Metamaterials operating at frequency ranges in which the dielectric permittivity is close to zero have been discussed for use across a wide range of optical applications.
Sean Molesky and colleagues from the University of Alberta in Canada, have now proposed methods for engineering thermally excited far-field electromagnetic radiation using epsilon-near-zero metamaterials. In the same paper, the researchers also introduce epsilon-near-pole metamaterials, where there selected wavelength is that corresponding to the resonance pole, rather than the wavelength that gives zero permittivity.
The researchers showed that these concepts may be useful for high-temperature applications such as capturing lost thermal energy in photovoltaic and other energy-conversion devices. In particular, they claim that photovoltaic devices with metamaterial emitters near temperatures of 1,500 K may surpass the Shockley–Queisser efficiency limit of 41%. They propose two metamaterial structures that should be able to be fabricated using current technology. One structure consists of simple layers of metal and dielectric films, and the other is a two-dimensional array of metallic nanowires in a dielectric matrix.
The main advantages of thermal emitters based on these metamaterials include omnidirectional thermal emission, narrowband emissivity and polarization insensitivity. Importantly, the epsilonnear-zero and epsilon-near-pole emitters also function in reverse as highly effective thin absorbers.