Matthias Ruther

Strong optical activity from twisted-cross photonic metamaterials

Following a recent theoretical suggestion and microwave experiments, we fabricate photonic metamaterials composed of pairs of twisted gold crosses using two successive electron-beam-lithography steps and intermediate planarization via a spin-on dielectric. The resulting two effective resonances of the coupled system lie in the 1-2 microm wavelength regime and exhibit pronounced circular dichroism, while the circular polarization conversion is very small. In between the two resonances, we find a fairly broad spectral regime with strong optical activity, i.e., with a pure rotation of incident linear polarization. The measured optical transmittance spectra agree well with theory.

Electrochemical Modulation of Photonic Metamaterials

[∗] L.-H. Shao ,[+] M. Ruther ,[+] Prof. S. Linden ,[+] Prof. J. Weissmuller , Prof. M. Wegener Institut fur Nanotechnologie Karlsruhe Institute of Technology (KIT) Postfach 3640, D-76021 Karlsruhe (Germany) E-mail: matthias.ruther@kit.edu L.-H. Shao , M. Ruther , Prof. S. Linden , S. Essig , Prof. K. Busch , Prof. J. Weissmuller , Prof. M. Wegener DFG-Center for Functional Nanostructures (CFN) Karlsruhe Institute of Technology (KIT) D-76131 Karlsruhe (Germany) M. Ruther , Prof. S. Linden , Prof. M. Wegener Institut fur Angewandte Physik Karlsruhe Institute of Technology (KIT) Wolfgang-Gaede-Strasse 1, D-76128 Karlsruhe (Germany) S. Essig , Prof. K. Busch Institut fur Theoretische Festkorperphysik Karlsruhe Institute of Technology (KIT) Wolfgang-Gaede-Strasse 1, D-76128 Karlsruhe (Germany) Prof. S. Linden Current Address: Physikalisches Institut, Universitat Bonn, Nusallee 12, D-53113 Bonn (Germany) [+] These authors have contributed equally to this work

Distance-dependence of the coupling between split-ring resonators and single-quantum-well gain

We present low-temperature femtosecond pump-probe experiments on arrays of silver split-ring resonators coupled to single quantum wells, in which we vary the geometrical separation between the two components to study the variation of coupling with distance. Its strength is found to decrease exponentially with increasing separation with a 1/e length on the order of 8 nm. We further link our experimental results to numerical calculations of the near fields which show the same distance-dependence as the coupling strength in the experiment. Together, this confirms the assumption of a near-field-assisted coupling mechanism.

On the mechanism of electrochemical modulation of plasmonic resonances

Recent electrochemical experiments on gold-based photonic metamaterials have shown a sizable reversible tuning and modulation of plasmonic resonances. Here, we study the mechanism of the electrochemical modulation by measuring the change of the resonance transmittance and resonance frequency during underpotential deposition of Pb,Cu, and electrosorption of OH. The electric resistance change of the resonators is identified as decisive for the resonance transmittance change, while the space-charge layer at the metal surface shifts the resonance frequency.

Electrochemical restructuring of plasmonic metamaterials

Recent electrochemical experiments on gold-based photonic metamaterials have shown reversible optical modulation as well as an irreversible reduction in the plasmonic damping. Here, we systematically study the latter aspect as a possible means of postprocessing plasmonic gold nanoantennae arrays aiming at loss reduction. We characterize the samples by optical spectroscopy, electron microscopy, and atomic-force microscopy. For sub-10 nm gold thicknesses, the measured damping decreases by factors exceeding 3; for 20–30 nm thin structures, the obtained loss reduction still amounts to about 30%.

Plasmonic metamaterials coupled to single InGaAs-Quantum-well gain

We present low-temperature femtosecond-pump-probe measurements on arrays of split ring resonators above a single InGaAs-quantum well in which we study the coupling mechanism between the metamaterial and gain medium upon variation of their separation.

Electromodulation of Photonic Metamaterials

By applying voltages of about 1V to very thin metal nanostructures via an aqueous electrolyte, we demonstrate reversible modulation of split-ring-resonator resonances by as much as 60 THz at around 300 THz resonance frequency.