Energy Streamlines in Near-Field Thermal Radiation

In the present paper, we investigate the energy propagation direction in near-field thermal radiation between two semi-infinite surfaces separated by a vacuum gap. Based on the fluctuational electrodynamics, we demonstrate in detail that Poynting vectors for each parallel wavevector component (β) are decoupled due to the randomness of thermal radiation. The results reveal that the spectral radiative energy is transferred in infinite directions. By separately tracing the Poynting vector for each given β, the energy propagation direction in the vacuum gap is visualized. Depending on β values, there exist considerable lateral shifts of the energy streamline. The range of β values dominantly contributing to the spectral energy flux is identified for different wavelengths. Furthermore, the effect of surface polaritons on the lateral shift is also discussed.