Anomalous photocarrier dynamics in free-standing carbon nanotubes driven by strong terahertz pulses

We demonstrate that free-standing multi-walled CNTs exhibit extraordinary nonlinear THz responses when the CNTs are optically excited. Strong THz pulses induce nonlinear absorption in unexcited CNTs due to the carrier generation via interband tunneling and impact ionization, where the field-induced absorption monotonically increases as the field gets stronger. The THz transmission of optically excited CNTs is, however, not monotonically decreasing as the field strength increases. The trend is reversed in the intermediate range of the THz field strength between 550 and 650 kV/cm, where the CNTs become more insulating. The transition is abrupt and the change in transmission is very large. There are two possible underlying mechanisms. One is that the field-induced band bending gives rise to a misalignment between the conduction and valence bands, preventing interband tunneling in this intermediate range, while the strong THz fields enhance the carrier-carrier scattering and lower the carrier mobility. Field-enhanced charge recombination is also possible, where strong electric fields mediate the Coulomb interaction between electronhole pairs and enhance charge recombination under non-equilibrium conditions.