Intrinsically ultrastrong light-matter interactions in crystalline films of carbon nanotubes

When optical emitters are strongly coupled to optical cavities, light and matter mix and lose their distinct characteristics. To date, strong light-matter coupling has only been observed in hybrid systems, where cavities and emitters are separate objects. Here, we show that carbon nanotubes can be crystallized into chip-scale superlattices and that this new material enables intrinsically ultrastrong light-matter interactions: rather than interacting with external cavities, nanotube excitons couple to the near-infrared Fabry-Perot plasmon resonances of the nanotubes themselves. Our crystallized nanotube films have a hexagonal crystal structure, ~25 nm domains, and a 1.74 nm lattice constant. This extremely high nanotube density allows plasmon-exciton coupling strengths to reach 0.5 eV, which is 75% of the bare exciton energy and a record for light-matter interactions at room temperature. Crystallized nanotube films provide a compelling foundation for high-ampacity conductors, low-power optical switches, tunable metamaterials, and thresholdless nanolasers.