Using hyperbolic media for efficient near-field radiative heat transfer

Hyperbolic materials (HMs), whose components of the permittivity tensor have opposite signs, can excite hyperbolic phonon polaritons in a wide frequency range, paving a way to control light. However, most HMs studied previously are artificial structures constructed with periodically stacked subwavelength metallic and dielectric layers, whose hyperbolic properties are limited by the tangential wavevector component. In comparison, the lattice constants of natural HMs are sub-nanometer in size, there is no need to consider this limitation. In this chapter, we investigated the near-field radiative heat transfer (NFRHT) between 2D natural HMs, including hBN and α-MoO3, whose excellent 2D properties can be obtained by mechanical exfoliation. The near-field radiative heat flux is calculated using the fluctuation-dissipation theorem and the modified 4 × 4 transfer matrix method. Numerical results show that the NFRHT between 2D natural HMs can be significantly enhanced in the hyperbolic region. Moreover, we pointed out the regions in the wavevector space where volume-confined hyperbolic polaritons (VHPs) and surface-confined hyperbolic polaritons (SHPs) can exist and proved that VHPs and SHPs excited in natural HMs is the main reason for the large radiative heat flux. In particular, we discussed the essential role of natural HMs in enhancing NFRHT, considering the effects of optical axis orientation, film thickness, and material types. We believe this chapter will open a novel path for the research on NFRHT and are expected to be applied to next-generation high-efficiency energy conversion devices.