Terahertz emission from vertically aligned multi-wall carbon nanotubes and their composites by optical excitation

Terahertz (THz) emission has been successfully observed from vertically aligned multi-wall carbon nanotube (VAMCNT) materials excited with polarized laser pulse in a reflection configuration. The effects of experimental parameters on THz generation are investigated systematically. The results indicate that the linear dependence of THz electric field on the pump power shows a typical second-order nonlinear optical effect and the transient photocurrent for generating THz waves is mainly originated from photon drag effect. By comparing with THz emission from polymer-coated VAMCNT composites, the detailed generation process of THz waves from VAMCNT materials is clarified: the free carriers in VAMCNT films could transport both along single nanotube and between the adjacent nanotubes, while free carriers in VAMCNT composite films transport mainly along the tube axis due to the isolated carrier transport between the adjacent nanotubes by the coated polymer. Thus, transient photocurrent for generating THz emission from VAMCNT composite films is only along the tube axis and its intensity depends on the component of the pump light electric field along the tube axis. In addition, THz emission from VAMCNT composites shows more regular response than that from pure VAMCNT films, which paves a way for stable performance of VAMCNT-based THz emitter. (c) 2018 Elsevier Ltd. All rights reserved.