Near- and far-field thermal radiation in plasmonic metamaterial emitters studied via fluctuation–dissipation FDTD

Renewed interest in near-field thermal radiation stems from its potential in energy harvesting, nanoscale thermal management, and thermophotovoltaics, driving efforts to understand and engineer its fundamental mechanisms. Plasmonic materials are well recognized for their ability to enhance fields proximal to their surfaces; however, detailed quantitative assessment of the field intensity and energy density of thermal radiation in the near field (relative to the far field) and potential practical impact is lacking. This work presents a detailed study of a prototypical metamaterial design supporting plasmon resonance in the mid-infrared spectrum. Significant intensification of thermal radiation in near field is investigated using a numerical finite-difference time-domain method that incorporates randomly fluctuating dipoles in accordance with the fluctuation-dissipation theorem that enables excitation of high-order spatial harmonics. Electromagnetic response of the metamaterial thermal emitter to both thermal and optical excitations is evaluated and compared. Findings indicate that the near field intensity at a mere 500 nm from the metamaterial surface exhibits an enhancement of up to 38-fold in near-field electromagnetic energy density relative to the corresponding far-field value at 4.8 μm lying near the edge of Brillouin zone under thermal excitation. In contrast, 6-fold enhancement is observed at 4.35 μm lying near the center of the Brillouin zone under optical excitation. Further, thermal excitation yields a 3.5-fold increase in peak intensity compared to conventional blackbody radiation at 2 μm away from the outermost layer of the metamaterial. Moreover, photothermal simulations further reveal rapid temperature rises exceeding 600 K within nanoseconds under low-power pulsed excitation. These results demonstrate that thermal excitation enables access to dark electromagnetic modes that dominate near-field radiation and highlight the importance of excitation mechanism in designing metamaterial-based thermal photonic devices.