Near-field thermal radiation in time-varying systems via Fourier modal method

This study addresses the theoretical challenges in near-field thermal radiation regulation of time-varying media by proposing an extended framework of fluctuational electrodynamics based on the Fourier modal method. Through temporal Fourier decomposition of permittivity and scattering matrix chain operations, we construct an operator-based model suitable for high-frequency modulated systems, systematically resolving the nonreciprocal regulation mechanism of near-field thermal radiation under temporal modulation. Our analysis demonstrates that temporal modulation induces frequency conversion of surface polaritons, effectively creating multiple photon transport channels via sideband generation. The introduction of a phase difference between modulation layers breaks the time-reversal symmetry of photon propagation paths, leading to nonreciprocal heat transfer. Furthermore, by designing a multilayer structure that enhances the coupling between temporal and spatial modulation, we achieve a near-field thermal rectification coefficient of 0.5. The proposed theoretical framework provides a semi-analytical tool for investigating modal interactions in infrared time-varying media, opening avenues for nonmagnetic, actively controllable thermal management devices.