Broadband super-Planckian thermal emission from hyperbolic metamaterials

Abstract

Abstract:We develop the fluctuational electrodynamics of metamaterials with hyperbolic dispersion and show the existence of broadband thermal emission beyond the black body limit in the near field. This arises due to the thermal excitation of unique bulk metamaterial modes, which do not occur in conventional media. We consider a practical realization of the hyperbolic metamaterial and estimate that the effect will be observable using the characteristic dispersion (topological transitions) of the metamaterial states. Our work paves the way for engineering the near-field thermal emission using metamaterials.Engineering the black body thermal emission using artificial media promises to impact a variety of applications involving energy harvesting 1 , thermal management 2 and coherent thermal sources 3 . The usual upper limit to the black-body emission is not fundamental and arises since energy is carried to the far-field only by propagating waves emanating from the heated source. If one allows for energy transport in the near-field using evanescent waves, this limit can be overcome. Thus thermal emission beyond the black body limit is expected due to surface electromagnetic excitations 4 or at the edge of the bandgap in photonic crystals 5 where there is a large enhancement in the photonic density of states. Advances in near field scanning and probing techniques 6-8 have led to conclusive demonstrations of these effects.One limitation of the above mentioned approaches using photonic crystals or surface electromagnetic excitations is that the energy transfer beyond the black body limit (superplanckian 9,10 thermal emission) only occurs in a narrow bandwidth. In this paper, we show that artificial media (metamaterials) with engineered dielectric properties can overcome the limitation of super-planckian thermal emission at a single resonant frequency. Our work rests on the recently discovered singularity in the bulk density of states of metamaterials with hyperbolic dispersion [11][12][13] . The unique property which sets these hyperbolic metamaterials (HMMs) apart from conventional approaches of engineering the photonic density of states (PDOS) is the broad bandwidth in which the PDOS is enhanced.

Super-Planckian near-field thermal emission with phonon-polaritonic hyperbolic metamaterials

“…In general amorphous SiO 2 supports surface modes in the infrared as well as SiC, but to get results which are comparable with the calculations done in Ref. 30 we assume that ǫ SiO 2 = 3.9 adding a vanishingly small absorption. The optical properties of SiC are taken from Ref.…”

“…In particular, we will show that for the realization of a hyperbolic metamaterial as studied in Ref. 30 the main contribution to super-Planckian radiation is not necessarily due to hyperbolic modes but can be due to surface modes depending on the choice of the topmost layer. We will show that in order to allow for broad-band super-Planckian emission by hyperbolic modes, mainly, it is important to use a material for that topmost layer which does not support surface modes in the thermal freqeuency range.…”

“…Finally, Guo et al. have studied the energy density produced by the thermally fluctuating fields close to a hyperbolic structure and found a broadband near-field contribution from which they have concluded that super-Planckian emission will be broad-band for hyperbolic materials 30 . This is in accordance with the findings in Ref.…”

“…We choose to consider the structure in Ref. 30 which is made of layers of SiC and SiO 2 . In general amorphous SiO 2 supports surface modes in the infrared as well as SiC, but to get results which are comparable with the calculations done in Ref.…”

Enhanced energy transfer by near-field coupling of a nanostructured metamaterial with a graphene-covered plate

“…Recent studies on hyperbolic metamaterials (HMMs) [12][13][14][15][16][17][18][19][20], which exhibit large photonic local density of states, show the promise in enhancing the near-field radiative transfer by means other than resonant coupling, while the strong enhancement also requires matching hyperbolic behaviors for the emitter and receiver materials. It is still a challenge to greatly enhance the performance of near-field thermophotovoltaic (TPV) by either resonance coupling of SPP/SPhP or strong hyperbolic modes because of inherent mismatch in the dissimilar optical properties of the emitter and cell.…”