Study of magnetic polaritons in deep gratings for thermal emission control

Abstract

Recently, it has been shown that convex cavities or 2D grating structures can enhance thermal emission for energy conversion systems. The mechanisms, however, cannot be well explained by either the conventional cavity resonance modes or surface plasmon polaritons. The present study elucidates the fundamental mechanism by considering the excitation of magnetic polaritons (MPs) in deep gratings. Rigorous coupled-wave analysis (RCWA) is employed to calculate the radiative properties by solving Maxwell's equations numerically. The LC-circuit model is employed to predict the resonance conditions. The current and field distributions further confirm the excitation of magnetic resonances. It is shown that MPs and cavity modes agree with each other when the kinetic inductance is negligibly small. However, when the kinetic inductance is sufficiently large, the maximum resonance wavelength can be more than twice that predicted by the cavity mode. Furthermore, different materials are considered and the frequency range is extended from the near-infrared to the microwave region to illustrate the scalability of the MPs. This study clarifies one of the underlying mechanisms of optical resonance in deep gratings and will benefit the design of wavelength-selective thermal emitters.