Excitation of surface plasmon polaritons helps to increase the near-field heat flux by orders of magnitude beyond the limit governed by Stefan−Boltzmann law. However, the photon tunneling probability is rather low, except for modes satisfying the resonance condition of surface plasmon polaritons. Broadband hyperbolic metamaterials can broaden the frequency region for the enhancement of nearfield heat transfer, but can hardly maintain a high tunneling probability for large wavevectors since no resonances are excited to overcome the inherent exponential decay. In this letter, perfect photon tunneling with near-unity probability across broad frequency and k-space region is demonstrated based on the hybridization of graphene plasmons and hyperbolic modes. As a result, the near-field heat transfer coefficient between doped-silicon-nanowire hyperbolic metamaterials can be further improved several fold when covered by a graphene sheet, approaching to a theoretical limit.

During the past decade, research on carbon nanotubes has revealed potential advances in thermal engineering applications. The present study investigates the radiative absorption and reflection of vertically aligned carbon nanotubes (VACNTs) in the broad spectrum from the near-infrared to far-infrared regions. The optical constants of VACNT are modeled based on the dielectric function of graphite and an effective medium approach that treats the CNT film as a homogenized medium. Calculated radiative properties show characteristics of near-unity index matching and high absorptance up to around 20 μm wavelength. The packing density and degree of alignment are shown to affect the predicted radiative properties. The Brewster angle and penetration depth of VACNTs are examined in the infrared spectrum. The radiative properties for VACNT thin films are also evaluated, showing some reduction of absorptance in the near-infrared due to transmission for film thicknesses less than 50 μm. This study provides a better understanding of the infrared behavior of VACNT and may guide the design for its applications in energy harvesting, space-borne detectors, and stealth technology.

Due to the negative coefficient of thermal expansion of graphene, temperature changes of graphene-coated photonic surfaces could induce resonant mode shifts in diffractive optical absorptance and emission. This study focuses on the modification of optical properties through folding, or “origami,” of graphene covering a plasmonic metal channel grating. This work is especially critical to understanding tailored deep plasmon emission from geometrically-modulated conducting sheets such as graphene. Conformational changes in graphene on gratings are found to tailor cavity resonance emission and plasmonic oscillations such as magnetic polaritons (MPs) and surface plasmon polaritons (SPPs), respectively. Up to 46% reduction in radiative absorptance was observed through retarded MP. Excited SPP modes can increase narrowband absorptance of 0.5 through folding of graphene. Tailoring of optical absorptance can be used for applications such as photodetectors and thermal emitters.

It has been shown that at small separation distances, thermal radiation between hyperbolic metamaterials is enhanced over blackbodies. This theoretical study considers near-field radiation when graphene is covered on the surfaces of two semi-infinite vertically aligned carbon nanotube (VACNT) arrays separated by a sub-micron vacuum gap. Doped graphene is found to improve photon tunneling in a broad hyperbolic frequency range, due to the interaction with graphene-graphene surface plasmon polaritons (SPP). In order to elucidate the SPP resonance between graphene on hyperbolic substrates, vacuum-suspended graphene sheets separated by similar gap distances are compared. Increasing the Fermi energy through doping shifts the spectral heat flux peak toward higher frequencies. Although the presence of graphene on VACNT does not offer huge near-field heat flux enhancement over uncovered VACNT, this study identifies conditions (i.e., gap distance and doping level) that best utilize graphene to augment near-field radiation. Through the investigation of spatial Poynting vectors, heavily doped graphene is found to increase penetration depths in hyperbolic modes and the result is sensitive to the frequency regime. This study may have an impact on designing carbon-based vacuum thermophotovoltaics and thermal switches.

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