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Hyperbolic Metamaterials for High-Temperature Applications: Insights from Material Science
Abstract
Hyperbolic metamaterials (HMMs) have gained significant attention due to their engineered extreme optical anisotropy, which allows for novel applications in sensing, photovoltaics, and health care. However, so far concerns about their limited thermal stability prevent the widespread implementation of HMMs in applications requiring high-temperature stability, such as thermophotovoltaics. This thesis addresses this challenge by examining the thermal stability of HMMs through a series of selected systems involving different material systems and architectures. The investigation begins with the silver/amorphous silicon system (Ag/a-Si), a well-studied binary material pair. It is shown that due to thermal instability, the system undergoes an order-disorder transition when annealed at 575 K. Despite this instability, the system retains its hyperbolic dispersion— key to its optical properties—beyond this temperature. At elevated temperatures, however, silver migration and subsequent sublimation alter the optical response. Finite element simulations and tomographic data analysis are used to compare the contributions of interfacial and elastic strain energies to this instability. Depending on stacking order the main driving force to the instability is either the elastic strain energy due to
thermal expansion mismatch or by the minimization of interfacial energy. The research progresses to explore a quaternary system, namely molybdenum disulfide (MoS2), paired with zirconium nitride (ZrN). MoS2 was chosen as it offers thickness- and phase-dependent optical properties in the nanometric regime, and ZrN was selected due to its chemical stability at the high temperatures necessary for MoS2 crystallization. We demonstrate that the variations in the optical properties of MoS2 can be leveraged for rapid monitoring of the thickness and phase of MoS2 during its fabrication. Such monitoring enabled a precise characterization of the functional properties of MoS2 across the amorphous-crystalline transition, culminating in the synthesis of MoS2/ZrN HMMs stable up to 975 K.
Furthermore, the thesis delves into chemically modulated zirconiumnitride/zirconiumoxide (ZrN/ZrO2) chemically modulated hyperbolic metamaterial (CM-HMMs). Due to their high average melting point of 3120 K, the system is a potential candidate as a selective emitter for thermophotovoltaics (TPV). These structures remain chemically stable upon annealing in vacuum up to 1200 K. Using calculation-assisted design, we demonstrate that these systems could provide a spectral efficiency up to 60% superior to that of a black body at 1200 K. The study highlights how optimized deposition techniques and simulation-guided design can enhance the photonic efficiency of such systems. Based on these case studies, the thesis examines the interplay between material selection, architectural design, and thermal degradation pathways in HMMs. It provides insights into the thermal response of various material systems and the underlying mechanisms of their thermal instability. The findings aim to guide the future design of robust metamaterials for high-temperature applications. Show more
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Contributors
Examiner: Galinski, Henning
Examiner: Spoleank, Ralph
Examiner: Alarcon-Lladó, Esther
Examiner: Dufresne, E.R.
Subject
metamaterial; Thermal stability; optics; scalability; Materials science; Photonics; thin filmOrganisational unit
03692 - Spolenak, Ralph / Spolenak, Ralph
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