Strain-Driven Thermal and Optical Instability in Silver/Amorphous-Silicon Hyperbolic Metamaterials

TL;DR

This study investigates the thermal and optical stability of Ag/a-Si hyperbolic metamaterials, revealing their persistence up to 500°C and identifying strain and interfacial energies as key instability factors.

Contribution

It provides a detailed analysis of the thermal stability of Ag/a-Si multilayers using nanotomography, simulations, and spectroscopy, highlighting the roles of strain and interfacial energies.

Findings

Hyperbolic dispersion persists up to 500°C.

Thermal instability begins at 300°C.

Instability driven by elastic strain or interfacial energy depending on stacking order.

Abstract

Hyperbolic metamaterials show exceptional optical properties, such as near-perfect broadband absorption, due to their geometrically-engineered optical anisotropy. Many of their proposed applications, such as thermophotovoltaics or radiative cooling, require high-temperature stability. In this work we examine Ag/a-Si multilayers as a model system for the thermal stability of hyperbolic metamaterials. Using a combination of nanotomography, finite element simulations and optical spectroscopy, we map the thermal and optical instability of the metamaterials. Although the thermal instability initiates at 300C, the hyperbolic dispersion persists up to 500C. Direct finite element simulations on tomographical data provide a route to decouple and evaluate interfacial and elastic strain energy contributions to the instability. Depending on stacking order the instability's driving force is either dominated by changes in anisotropic elastic strain energy due thermal expansion mismatch or by minimization of interfacial energy. Our findings open new avenues to understand multilayer instability and pave the way to design hyperbolic metamaterials able to withstand high temperatures.