Enhancing Near-Field Radiative Heat Transfer with Si-based Metasurfaces

Abstract

We demonstrate in this work that the use of metasurfaces provides a viable strategy to largely tune and enhance near-field radiative heat transfer between extended structures. In particular, using a rigorous coupled wave analysis, we predict that Si-based metasurfaces featuring two-dimensional periodic arrays of holes can exhibit a room-temperature near-field radiative heat conductance much larger than any unstructured material to date. We show that this enhancement, which takes place in a broad range of separations, relies on the possibility to largely tune the properties of the surface plasmon polaritons that dominate the radiative heat transfer in the near-field regime. DOI: 10.1103/PhysRevLett.118.203901 Thermal radiation is one of the most ubiquitous physical phenomena. In recent years, there has been a renewed interest in this topic due to the confirmation of the prediction that radiative heat transfer can be drastically enhanced for bodies separated by small gaps [1,2]. This occurs when the gap is smaller than the thermal wavelength (9.6 μm at room temperature), and it is due to the contribution of evanescent waves that dominate the near-field regime. The fact that this near-field radiative heat transfer (NFRHT) can overcome the far-field limit set by the Stefan-Boltzmann law has now been verified in a variety of experiments exploring different materials, geometrical shapes, and gaps ranging from micrometers to a few nanometers [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. These experiments have also triggered off the hope that NFRHT could have an impact in different thermal technologies [18] such as thermophotovoltaics [19], heat-assisted magnetic recording [20,21], scanning thermal microscopy [22][23][24], nanolithography [25], thermal management [26,27], or coherent thermal sources [28,29].In this context, the question on the fundamental limits of NFRHT is attracting a lot of attention [30]. So far, the largest NFRHT enhancements in extended structures have been reported for polar dielectrics (SiC, SiO 2 , SiN, etc.), in which the NFRHT is dominated by surface phonon polaritons (SPhPs) [31,32]. There has not been any report of an extended structure that has a heat transfer coefficient exceeding that between two planar polar dielectric surfaces, and that includes metamaterials like hyperbolic ones [33,34]. In an attempt to tune NFRHT, several calculations of NFRHT between periodic metallic nanostructures in both 1D [35][36][37][38] and 2D [39] have been reported. These calculations have shown some degree of tunability and a NFRHT enhancement over the corresponding material without nanostructuration. However, the reported NFRHT in these structures is still smaller than in the case of parallel plates made of polar dielectrics. There have also been theoretical studies of the NFRHT between photonic crystals and periodic metamaterials made of dielectrics [40][41][42] that show how the radiative properties can be enhanced with respect to the bulk counterpart. However, the resulting NFRHTs are agai...

Fundamental Limits to Radiative Heat Transfer: The Limited Role of Nanostructuring in the Near-Field

“…Finally, we compare the power spectrum Φ planar × d 2 /A associated with identical planar films [6,12] to the exact and Born bounds in Fig. 3, specifically considering gold (Au), doped silicon (Si), and silicon carbide (SiC) as representative materials, as well as to the largest heat transfer observed in specific nanostructured Au [16] and Si [7] surfaces studied in the past. (We employ Drude dispersions for Au [16] and Si [7], and a phonon polaritonic dispersion for SiC [17].)…”

“…3, specifically considering gold (Au), doped silicon (Si), and silicon carbide (SiC) as representative materials, as well as to the largest heat transfer observed in specific nanostructured Au [16] and Si [7] surfaces studied in the past. (We employ Drude dispersions for Au [16] and Si [7], and a phonon polaritonic dispersion for SiC [17].) In particular, in the infrared where the Planck function is considerable (at typical experimental temperatures, T 1000 K), Φ Born for all of these materials is significantly larger than the corresponding Φ opt and is highly sensitive to material dispersion; as a specific example, the Born bound for Au lies significantly above the upper limits of the plot over the entire range of frequencies shown.…”

“…However, after accounting for the effects of such frequency shifts, the degree to which the spectrum Φ at a given frequency can be enhanced remains an open question. The inability of trial-and-error explorations and optimization procedures [6,7] to saturate prior bounds on Φ based on modal analyses [8][9][10][11] or energy conservation [12] suggests that these prior bounds may be too loose.…”

Fundamental limits to radiative heat transfer: Theory

“…While near-field RHT between dipolar objects can attain these bounds in a dilute limit, such a universal scaling has yet to be observed in largearea structures. This naïvely suggests room for improvement in Φ through nanostructuring via enhancements in the density of states or equivalently, via saturation of modal contributions, yet trial-and-error explorations and optimization procedures [34,35] have failed to produce nanostructured geometries that bridge this gap, leading to the alternative possibility that existing bounds are too loose.In this paper, we derive new algebraic bounds on RHT, valid in the near-, mid-, and far-field regimes, through analysis of the singular value decompositions of relevant response quantities. In contrast to prior limits, our bounds incorporate constraints imposed by material losses and multiple scattering, and are therefore tighter; moreover, they are formulated to be independent of object shapes while simultaneously accounting for finite size effects.…”

Active control of near-field radiative heat transfer through nonreciprocal graphene surface plasmons

“…To visualize the contribution of the nanostructure to the near-field radiative heat transfer, Figure 2(a) presents the spectral HTC between two graphene sheets with a lower chemical potential of μ = 0 eV at vacuum gap distance of d = 10 nm with different drift-current velocity vd from 0 to 0.9 vf. This spectral HTC is defined as the HTC per unit of frequency or photon energy [4]. Notice that the maximum of spectral HTC increases drastically with the drift-current velocity, reaching a maximum at vd = 0.9 vf.…”