  In cavity quantum electrodynamics, optical emitters that are strongly coupled
to cavities give rise to polaritons with characteristics of both the emitters
and the cavity excitations. We show that carbon nanotubes can be crystallized
into chip-scale, two-dimensionally ordered films and that this new material
enables intrinsically ultrastrong emitter-cavity interactions: rather than
interacting with external cavities, nanotube excitons couple to the
near-infrared plasmon resonances of the nanotubes themselves. Our
polycrystalline nanotube films have a hexagonal crystal structure, ~25 nm
domains, and a 1.74 nm lattice constant. With this extremely high nanotube
density and nearly ideal plasmon-exciton spatial overlap, plasmon-exciton
coupling strengths reach 0.5 eV, which is 75% of the bare exciton energy and a
near record for room-temperature ultrastrong coupling. Crystallized nanotube
films represent a milestone in nanomaterials assembly and provide a compelling
foundation for high-ampacity conductors, low-power optical switches, and
tunable optical antennas.
