Efficient ultrafast THz modulators based on negative photoconductivity in controllably doped carbon nanotubes

Large diameter single-walled carbon nanotubes films are shown to have a doping level and THz conductivity that can be readily controlled using adsorption doping or electrostatic gating. The giant negative photoconductivity observed upon optical excitation is due to a rapid reduction in mobile carrier density, synchronously to a lowering of the momentum scattering rate. The large modulation depth and tunable recovery time open prospects for carbon nanotube films in ultrafast and THz devices.