarization conversion of metal-helix based metamaterials can be eliminated by recovering four-fold rotational symmetry. Symmetry considerations and current progress in the fabrication of N-helix optical metamaterials as broadband circular polarizers is presented.

Metamaterial metal-based bolometers

We demonstrate metamaterial metal-based bolometers, which take advantage of resonant absorption in that a spectral and/or polarization filter can be built into the bolometer. Our proof-of-principle gold-nanostructure-based devices operate around 1.5 μm wavelength and exhibit room-temperature time constants of about 134 μs. The ultimate detectivity is limited by Johnson noise, enabling room-temperature detection of 1 nW light levels within 1 Hz bandwidth. Graded bolometer arrays might allow for integrated spectrometers with several octaves bandwidth without the need for gratings or prisms and for integrated polarization analysis without external polarization optics.

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.

Polarizing properties of three-dimensional helical metamaterials

We investigate the polarizing properties of chiral photonic metamaterials composed of three-dimensional metal helices. The calculated spectra reveal pronounced circular dichroism. Our geometry parameters are compatible with fabrication via direct laser writing and electrodeposition.