Perovskite Lanthanum-Doped Barium Stannate: A Refractory Near-Zero-Index Material for High-Temperature Energy Harvesting Systems

The recent interests in bridging intriguing optical phenomena and thermal energy management has led to the demonstration of controlling thermal radiation with epsilon-near-zero (ENZ) and the related near-zero-index (NZI) optical media. In particular, the manipulation of thermal emission using phononic ENZ and NZI materials has shown promise in mid-infrared radiative cooling systems operating under low-temperature environments (below 100 °C). However, the absence of NZI materials capable of withstanding high temperatures has limited the spectral extension of these advanced technologies to the near-infrared (NIR) regime. Herein, a perovskite conducting oxide, lanthanum-doped barium stannate (La:BaSnO₃ [LBSO]), as a refractory NZI material well suited for engineering NIR thermal emission is proposed. This work focuses on the experimental demonstration of superior high-temperature stability (of at least 1000 °C) of LBSO films in air and its durability under intense UV-pulsed laser irradiation below peak power of 9 MW cm⁻². Based on the low optical-loss in LBSO, a selective narrow-band thermal emission utilizing a metal-insulator-metal (MIM) Fabry–Pérot nanocavity consisting of LBSO films as metallic component is demonstrated. This study shows that LBSO is an ideal candidate as a refractory NZI component for thermal energy conversion operating at high temperatures in air and under strong light irradiations.