Nonlinear Terahertz Carrier Dynamics in Free-Standing Carbon Nanotubes

Carbon nanotubes (CNTs) have exceptional electrical and optical properties which have inspired unique applications in nanoscale optoelectronics. High speed electronics demands the studies of CNTs at THz frequencies, particularly in high-field regime. We present an experimental study on nonlinear anisotropic response of unidirectionally aligned multi-walled CNTs (MWCNTs). Free-standing MWCNTs form a quasi-one-dimensional semi-metallic structure and exhibit highly nonlinear THz responses when the THz polarization is parallel to the CNT axis, while no nonlinear THz effects are observed for the perpendicular polarization. Unlike a typical conducting medium in which strong THz pulses induce transparency, intense THz fields enhance absorption in MWNTs, which suggests that strong THz fields efficiently generate charge carriers in MWNTs. The experimental data of THz time-domain spectroscopy and the theoretical analysis based on Drude-Lorentz model indicates that intense THz pulses enhance permittivity in CNTs via carrier generation and multiplication. The nonlinear THz effects undergo an extraordinary transition when CNTs are optically excited. THz fields of the intermediate range (~600 kV/cm) suppress the mobility of the photocarriers in CNTs.