Electrically Tunable Excitonic-Hyperbolicity in Chirality-Pure Carbon Nanotubes.

Abstract

Metamaterials exhibiting hyperbolic dispersion achieve a large degree of control over light-matter interactions, from subdiffraction imaging to enhanced spontaneous emission. However, conventional plasmonic hyperbolic metamaterials suffer from limited tunability and lack intrinsic emission capabilities, constraining their utility for active photonic devices. Here, we demonstrate a room-temperature, electrically tunable, excitonic hyperbolic metamaterial using aligned films of chirality-pure semiconducting carbon nanotubes. Unlike plasmonic systems, these excitonic metamaterials of aligned nanotubes combine strong optical anisotropy with dynamic electrostatic tunability. Spectroscopic ellipsometry reveals that the hyperbolic dispersion window can be electrically shifted by 53 meV, enabling real-time switching between hyperbolic and elliptical regimes. Theory predicts that this tunability translates to the propagation angle being modulated by 34°, driven by a momentum enhancement 3.11 times that of free space, limited primarily by material losses that can be mitigated through improved alignment. In addition, simulations of the system exhibit a high Purcell factor of 1550 and a modulation of 37% without an optical cavity for a dipole placed 5 nm above the aligned nanotubes. These findings establish excitonic carbon nanotubes as a versatile platform for dynamically reconfigurable photonic metamaterials such as adaptive optical devices, electrically controlled spontaneous emission, and tunable hyper-lenses operating at room temperature.