Near-field thermal diode based on 2D gratings

Abstract

This paper proposes a near-field thermal diode (NFTD) based on VO2 and cBN 2D gratings to improve the rectification ratio. Different from previous studies, this paper is the first to verify the positive effect of 2D gratings on NFTD. The exact calculations are based on rigorous coupled wave analysis (RCWA). After optimization, the rectification ratio of the 2D grating-based NFTD reaches the maximum R = 29.5 at filling factor f = 0.04 and gap distance d = 100 nm, which is nearly four times larger than the optimal result of the plain-based one. The reason for the improvement of rectification ratio is that the reverse spectral heat flux is weakened by more than an order of magnitude in nearly the whole spectrum, while the forward spectral heat flux is influenced little in the hyperbolic regions. Besides, the lattice period P = 100 nm and grating height h = 1 μm are feasible solutions for symmetric configurations. The validity of effective medium theory (EMT) for calculating 2D grating-based NFTD is also investigated. The EMT is consistent with RCWA in predicting the optimal filling factor and grating height at d = 100 nm, but is invalid in predicting the optimal gap distance and the maximum rectification ratio. The longitudinal asymmetric configuration is further proposed by changing the structural parameters. When the size ratios of cBN grating and VO2 grating are selected as 0.6 and 0.1, respectively, the rectification ratio is better than the symmetric configuration due to the broadening of the hyperbolic region during forward bias. After optimizing grating height, the rectification ratio further reaches 54. Our study deepens the understanding of the rectification mechanism of 2D grating-based thermal diodes and paves the way for potential applications of thermal diodes.