Tailoring Infrared Absorption and Thermal Emission with Ultrathin-film Interferences in Epsilon-Near-Zero Media

TL;DR

This paper uses simulations to explore how ultrathin epsilon-near-zero films can be engineered to control infrared absorption and thermal emission through interference effects, enabling customizable optical coatings.

Contribution

It develops a physical model of ultrathin-film interference in ENZ media, revealing universal wave phenomena and guiding design for tailored infrared absorption and emission.

Findings

Identified wavelength and angular ranges for efficient ENZ absorption and emission.

Developed a model explaining resonant interferences below quarter-wavelength thickness.

Demonstrated control over spectral and directional properties of thermal radiation.

Abstract

Engineering nanophotonic mode dispersions in ultrathin, planar structures enables significant control over infrared perfect absorption (PA) and thermal emission characteristics. Here, using simulations, the wavelength and angular ranges over which ultrathin, low loss, epsilon-near-zero (ENZ) films on a reflecting surface most efficiently absorb and re-radiate are identified, and the design parameters that tailor the ENZ mode dispersion within these limits are investigated. While the absorption is spectrally limited to wavelengths where the refractive index ($n$) lies below unity, the angular limits are determined by the ENZ material dispersion in this range. A model of ultrathin-film interference is developed to provide physical insight into the absorption resonances in this regime, occurring well below the conventional quarter-wavelength thickness limit. Driven by non-trivial phase shifts incurred on reflection at the $n<1$ surface, these resonant interferences are shown to be universal wave phenomena in planar structures having appropriate index contrast, extending beyond ENZ materials. Selective choice of material, film thickness and loss allows fine-tailoring the mode dispersions, enabling wide variation in spectral range ($ \sim 0.1 - 1.0 \mu m$) and precise directional control of spectrally and angularly narrow-band PA and thermal radiation, paving the way towards efficient ENZ-based infrared optical and thermal coatings.