Tailoring Infrared Absorption and Thermal Emission with Ultrathin Film Interferences in Epsilon‐Near‐Zero Media

Abstract

Engineering modal dispersions of ultrathin, planar structures enables significant control over infrared perfect absorption (PA) and thermal emission characteristics. Herein, the optical response of ultrathin, low loss, epsilon‐near‐zero (ENZ) films on reflecting surfaces is simulated to investigate the wavelength and angular ranges over which they absorb and emit radiation most efficiently and identify the design parameters that tailor the ENZ mode dispersion of the system. A generic interference model is shown to elucidate the underlying physics of these ultrathin film absorption resonances, occurring well below the conventional quarter‐wavelength thickness limit. While the absorption is spectrally limited to wavelengths where the ENZ film is optically a dielectric with refractive index below unity, the angular spread is delimited by the material dispersion. Further, these resonant interferences are understood to arise from universal wave phenomena, realizable in simple planar structures having appropriate refractive index contrast, not restricted to ENZ materials. The results show that appropriate choice of material, film thickness and loss allows tailoring the modal dispersions, which enables precise directional control and wide tunability in spectral range (width ≈0.1–1.0 μm) of PA and thermal emission, paving the way towards efficient ENZ‐based infrared optical and thermal coatings.

Epsilon-Near-Zero Metal Oxide-Based Spectrally Selective Reflectors

“…Consequently, ENZ systems are transparent for λ < λ ENZ and become reflective progressively at longer wavelengths. Furthermore, they sustain the nonradiative ENZ modes around λ ENZ , and ultrathin layers (thickness ∼10 nm) evidence perfect absorption under p -polarized illumination . Spectrally, perfect absorption is realized at wavelengths λ PA ≲ λ ENZ and is coupled with excitation of the ENZ modes accompanied by extreme field amplification and confinement .…”

“…Furthermore, they sustain the nonradiative ENZ modes around λ ENZ , and ultrathin layers (thickness ∼10 nm) evidence perfect absorption under ppolarized illumination. 16 Spectrally, perfect absorption is realized at wavelengths λ PA ≲ λ ENZ and is coupled with excitation of the ENZ modes accompanied by extreme field amplification and confinement. 17 Here, we show that the high reflectivity (R → 1) for λ > λ ENZ , together with perfect absorption (R → 0) around λ ENZ and plasmonic resonances in ITO nanostructures (NSs), enables a niche application of ENZ media in developing spectrally selective coatings of subwavelength dimensions.…”

Epsilon-near-zero characteristics of near-field radiative heat transfer between α-MoO3 slabs

“…42 Moreover, thinfilm media at the ENZ frequency exhibit some unique properties, including various nonlinear optical effects and nearly zero dispersion. [43][44][45] These ENZ frequencies take place when the real part of the permittivity crosses zero (Re[e] = 0). The permittivity of some natural hyperbolic materials follows the Drude model or a Lorentz dispersion relation, and these materials generally exhibit the real part of epsilon to be zero at several specific frequencies.…”