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Near-field thermophotovoltaics (NF-TPV) offers the potential for achieving elevated power density and conversion efficiency by leveraging the amplification of thermal radiation within a nanoscale gap. Here, we propose an NF-TPV device with a sandwich emitter composed of calcite film and graphene layer. The results show that this sandwich configuration can significantly enhance output power, outperforming monolayer-graphene-covered heterostructures and the single calcite film. These are because the sandwich configuration can enhance hybrid polaritons, which are formed by the coupling between surface plasmon polaritons in graphene and hyperbolic phonon polaritons in calcite. In addition, the effects of graphene chemical potentials on the performance of NF-TPV devices are also studied. The tunable power density range of the sandwich structure can be up to 3.26 times that of other structures by altering the chemical potential of graphene. The findings presented here may unpack a promising path for enhancing and manipulating the performance of NF-TPV at the nano- and microscale. [ABSTRACT FROM AUTHOR]

Near-field radiative heat transfer (NFRHT) between multilayer graphene/hBN heterostructures has been demonstrated to exceed the blackbody limit due to the coupling mechanism of surface plasmon polaritons and hyperbolic phonon polaritons, opening the door to applications in thermal management, thermophotovoltaics, and nanoscale metrology. Recent studies have shown that adding vacuum layers within multilayer structures can effectively promote surface modes and thus enhance NFRHT. However, the influence of vacuum layers on NFRHT between multilayer graphene/hBN heterostructures has not been investigated. Moreover, the influence of vacuum layers on coupled resonance modes excited in multilayer structures is worth discussing. In this work, we study the NFRHT based on multilayer graphene/vacuum/hBN/vacuum structures. The results show that as the gap distance increases from 20 nm to 100 nm, the NFRHT of three-cell and six-cell configurations is enhanced, while that of unit-cell configuration is suppressed. The potential mechanism can be identified as the excitation of surface plasmon-phonon polaritons (SPPPs) and hyperbolic plasmon-phonon polaritons (HPPPs) in multilayer structures. The enhancement factor of the six-cell configurations is up to 4.82 when the gap distance is 80 nm. Moreover, the influences of the chemical potential of graphene and the layer thickness on the NFRHT are discussed. The interesting results in this work indicate the perspectives for future near-field research involving coupling of SPPPs and HPPPs, and shed new light on high-performance devices introducing vacuum layers based on near-field radiative heat transfer. [ABSTRACT FROM AUTHOR]

Coupling of surface plasmon polaritons (SPPs) supported by graphene and hyperbolic phonon polaritons (HPPs) supported by hyperbolic materials (HMs) could effectively promote photon tunneling, and hence the radiative heat transfer. In this work, we investigate the polariton hybridization phenomena on near-field radiative heat transfer (NFRHT) in multilayer heterostructures, which consist of periodic graphene/α-MoO3 cells. Numerical results show that increasing the graphene/α-MoO3 cells can effectively enhance the NFRHT when the vacuum gap is less than 50 nm, but suppresses the enhanced performance with larger gap distance. This depends on the coupling of SPPs and HPPs in the periodic structure, which is analyzed by the energy transmission coefficients distributed in the wavevector space. The influence of the thickness of the α-MoO3 film and the chemical potential of graphene on the NFRHT is investigated. The findings in this work may guide designing high-performance near-field energy transfer and conversion devices based on coupling polaritons. [ABSTRACT FROM AUTHOR]

Near-field radiative heat transfer (NFRHT) could surpass the blackbody limit defined by Stefan-Bolzmann's law by several orders of magnitude, which has potential applications in thermal switching, thermal management, and photovoltaics. To further develop the NFRHT from theory to application, the substrate, which could enhance the stability of the structure, is a critical factor not to be ignored. However, the substrate effect on the NFRHT is still rarely discussed. In this work, we investigate the NFRHT between hexagonal boron nitride (hBN) films with different permittivities of the substrate. Results demonstrate that when the thickness of the film is 1 nm, increasing the permittivity of the substrate will suppress the NFRHT. In contrast, when the thickness of the film is larger (>2 nm), the high-permittivity substrate could enhance the NFRHT. The spectral heat flux (SHF) corresponding to substrates with different permittivities was investigated. The SHF in Type I hyperbolic band of hBN increases with the increase in the permittivity of the substrate, while that in Type II hyperbolic band is completely opposite. This competitive relationship leads to the above-mentioned phenomenon of NFRHT. The underlying physics mechanism can also be explained by the hyperbolic phonon polaritons (HPPs), which are analyzed by the energy transmission coefficients and dispersion relations. The findings in this work will deepen the understanding of the substrate on HPPs and pave a novel way for near-field radiation devices based on hyperbolic materials. [ABSTRACT FROM AUTHOR]

