Ultrafast terahertz photoresponse of single and double-walled carbon nanotubes: Optical pump-terahertz probe spectroscopy

Photocarrier excitation in conventional semiconductors enhances the conductivity due to increased intraband absorption of terahertz (THz) radiation. We report frequency dependent photoconductivity in terahertz range after 800 nm optical pump excitation in (6,5) semiconducting single-walled carbon nanotubes (SWCNT) and double-walled carbon nanotubes (DWCNT) containing both metallic and semiconducting tubes. The real and imaginary parts of photoconductivity (Delta sigma(omega)) show non-Drude behavior. In SWCNT, the real part Delta sigma(Re)(omega) is positive for low frequency and negative on the high-frequency side of the terahertz spectra. In contrast, DWCNTs show negative Delta sigma(Re)(omega) on low frequency and positive on the high-frequency side. This contrasting behavior is explained using Boltzmann transport theory, where the carrier scattering rate is energy dependent. Taking the scattering rate to be dominated by short-range disorder scattering, we show that the Boltzmann transport model captures the unique experimental features of Delta sigma(omega), for SWCNT as well as DWCNT. Both the semiconducting and metallic nanotubes in DWCNT are shown to contribute to the observed photoconductivity. (C) 2018 Published by Elsevier Ltd.