Tight bounds and the role of optical loss in polariton-mediated near-field heat transfer

We introduce an analytical framework for near-field radiative heat transfer in bulk plasmonic and polar media. Considering material dispersion, we derive a closed-form expression for the radiative thermal conductance, which disentangles the role of optical loss from other material dispersion characteristics, such as the spectral width of the Reststrahlen band in polar dielectrics, as well as from the temperature. We provide a universal condition for maximizing heat transfer that defines the optimal interplay between a material's optical loss and polariton resonance frequency, based on which we introduce tight bounds to near-field heat transfer. With this formalism, one can quantitatively evaluate all polaritonic materials in terms of their performance as near-field thermal emitters

Experimental demonstration of tunable graphene-polaritonic hyperbolic metamaterial

Tuning the macroscopic dielectric response on demand holds potential for actively tunable metaphotonics and optical devices. In recent years, graphene has been extensively investigated as a tunable element in nanophotonics. Significant theoretical work has been devoted on the tuning the hyperbolic properties of graphene/dielectric heterostructures; however, until now, such a motif has not been demonstrated experimentally. Here we focus on a graphene/polaritonic dielectric metamaterial, with strong optical resonances arising from the polar response of the dielectric, which are, in general, difficult to actively control. By controlling the doping level of graphene via external bias we experimentally demonstrate a wide range of tunability from a Fermi level of E_F=0 eV to E_F=0.5 eV, which yields an effective epsilon-near-zero crossing and tunable dielectric properties, verified through spectroscopic ellipsometry and transmission measurements

Retrieval of material parameters for uniaxial metamaterials

We present a general method for retrieving the effective tensorial permittivity of any uniaxially anisotropic metamaterial. By relaxing the usually imposed condition of non-magnetic metal/dielectric metamaterials, we also retrieve the permeability tensor and show that hyperbolic metamaterials exhibit a strong diamagnetic response in the visible regime. We obtain global material parameters, directly measurable with spectroscopic ellipsometry and distinguishable from mere wave parameters, by using the generalized dispersion equation for uniaxial crystals along with existing homogenization methods. Our method is analytically and experimentally verified for Ag/SiO2 planar metamaterials with varying number of layers and compared to the effective medium theory. We also propose an experimental method for retrieving material parameters using methods other than ellipsometry.Comment: 17 pages, 9 figure

Thermodynamic performance bounds for radiative heat engines

This paper discusses the performance limits of heat engines exchanging heat radiatively with a hot source while in thermal contact with a cold sink. Starting from solar energy conversion models, we derive power-versus-efficiency upper bounds for both reciprocal and nonreciprocal radiative heat engines. We find that nonreciprocal engines may allow significantly better performance than reciprocal ones, particularly for low emitter temperatures or when operating close to Carnot efficiency. The results give valuable guidelines for the design and optimization of thermophotovoltaic systems.Comment: 6 pages, 5 figure

Mimicking surface polaritons for unpolarized light with high-permittivity materials

Tailoring near-field optical phenomena often requires excitation of surface plasmon polaritons (SPPs) or surface phonon polaritons (SPhPs), surface waves at the interface between media with electric permittivities of opposite sign. Despite their unprecedented field confinement, surface polaritons are limited by polarization: only transverse magnetic fields enable their excitation, leaving transverse electric fields unexploited. By contrast, guided modes in positive permittivity materials occur for both linear polarizations, however, they typically cannot compete with SPPs and SPhPs in terms of confinement. Here we show that omnipolarization guided modes in materials with high-permittivity resonances can reach confinement factors similar to SPPs and SPhPs, while surpassing them in terms of propagation distance. We explore the cases of silicon carbide and transition-metal dichalcogenides near their permittivity resonances, and compare with SPhPs in silicon carbide and SPPs in silver, at infrared and visible frequencies, respectively