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The long-term operation of refractory-metal-based metamaterials is crucial for applications such as thermophotovoltaics. The metamaterials based on refractory metals like W, Mo, Ta, Nb, and Re fail primarily by oxidation. Here, the use of the noble metal Ir is proposed, which is stable to oxidation and has optical properties comparable to gold. The thermal endurance of Ir in a 3-layer-system, consisting of HfO2/Ir/HfO2, by performing annealing experiments up to 1240 °C in a pressure range from 2 × 10−6 mbar to 1 bar, is demonstrated. The Ir layer shows no oxidation in a vacuum and inert gas atmosphere. At temperatures above 1100 °C, the Ir layer starts to agglomerate due to the degradation of the confining HfO2 layers. An in situ X-ray diffraction experimental comparison between 1D multilayered Ir/HfO2 and W/HfO2 selective emitters annealed at 1000 °C, 2 × 10−6 mbar, over 100 h, confirms oxidation stability of Ir while W multilayers gradually disappear. The results of this work show that W-based metamaterials are not long-term stable even at 1000 °C. However, the oxidation resistance of Ir can be leveraged for refractory plasmonic metamaterials, such as selective emitters in thermophotovoltaic systems with strong suppression of long wavelength radiation.

Refractory tungsten (W) plays an important role in high temperature photonic/plasmonic applications. Previously room temperature bulk single/poly-crystalline optical constants were extensively used to calculate the optical properties of W nanostructures at high temperatures. This might lead to a significant deviation between the predicted and measured optical properties due to the exclusion of electron–phonon interaction, as well as grain-boundary and surface-scattering. Herein, we show experimentally, how film thickness and temperature affects the optical losses in W. A drastic increase in the effective electron collision frequency is observed with decreasing the film thickness down to 5 nm, due to the grain-boundary and surface-scattering mechanisms. At sufficiently high temperatures (greater than 200 °C for W), the electron–phonon interaction eventually becomes the dominant mechanism, linearly increasing collision frequency with temperature, and it is independent of the geometry of the thin film structure. The impact of thickness and temperature-dependent optical properties of W is showcased with a hyperbolic 1D metamaterial structure acting as a thermophotovoltaic emitter. This work opens new directions in accurate prediction of the optical properties of nanostructures and design of efficient devices in various applications, such as aerospace, energy-efficient lighting, radiative cooling and energy harvesting, by incorporating thickness and temperature-dependent optical constants., Deutsche Forschungsgemeinschaft (DFG)

Titanium nitride (TiN) has emerged as one of the most promising refractory materials for plasmonic and photonic applications at high temperatures due to its prominent optical properties along with mechanical and thermal stability. From a high temperature standpoint, TiN is a substitution for Au and Ag in the visible to near-infrared wavelength range, with potential applications including thermophotovoltaics, thermoplasmonics, hot-electron and high temperature reflective coatings. However, the optical properties and thermal stability of TiN films strongly depend on the growth conditions, such as temperature, partial pressure of the reactive ion gas, ion energy, and substrate orientation. In this work, epitaxial TiN films are grown at 835 °C on an Al2O3 substrate using a radio frequency sputtering method. The oxidization behavior of TiN is investigated at 1000 °C under a medium vacuum condition of 2 × 10–3 mbar, which is relevant for practical technical applications, and the thermal stability at 1400 °C under a high vacuum condition of 2 × 10–6 mbar. The TiN film structure shows an unprecedented structural stability at 1000 °C for a minimum duration of 2 h under a medium vacuum condition, and an exceptional thermal stability at 1400 °C, for 8 h under a high vacuum condition, without any protective coating layer. The work reveals, for the first time to the authors’ knowledge, that the TiN film structure with columnar grains exhibits remarkable thermal stability at 1400 °C due to low-index interfaces and twin boundaries. These findings unlock the fundamental understanding of the TiN material at extreme temperatures and demonstrate a key step towards fabricating thermally stable photonic/plasmonic devices for harsh environments.

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 frequency of a metamaterial and the connected optical topological transition (OTT) are adjusted to selectively enhance and suppress 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 OTT in a thermally stable metamaterial at high temperatures of 1,000 °C.