Near-Field Radiative Heat Transfer Between Two α-MoO3 Biaxial Crystals

The near-field radiative heat transfer between two semi-infinite a-MoO3 biaxial crystals is investigated numerically based on the fluctuation-dissipation theorem combined with the modified 4?4 transfer matrix method in this paper. In the calculations, the near-field radiative heat flux along each of the crystalline directions of a-MoO3 is obtained by controlling the orientation of the biaxial crystals. The results show that much larger heat flux than that between two semi-infinite hexagonal boron nitride can be achieved in the near-field regime, and the maximum heat flux is along the [001] crystalline direction. The mechanisms for the large radiative heat flux are explained as due to existence of hyperbolic phonon polaritons (HPPs) inside a-MoO3 and excitation of hyperbolic surface phonon polaritons (HSPhPs) at the vacuum/a-MoO3 interfaces. The effect of relative rotation between the emitter and the receiver on the heat flux is also investigated. It is found that the heat flux varies significantly with the relative rotation angle. The modulation contrast can be as large as 2 when the heat flux is along the [010] direction. We attribute the large modulation contrast mainly to the misalignment of HSPhPs and HPPs between the emitter and the receiver. Hence, the results obtained in this work may provide a promising way for manipulating near-field radiative heat transfer between anisotropic materials.