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• The NFRHT between α -MoO 3 films with different substrate permittivities is studied. • The substrate effect on the NFRHT highly relies on film thickness and gap distance. • Anisotropic hyperbolic polaritons can explain the underlying physical mechanism. • This study provides guidance for designing near-field thermal radiation devices. Near-field radiative heat transfer (NFRHT) has promising prospects in modern nanotechnology, such as near-field thermal microscopy, nanoscale non-contact thermal management, and information processing. Experimentally, supported substrates are crucial in ensuring structural stability, especially in ultrathin structures. However, the effect of the substrate on the NFRHT has seldom been explored. Here, the NFRHT between α -MoO 3 films with different permittivities of substrate is studied. For lossless substrates, the NFRHT is suppressed as the permittivity of the substrate increases when the heat transfer is along the [010] and [100] crystal directions of α -MoO 3. When the NFRHT is along the [001] crystal direction of α -MoO 3 , high-permittivity substrates suppress the NFRHT when the film is thin (< 10 nm), while enhancing the NFRHT in thicker films (> 10 nm). Moreover, we find that the effect of the substrate on the NFRHT highly relies on film thickness. The effect of lossy substrate on NFRHT is also discussed. We find that the loss of the substrate is more significant for the enhancement of heat flux at a small gap distance (20 nm). However, when the gap distance is large (100 nm), the excessive loss suppresses the NFRHT. Hyperbolic polaritons (HPs) can effectively explain the above phenomenon, as confirmed by energy transmission coefficients distribution and dispersion relation in wavevector space. In particular, the volume-confined hyperbolic polaritons play a dominant role in the flux variation between the thin films (1 nm and 10 nm). For the 100-nm film, surface-confined hyperbolic polaritons are more important. This study sheds light on the effect of substrate on HPs and provides theoretical guidance for designing near-field thermal radiation devices. [ABSTRACT FROM AUTHOR]

• Enhancement and modulation of NFRHT in α -MoO 3 three-body NPs system is studied. • An enhancement ratio of over 20,000 is achieved. • The physical mechanism is attributed to the high directionality of the LHPs. • This study demonstrates the potential of anisotropic HPs in NFRHT modulation. Anisotropic hyperbolic polaritons (HPs) provide novel opportunities to control light at nanoscale because of their promising ability of subdiffractional confinement, low losses, and polarization sensitivities. However, it has not been explored in the modulation of many-body near-field radiative heat transfer (NFRHT). In this work, we investigate the NFRHT in an α -phase molybdenum trioxide three-body nanoparticles (NPs) system. The results show that the near-field radiative conductance (NFRHC) can be significantly enhanced by adding the third NP between two NPs due to the excitation of the localized hyperbolic polaritons (LHPs). The enhancement effect can be actively modulated by rotating the intermediate NP, which is attributed to the high directionality of the LHPs. In addition, the effects of interparticle distance and vertical displacement on the NFRHC are also studied. We achieved an enhancement ratio of over 20,000 when the interparticle distance and vertical displacement is 4 times and 1.2 times the radius, 38 times higher than that of previous works. This work demonstrates the potential of anisotropic HPs in NFRHT modulation within the many-body system. [ABSTRACT FROM AUTHOR]

