Broadband thermal beaming using multiple epsilon near zero materials

Controlling the directionality of an absorptive or thermally emissive material is a fundamental challenge in contemporary materials and photonics research. Over the past two decades, a range of photonic strategies have enabled angular beaming of thermal emission, but only over narrow sets of bandwidths. However, thermal radiation is inherently a broadband phenomenon, and many emerging applications in imaging and energy require broad spectrum control of emission and absorption. We currently lack the ability to constrain thermal emission, as well as absorption, over arbitrary angles of incidence, but consistently over a broad range of wavelengths. Here, we theoretically propose and experimentally demonstrate a mechanism for broadband angular selectivity in absorption and emission that leverages multiple materials that exhibit a permittivity near zero, ENZ materials. We experimentally realize a broadband directional thermal emitter by introducing a subwavelength photonic film consisting of multiple oxides that exhibit epsilon near zero (ENZ) regions in the long-wave infrared region. We obtain high emissivity (> 0.6) in the p polarization from 7.7 to 11.5 micron over an angular range between 65 deg and 85 deg. Outside this range the photonic film is highly reflective. The broadband nature of angular control enables strong radiative heat transfer only at large angles and is directly observed through thermal imaging. The materials and photonic strategy employed use abundant, low-cost materials that are fully compatible with large-area deposition. By decoupling conventional limitations on angular and spectral response, our approach opens new possibilities for radiative heat transfer in a range of applications, including thermal camouflaging, solar heating, radiative cooling and waste heat recovery.