Intense terahertz pulse induced exciton generation in carbon nanotubes.

We have investigated the highly nonlinear terahertz (THz) light-matter interaction in single-walled carbon nanotubes (SWNTs). The high-peak THz electric-field (∼0.7 MV/cm) and the low effective mass of carriers result in their ponderomotive energy exceeding the bandgap energy of semiconducting SWNTs. Under such an intense THz pulse irradiation, the interband excitation that results in the generation of excitons occurs, although the THz photon energy (∼4 meV) is much smaller than the gap energy of SWNTs (∼1 eV). The ultrafast dynamics of this exciton generation process is investigated by THz pump and optical probe spectroscopy. The exciton generation mechanism is described by impact excitation process induced by the strong THz E-field. Such intense THz pulse excitation provides a powerful tool to study nonlinear terahertz optics in non-perturbative regime as well as nonlinear transport phenomena in solids with ultrafast temporal resolution.