Ultrafast terahertz probes of interacting dark excitons in chirality-specific semiconducting single-walled carbon nanotubes.

Ultrafast terahertz spectroscopy accesses the dark excitonic ground state in resonantly excited (6,5) single-walled carbon nanotubes via internal, direct dipole-allowed transitions between the lowest-lying dark-bright pair state of ∼6  meV. An analytical model reproduces the response that enables the quantitative analysis of transient densities of dark excitons and e-h plasma, oscillator strength, transition energy renormalization, and dynamics. Nonequilibrium, yet stable, quasi-one-dimensional quantum states with dark excitonic correlations rapidly emerge even with increasing off-resonance photoexcitation and experience a unique crossover to complex phase-space filling of both dark and bright pair states, different from dense two- and three-dimensional excitons influenced by the thermalization, cooling, and ionization to free carriers.