Plasmon resonance in carbon nanotubes is an exciting resource for confining light to the nanometer scale . In this talk, I will discuss how dense films of carbon nanotubes have emergent optical properties related to these resonances. We assemble our films with a simple vacuum filtration technique and find that optimal assembly leads to the nanotubes crystallizing into monolithic, two-dimensionally ordered films. Using Mueller-matrix ellipsometry, we then characterize the films’ optical properties and find a broadband, electrostatically tunable hyperbolic region in the mid-infrared . Another fascinating property of dense nanotube films is that excitons in the nanotubes can couple to the nanotubes’ own plasmon resonances – that is, cavity quantum electrodynamics (cQED) without an external cavity . In fact, this coupling strength can reach the ultrastrong light-matter interaction regime. Technologically, crystallized nanotube films could provide a materials-driven pathway from fundamental concepts in cQED to advanced telecommunications hardware, including tunable thermal emitters, super-resolution imaging systems, and active, nonlinear optical devices like modulators and nanolasers.  Falk et al, “Coherent plasmon and phonon-plasmon resonances in carbon nanotubes,” Phys. Rev. Lett. 118, 257401 (2017).  Roberts et al, “Tunable Hyperbolic Metamaterials Based on Self-Assembled Carbon Nanotubes,” Nano Lett. 19, 3131 (2019).  Ho et al, “Intrinsically ultrastrong plasmon–exciton interactions in crystallized films of carbon nanotubes,” Proc. Nat. Acad. Sci. 115, 12662 (2018).