Abstract
In order to tailor thermophotovoltaic emitters to match specific photovoltaic receivers a spectrally selective emitter is required that have close to black body emission at short wavelengths and substantially reduced emission at long wavelengths. We propose a hyperbolic metamaterial for this purpose which changes its emission properties close to the topological transition of its isofrequency surface. At short wavelength the metamaterial has permittivity close to one and thus e±ciently absorbs and emits radiation. At longer wavelength, after the topological transition, the thermally excited hyperbolic modes have large wavevectors and cannot leave metamaterial due to total internal re°ection. To emit significant power at the wavelengths usable for photovoltaic conversion (below 2 ¹m) the far-field emitter should be heated to high temperatures and thus should be thermally stable. We demonstrate selective band-edge emitters based on a W-HfO2 layered metamaterial [1]. The thicknesses of tungsten and hafnium oxide are 20 and 100nm correspondingly. The metamaterial selectivity comes from the change in e®ective permittivity and does not rely on the phase matching condition. Thus the metamaterial exhibits almost angle independent selective emission. Stability up to 1000±C is demonstrated in vacuum conditions. At higher temperature residual oxygen in vacuum diffuses through HfO2 cap layer and oxidizes the upper W layer of the metamaterial
leading to degradation of the selective emission.
