Pt/Alumina Hyperbolic Metafilms with High-Temperature Stability, Wide Wavelength Tunability, and Omnidirectional Absorption

Abstract

The authors report high-temperature stable, spectrally tunable, Pt/alumina hyperbolic metafilms with broadband absorption. Two uniform pairs of Pt (10nm)/alumina (100, 150, or 200nm) multilayer films with a top alumina passivation layer and a bottom Pt mirror and are prepared on sapphire substrates by sputtering. These Pt/alumina multilayer films yielded strong light absorption at visible to near-infrared wavelengths that are spectrally tunable by modulating the thickness of each alumina layer. Based on effective medium theory for 1D metafilms, such tuning behavior is well understood; the maximum absorption occurred at a specific range of wavelengths wherein the real part of the effective permittivity was near-zero. The broadband absorption spectrum is only marginally altered over a wide range of incident angles from 0 to 70 degrees, a finding supported by simulations using the transfer matrix method using the experimentally derived optical constants of Pt and alumina. The fabricated Pt/alumina multilayer films maintained their initial absorption spectra after a heating test at 1450K for 12h. Transmission electron microscopy images and fast Fourier transform diffraction patterns show pristine Pt/alumina interfaces, devoid of any features related to oxidation, interdiffusion, and deformation after the heating test. These pristine interfaces are further verified with the elemental mapping results obtained by energy dispersion X-ray spectroscopy. This work demonstrates that a one-dimensional stack of Pt and alumina layers with deep-submicron thicknesses functions as a refractory hyperbolic metamaterial with a large absorption at specific wavelengths, which will be extensively utilized in a diverse range of thermal radiation-engineered applications.