The refractory hyperbolic material described in the ACS Photonics journal under the title “Macroscopically Aligned Carbon Nanotubes as a Refractory Platform for Hyperbolic Thermal Emitters”, consists of stacks of highly aligned and closely packed single-wall carbon nanotubes in deeply subwavelength-sized cavities. The macroscopic alignement and stacking of the nanotubes is obtained through a simple vacuum filtration method the group reported back in 2016.
Due to the strong quantum confinement at play in the nanometer-scale features of the one-dimensional material, the researcher were able to exploit its extremely anisotropic electronic, optical and thermal properties. In fact, each stack of aligned CNTs behaves as an uniaxial anisotropic medium, highly conductive along the nanotubes’s main axis but insulating in the perpendicular plane. It was observed that when thermal photons hit the stacks (from any direction), they can only leave via one, turning IR frequencies from a broad spectrum into a narrow bandwidth IR radiation that would be more efficiently converted into electricity.
"Any hot surface emits light as thermal radiation," explains corresponding author Gururaj Naik. "The problem is that thermal radiation is broadband, while the conversion of light to electricity is efficient only if the emission is in a narrow band. The challenge was to squeeze broadband photons into a narrow band".