Terahertz Spectroscopy of Metallic Single-Wall Carbon Nanotubes

Optically generated, Coulombically bound electron-hole pairs, known as excitons, are rarely observed in metals due to strong electrostatic screening. However, in quantum-confined systems, such as one-dimensional (1D) single-wall carbon nanotubes (SWCNTs), screening effects are suppressed giving rise to exciton-dominated optical spectra in both semiconducting and metallic SWCNTs. Because of the difficulty of creating highly isolated 1D metallic environments, these metallic excitons are poorly studied. Here, we use terahertz absorption of single-chirality enriched SWCNTs at low temperatures to examine collective phenomena in 1D. We prepared, single-chirality (5,5) metallic and enriched (6,5) semiconducting SWCNTs in high-purity using aqueous two-phase extraction and characterized these fractions by optical methods. Enriched SWCNTs were immersed in a broadly transparent polymer matrix which preserved SWCNT individualization for cryogenic measurement. Using terahertz time-domain spectroscopy, we observed low-frequency plasmon absorption in metallic SWCNTs across a broad temperature range. This work provides the foundation for in-depth study of excitonic and plasmonic phenomena in single-chirality SWCNTs.