Coupling into Hyperbolic Carbon-Nanotube Films with a Deep-Etched Antenna Grating

Macroscopic films of aligned and packed carbon nanotubes (CNTs) are known to act as broadband hyperbolic materials in the infrared, but methods for efficiently coupling light with high-k modes supported by such materials are currently absent. Here, we describe a deep antenna grating structure fabricated by directly etching a thick wafer-scale film of densely aligned CNTs. This novel architecture displayed hyperbolic dispersion relations in an ultrabroadband infrared spectral range, with an epsilon near zero point tunable via doping and annealing. Using systematic finite element analysis, we obtained the key geometrical and optical parameters that determine the absorption and emission efficiency of the structure, clarifying the important role of cavity mode generation in the deep-etched grating via confined hyperbolic plasmon polariton excitation. While addressing the question of how to satisfy coupling requirements and further enhance the light-matter interaction strength, we found that properly designed deep grating grooves should allow optical coupling with arbitrarily thick hyperbolic metamaterials, suggesting a strategy for large-scale applications.