Epsilon-near-zero enhancement of near-field radiative heat transfer in BP/hBN and BP/α-MoO3 parallel-plate structures

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Applied Physics Letters
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AIP Publishing LLC
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Black phosphorous (BP) is a well-known two-dimensional van der Waals (vdW) material with in-plane anisotropy and remarkable electronic and optical properties. Here, we comprehensively analyze the near-field radiative heat transfer (NFRHT) between a pair of parallel non-rotated BP flakes that occurs due to the tunneling of the coupled anisotropic surface plasmon polaritons (SPPs) supported by the flakes. It is demonstrated that the covering of the BP flakes with hexagonal boron nitride (hBN) films leads to the hybridization of the BP's SPPs with the hBN's hyperbolic phonon polaritons and to the significant enhancement of the NFRHT at the hBN's epsilon-near-zero frequencies. It is also shown that the NFRHT in the BP/hBN parallel-plate structure can be actively switched between the ON and OFF states by changing the chemical potential of the BPs and that the NFRHT can be modified by altering the number of the BP layers. Finally, we replace hBN with α-MoO3 and explore how the NFRHT is spectrally and strongly modified in the BP/α-MoO3 parallel-plate structure. We believe that the proposed BP/polar-vdW-material parallel-plate structures can prove useful in the thermal management of optoelectronic devices. Following the pioneering work of Polder and Hove,1 the near-field radiative heat transfer (NFRHT) has attracted considerable attention in the last two decades due to its promising applications in thermophotovoltaics,2,3 thermal rectification,4 electroluminescent cooling,5 thermal diodes,6 and transistors.7 While the propagating waves contribute to the far-field radiative heat transfer,8 the evanescent waves are responsible for the heat flux in the NFRHT—also referred to as photon tunneling. It is known that the NFRHT is considerably enhanced due the excitation of surface polaritons.9,10 The efficiency of the NFRHT can exceed the blackbody limit by several orders in magnitude via the resonant coupling of surface plasmon polaritons (SPPs) in structures based on metals,11,12 doped Si,1–14 and surface phonon polaritons in heat transfer devices composed of SiO2,15 Al2O3,16 and SiC.17,18 Moreover, it was shown that due to the thermal excitation of isotropic graphene SPPs, NFRHT between two closely spaced parallel-plates of graphene can be strongly mediated, enhanced, and tuned via the modification of the chemical potential of graphene in the infrared range.19–22 Black phosphorous (BP) is often used as an anisotropic plasmonic van der Waals (vdW) material for the realization of enhanced NFRHT.23,24 It has a tunable bandgap, ranging from 1.51 eV for a monolayer BP to 0.59 eV for a five-layer BP, and a thickness-dependent anisotropic absorption coefficient.25 The latter feature implies that, unlike graphene, the SPPs of BP exhibit anisotropic behavior.26 Moreover, it has been reported that the thermal conductivities of BP along the zigzag and armchair directions are three orders of magnitude lower than that of graphene at 300 K.27 This makes BP a better candidate than graphene for the heat management via NFRHT. Recent studies have revealed that the NFRHT is greatly enhanced by the out-of-plane hyperbolic plasmon polaritons or phonon polaritons (HPPs) of metamaterials28–32 as well as by the in-plane modes of the graphene-based33,34 or BP-based35 metasurfaces. However, the dependence of the maximum wavenumber of HPPs on the size of the unit cell and the associated fabrication complexity (due to electron beam lithography or several film deposition processes) of the hyperbolic metamaterials or metasurfaces challenge their practical realization for the NFRHT purposes. Natural hyperbolic vdW materials, such as hexagonal boron nitride (hBN)36–38 and α-MoO3,39,40 have also been used to enhance the NFRHT. It has been demonstrated that the NFRHT can be mechanically tuned in twisted hBN films37 or actively modulated in graphene-hBN heterostructures.41–43 Because α-MoO3 has different optical responses along its three crystallographic directions, a similar mechanical modulation of the NFRHT is observed for the twisted films of α-MoO3 with differently aligned surfaces.40 Despite a great deal of recent studies on this topic, the active modulation of enhanced NFRHT in non-rotated epsilon-near-zero BP/hBN and BP/α-MoO3 parallel-plate structures has not been analyzed so far to the best of our knowledge. In the present paper, we comprehensively analyze the NFRHT between two parallel non-rotated BP flakes covered with hBN films [Fig. 1(b)]. It is found that the coupling of the anisotropic plasmons of BP with the hyperbolic phonons of hBN considerably enhances the NFRHT near the edges of the Reststrahlen bands (RBs) of hBN, where hBN exhibits the epsilon-near-zero (ENZ) feature. We demonstrate the possibilities of the active and passive tunings of the NFRHT through changing the chemical potential and altering the number of layers of the BP flakes. It is also shown that the replacement of hBN with α-MoO3 allows one to strongly manipulate the spectrum of the NFRHT in the structure.

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