Within the next decade photonic nanomaterials will make an impact on photonics and optics comparable to the influence semiconductors have had on microelectronics development. In this context both the electronic properties of the constituting materials such as the complex electromagnetic susceptibility and structural aspects of material arrangements such as shapes of nano-sized structural elements as well as their orientational and positional distribution in space will crucially determine the photonic nanomaterial properties.
The Division Materials Technology at HZG in collaboration with Hamburg University of Technology (TUHH) directs its research on both the characterization of optical material properties as well as on the theory, modelling, fabrication and characterization of structural realizations of photonic nanomaterials. In particular, plasmonic materials and dielectrics are investigated which can be combined to form optical metamaterials. Such metamaterials employ feature sizes much smaller than the vacuum wavelength of light and thus can act as effective materials. Depending on the topology combinations of plasmonic and dielectric media can form so called hyperbolic optical media, stacks of nanoscaled layers, which allow suppressing the emission of radiation in certain wavelength ranges. Such hyperbolic materials, if constructed from refractory constituents, may serve as selective emitters in high temperature thermophotovoltaic applications (TPV) where they allow enhancing the conversion efficiency, significantly. If, however, plasmonic and semiconductor constituents are arranged in nanoporous three-dimensional strucures a metamaterial results, which, due to its broadband optical absorption properties combined with an extremely large inner surface, can be used as broadband absorbing and effective novel photocatalytic nanomaterials for solar water splitting into oxygen and into the clean fuel hydrogen.
Photonic Nanomaterials: Novel High Temperature Stable Metamaterials as Selective Emitters for Thermophotovoltaics
Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO2 metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero (ENZ) frequency of a metamaterial and the connected optical topological transition are adjusted to selectively shape the thermal emission in the near-infrared spectrum, crucial for improved TPV efficiency. The near-omnidirectional and spectrally selective emitter is obtained as the emission changes due to material properties and not due to resonances or interference effects, marking a paradigm shift in thermal engineering approaches. We experimentally demonstrate the optical topological transition effect in a thermally stable metamaterial at high temperatures of 1,000 °C.
Photonic Nanomaterials: Broadband Absorbing Nanoporous Gold for Photoelectrochemical Water Splitting
We present an explanation for the spectral properties of nanoporous gold (NPG) in the visible with the help of an effective medium model based on a cubic mesh of gold wires. With very few input parameters this model yields spectra that closely resemble those of NPG. From our analysis we see that the spectral response of NPG shows a characteristic overall peak attributed to an optical metamaterial based on ″diluted″ gold with a dip in the transmission due to the averaged electric field approaching zero. We further study the electrochemical tuning of the described transmission spectrum of such metamaterial NPG films from using optical in-situ measurements in an electrochemical environment, including the effect of the ligament size. The long wavelength part of the transmission spectrum around 800 nm can be reversibly tuned via the applied electrode potential. The diluted metal NPG behaves as a plasmonic metamaterial with its transition from dielectric to metallic response shifted to longer wavelengths as compared to bulk gold. We find that due to the enormous surface-to-volume-ratio of typically 108 m²/m³ we can strongly alter the charge carrier density as function of the applied potential. Compared to plasmonic nanoparticles, a NPG optical metamaterial, due to its connected structure, shows a much stronger and more broadband change in optical transmission for the same change in charge carrier density. We were able to create a partially transmitting mirror of only sub-wavelength thickness of 200 nm of which we could tune the transmission by 30% applying a potential of less than 1 V. In combination with an electrolyte such a tunable NPG based optical metamaterial is expected to play an important role in sensor applications, for photoelectrochemical water splitting into hydrogen and oxygen and for solar water purification.