• We investigate the radiative heat transfer between stacked structures consisting of two different hyperbolic materials in the near-field and far-field. • In the far-field, the heat flux is suppressed by covering a bulk hyperbolic material with different hyperbolic film. • The near-field radiative heat flux decreases by 80% when bulk α -MoO 3 is covered with a 20-nm hBN film, while a 4-fold enhancement of heat flux is reached when bulk hBN is covered with a 20-nm α -MoO 3 film. • This work will guide us in designing the emitter and receiver based on hyperbolic materials when considering the radiative heat transfer in both near-field and far-field. Hyperbolic materials have attracted much attention in nanophotonics and thermal radiation because they can support high-wavevector modes. The near-field radiative heat transfer between single hyperbolic materials has been studied extensively, however, the radiative heat transfer (RHT) between multilayer hyperbolic materials in both near-field and far-field has seldomly been investigated. In this paper, we investigate the RHT between stacked structures consisting of two different hyperbolic materials in the near-field and far-field, taking uniaxial hBN, and biaxial α -MoO 3 for instance. The influence of the arrangement and thicknesses of the two materials on the RHT have been fully studied. The numerical results demonstrate that the RHT is dominated by the substrate material of the stacked structures in the far-field, and the contribution of the hyperbolic film is limited. However, in the near-field, the RHT is dominated by the hyperbolic film. Besides, when the substrate material is hBN, the addition of α -MoO 3 will greatly enhance the RHT. When the substrate is α -MoO 3 , the addition of hBN will be greatly suppressed. The performance of RHT in different cases is attributed to the coupling of hyperbolic polaritons supported by hBN and α -MoO 3 , which is verified by investigating the energy transmission coefficients. Our results in this work may guide us in designing the emitter and receiver based on hyperbolic materials when considering the RHT in both near-field and far-field. [ABSTRACT FROM AUTHOR]

• The HTC between α -MoO 3 films along different crystal directions is studied. • The HTC and MC are strongly related to the crystal direction. • The excitation and mismatch of HPhPs can explain the underlying physical mechanism. • This work paves novel way to design non-contact thermal modulators based on HMs. Near-field radiative heat transfer (NFRHT) has great potential in thermal management and energy harvesting due to the giant radiative heat flux. Modulation of NFRHT is another important research direction, where relative rotation between the emitter and receiver is a typical method. However, the enhancement of modulation contrast (MC) is mainly concerned in previous works, while large heat flux is not taken into account. In this work, we investigate the heat transfer coefficient (HTC) between α -MoO 3 films along different crystal directions of α -MoO 3. We take account of the modulator with high heat flux and high MC. The results show that the HTC and MC can reach 223.2 W/(m2·K) (36 times of blackbody limit) and 6.33 at a gap distance of 100 nm when NFRHT is along [010] crystal direction of α -MoO 3 , respectively. These are attributed to the excitation and mismatch of hyperbolic phonon polaritons (HPhPs). Furthermore, when NFRHT is along [100] and [001] crystal directions of α -MoO 3 , MCs reach 5.84 and 2.38, respectively. The results are strongly related to the crystal direction. In addition, the impact of film thickness on the MC for different crystal directions is investigated. This work is conducive to the applications of hyperbolic materials in non-contact thermal modulators. [ABSTRACT FROM AUTHOR]

• The near-field radiative heat transfer between multilayer structures composed of different hyperbolic materials is investigated. • The total heat flux between multilayer structures is enhanced compared to that between hBN (19-fold) and α -MoO 3 (1.46-fold) films. • The underlying physical mechanism is that vacuum layers promote the strong coupling of hyperbolic phonon polaritons supported by different hyperbolic materials. Near-field radiative heat transfer (NFRHT) has drawn significant interest in the recent years, including thermal management and energy harvesting. The NFRHT between single hyperbolic materials (HMs) has been thoroughly investigated. However, the research on the NFRHT between periodic multilayer structures composed of different HMs is rarely discussed. Moreover, the coupling effect of hyperbolic phonon polaritons (HPPs) supported by different HMs is still unclear. In this work, we investigated the NFRHT between multilayer structures composed of hexagonal boron nitride (hBN) and α -phase molybdenum trioxide (α -MoO 3), separated by vacuum layers. The influence of the gap distance, the unit-cell number, the thickness of the HMs and thickness of the vacuum layer on the NFRHT are discussed. The numerical results show that the maximal total heat flux (THF) between six-cell multilayer structures is 19 times compared to the THF between hBN films at the gap distance of 50 nm and is 1.46 times compared to the THF between α -MoO 3 films at the gap distance of 30 nm. Such enhancement can also be found at other similar gap distances. The vacuum layer promotes the coupling of HPPs supported by different HMs, which can be elucidated by the energy transmission coefficients. This work could benefit the application of near-field thermal radiative devices based on HMs. [ABSTRACT FROM AUTHOR]