Tunable quasi-monochromatic near-field radiative heat transfer in s and p polarizations by a hyperbolic metamaterial layer

The spectra of thermal radiation have been controlled for thermophotovoltaics and mid-infrared light sources, and the spectral heat flux has been shown to exceed the blackbody limit by utilizing near-field coupling. We show that a hyperbolic metamaterial layer enables quasi-monochromatic near-field radiative heat transfer between a metallic emitter and a dielectric receiver. The quasi-monochromatic transfer originates from the Fabry-Perot resonance in the hyperbolic layer, where evanescent waves in the vacuum gap become propagative. The Fabry-Perot resonance is excited in s and p polarizations, and the resonant condition is almost independent of the lateral wavenumber due to the large effective parallel permittivity of the hyperbolic metamaterial. The resonant frequency is tuned by the volume filling fraction and the thickness of the layer, while the frequency misalignment between polarizations is kept small. Furthermore, the resonant frequency is shown to be robust to the fluctuation of the gap width and the refractive index of the receiver dielectrics. The hyperbolic metamaterial layer is applied to near-field thermophotovoltaic energy generation, and both the power output and the efficiency are enhanced simultaneously. Numerical simulation reveals that the hyperbolic metamaterial layer can be realized by a hexagonal cylinder array.