Optimal azimuthal distribution of thermal emission for efficient radiative cooling

Directional control of thermal emission to enhance radiative cooling has recently attracted significant attention. However, the ideal emission directionality remains questionable and under-explored. This Letter employs a universal formalism to analyze typical emission patterns and their impact on radiative power, equilibrium temperature, and cooling power. Contrary to intuition, the optimal emission pattern is bipolar rather than unipolar toward the zero-degree zenith angle under conditions of reduced atmospheric transmittance and low non-radiative heat transfer coefficient, which coincides well with the intrinsic emission pattern of epsilon-near-zero (ENZ) optical metamaterials. This study lays a theoretical foundation for ENZ metamaterial applications in radiative cooling and demonstrates the potential for enhancing cooling through directional thermal emission.