Macroscopically Aligned Carbon Nanotubes as a Refractory Platform for Hyperbolic Thermal Emitters

Nanophotonic thermal emitters with large photonic density of states (PDOS) have the potential to significantly enhance the efficiency of radiative cooling and waste heat recovery. Because of their nearly infinite PDOS, refractory hyperbolic materials make a promising material platform for thermal emitters. However, it is challenging to achieve a prominent PDOS in existing refractory hyperbolic materials, especially in a broad bandwidth. Here, we demonstrate macroscopically aligned carbon nanotubes as an excellent refractory material platform for hyperbolic nanophotonic devices. Aligned carbon nanotubes are thermally stable up to 1600 °C and exhibit extreme anisotropy: metallic in one direction and insulating in the other two directions. Such extreme anisotropy results in an exceptionally large PDOS over a broadband spectrum range (longer than 4.3 μm) in the mid-infrared, manifesting as strong resonances in deeply subwavelength-sized cavities. We demonstrate polarized, spectrally selective, thermal emission from aligned carbon nanotube films and indefinite cavities of volume as small as ∼λ3/700 operating at 700 °C. These experiments suggest that aligned carbon nanotubes enhance PDOS and hence also thermal photon density by over 2 orders of magnitude, making them a promising refractory nanophotonics platform.