• The near-field radiative heat transfer between BP/α-MoO 3 heterostructures is studied. • The anisotropic polariton hybridization generates topological reconfiguration and transition. • The heat flux between multilayer heterostructures is further enhanced. • This work provides guidance for designing near-field thermal radiation devices. The effect of polariton hybridization in different materials have been proved to enhance near-field radiative heat transfer (NFRHT). However, the hybrid effect between anisotropic polaritons has not been studied yet. Here, we investigate the NFRHT between two black phosphorus (BP)/α-MoO 3 heterostructures. Compared with the individual BP and α-MoO 3 configurations, the NFRHT between heterostructures is significantly enhanced and can reach twice that of individual BP sheets. We find that the polariton hybridization in heterostructure exhibits complementary and competitive mechanisms, which leads to significant topological reconfiguration of hybrid polaritons, and modulates the intensity of photon tunneling. With the increase of α-MoO 3 film thickness, we observe that the topological structure of hybrid polaritons will undergo a transition of quasi-elliptic, quasi-hyperbolic and multiple resonance states. Moreover, we analyze the NFRHT between multilayer heterostructures, as the periodic cell number increases, the hybrid polaritons of multilayer heterostructures also exhibit obvious topological transitions, resulting in nearly twice the enhancement of NFRHT. Our work reveals the abundant phenomena of photon tunneling reconfiguration caused by anisotropic polariton hybridization and the accompanying enhancement of NFRHT, which may benefit the applications of near-field thermal management. [ABSTRACT FROM AUTHOR]

Spinning thermal radiation has demonstrated applications in engineering, such as radiation detection and biosensing. In this paper, we propose a new spin thermal radiation emitter composed of the twisted bilayer α-MoO3 metasurface; in our study, it provided more degrees of freedom to control circular dichroism by artificially modifying the filling factor of the metasurface. In addition, circular dichroism was significantly enhanced by introducing a new degree of freedom (filling factor), with a value that could reach 0.9. Strong-spin thermal radiation resulted from the polarization conversion of circularly polarized waves using the α-MoO3 metasurface and selective transmission of linearly polarized waves by the substrate. This allowed for extra flexible control of spinning thermal radiation and significantly enhanced circular dichroism, which promises applications in biosensing and radiation detection. As a result of their unique properties, hyperbolic materials have applications not only in spin thermal radiation, but also in areas such as near-field thermal radiation. In this study, hyperbolic materials were combined with metasurfaces to offer a new idea regarding modulating near-field radiative heat transfer. [ABSTRACT FROM AUTHOR]

• 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]

Isotope engineering, which has received much attention for significantly reducing the optical loss of phonon polaritons, opens new doors for thermal radiation manipulation. Recent studies have shown that the shift and broadening of phonon lines caused by isotopic mass effects significantly affect near-field radiative heat transfer. Near-field thermophotovoltaics (NF-TPV) is an efficient thermal-electric energy conversion device based on radiative heat transfer at the nano/micro scale. However, the impact of isotopes on NF-TPV remains to be further studied. Here, we have theoretically studied the performance of NF-TPV systems with hBN thermal emitters of distinct isotope compositions. The numerical results show that the 98.7% 10B hBN isotope enrichment configuration reaches 67.8 kW/m2 output power and 34.7% efficiency when the emitter temperature is 900 K, surpassing the natural hBN by 8.3 kW/m2 and 1.20%. Nevertheless, the performance is suppressed in 99.2% 11B hBN structure, reduced by 1.8 kW/m2 and 0.64% compared to natural hBN. This is attributed to the isotope-induced shift and broadening of the region of hyperbolic phonon polaritons. This study can provide theoretical guidance for designing NF-TPV systems based on isotope engineering and has great application prospects in renewable energy and energy storage fields. • The impact of isotope on performances of NF-TPV systems with hBN thermal emitters is investigated. • The 98.7% 10B hBN shows the best performance (67.8 kW/m2 and 34.7%), surpassing other isotope composition devices. • The varied performance is due to the isotope-induced shift and broadening of the region of HPhPs. • This study provides theoretical guidance for designing NF-TPV systems via isotope engineering. [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]