Accessing the Dark Exciton States in Semiconducting Single-Walled Carbon Nanotubes with Terahertz Pulses

Singled-walled carbon nanotubes (SWNTs) represent a model system to systematically investigate correlated charge excitation in 1-D limits. One of the most outstanding issues both in fundamental nanotube physics and for their technological development is to detect and understand optically-forbidden, dark collective states. Thus far supporting evidence of dark states has been demonstrated in static magneto-optics and light scattering. However, the unique internal transitions from dark excitonic ground states and their dynamic evolution remain highly elusive. We report our investigation of this problem using optical pump, terahertz probe spectroscopy of (6,5) and (7,5) SWNTs. We measure transient THz conductivity from 0.5-2.5 THz (2-10.5 meV) at low temperature down to 5 K with resonant and off-resonant excitation at the E$_{22}$ transitions of (6,5) and (7,5) nanotubes. These results reveal, for the first time, dynamics of lowest dark excitons and density-dependent renormalization of these many-particle states. The internal-excitonic spectroscopy with THz pulses represents a fundamentally new spectroscopy tools to study dark excitons and shine new lights on the correlation physics of excitonic ground states.