Near-field radiative heat transfer (NFRHT) between materials supporting surface polaritons in the infrared has potentially important applications in photonic thermal rectification, near-field thermophotovoltaics, noncontact refrigeration and thermal transistors. In contrast to far-field radiative heat transfer where only propagating waves make contribution to the heat flux, NFRHT is dominated by evanescent waves. Previous studies on NFRHT between two hyperbolic metamaterials have revealed that the near-field radiative heat flux can be greatly enhanced via hyperbolic phonon polaritons (HPPs). Regrettably, when the tangential wavevector component is larger than π/Λ, the hyperbolic properties do not hold any more. Consequently, this limits the application of metamaterials in NFRHT. Nonetheless, for natural hyperbolic materials, such as hexagonal boron nitride (hBN), since the lattice constant is on the order of sub-nanometer, such limitation on the wavevector for NFRHT is negligible. However, for hyperbolic biaxial crystals such as α-MoO3, the situation will become more complicated since such a material shows different optical responses in its three crystalline directions. Unfortunately, there exists minimal literature focusing on NFRHT between two α-MoO3 surfaces with the surfaces oriented in different crystalline directions.
Therefore, an important task is to reveal whether MoO3 can outperform hBN in achieving significantly enhanced NFRHF and how such a material can be used to control the NFRHF. On this account, Peking University researchers, Dr. Xiaohu Wu and Professor Ceji Fu, in collaboration with Professor Zhuomin M. Zhang at the Georgia Institute of Technology carefully investigated the NFRHT between two semi-infinite α-MoO3 biaxial crystals with the surfaces of the emitter and the receiver parallel to each other and separated by a vacuum gap. Their work is currently published in the Journal of Heat Transfer.
In their approach, the NFRHF was calculated when the surfaces of the emitter and the receiver were arranged perpendicular to each of the three crystalline directions of α-MoO3. The researchers also investigated the modulation of the NFRHF by controlling the relative rotation angle between the receiver and the emitter. Overall, in their study, the calculations were performed based on the fluctuation–dissipation theorem combined with the modified 4 x 4 transfer matrix method.
The research team found that by adopting the enhanced transmittance matrix approach, their method was capable of calculating the NFRHF between two homogeneous as well as stratified media efficiently while avoiding the problem of numerical overflow when dealing with evanescent waves. Specifically, the results showed that much larger heat flux than that between two semi-infinite hexagonal boron nitrides could be achieved in the near-field regime, and the maximum heat flux is along the  crystalline direction.
In summary, the study presented an in-depth numerical investigation of the NFRHT between two semi-infinite α-MoO3 biaxial crystals and the effect of the crystal orientation on the NFRHF. Remarkably, it was found that the NFRHF can be well controlled by changing the relative rotation angle. More so, the modulation contrast can be as large as two in the  direction. In a statement to Advances in Engineering, the authors said their findings provide a promising approach for manipulating near-field radiative transfer between anisotropic materials.
Xiaohu Wu, Ceji Fu, Zhuomin M. Zhang. Near-Field Radiative Heat Transfer Between Two α-MoO3 Biaxial Crystals. Journal of Heat Transfer, volume 142(7): 072802.