Controlling the directionality of emitted far-field thermal radiation is a fundamental challenge. Photonic strategies enable angular selectivity of thermal emission over narrow bandwidths, but thermal radiation is a broadband phenomenon. The ability to constrain emitted thermal radiation to fixed narrow angular ranges over broad bandwidths is an important, but lacking, capability. We introduce gradient epsilon-near-zero (ENZ) materials that enable broad-spectrum directional control of thermal emission. We demonstrate two emitters consisting of multiple oxides that exhibit high (>0.7, >0.6) directional emissivity (60° to 75°, 70° to 85°) in the p-polarization for a range of wavelengths (10.0 to 14.3 micrometers, 7.7 to 11.5 micrometers). This broadband directional emission enables meaningful radiative heat transfer primarily in the high emissivity directions. Decoupling the conventional limitations on angular and spectral response improves performance for applications such as thermal camouflaging, solar heating, radiative cooling, and waste heat recovery.

控制发射的远场热辐射的方向性是一项根本性的挑战。光子策略可以在较窄的带宽上实现热发射的角度选择性，但是热辐射是一种宽带现象。在宽带宽上将发射的热辐射约束到固定的窄角度范围的能力是重要但缺乏的能力。我们介绍了梯度ε接近零（ENZ）的材料，该材料可实现热辐射的广谱定向控制。我们演示了由多种氧化物组成的两个发射器，这些氧化物在一定波长范围内（10.0至14.3微米，7.7）在p极化中具有高（> 0.7，> 0.6）方向性发射率（60°至75°，70°至85°）。至11.5微米）。这种宽带定向发射主要在高发射率方向上实现了有意义的辐射热传递。取消对角度和光谱响应的常规限制，可以改善热伪装，太阳能加热，辐射冷却和废热回收等应用的性能。