Optical limiting behavior of carbon nanotubes exposed to infrared laser irradiations studied by the Z-scan technique.

The optical limiting behavior of multiwalled carbon nanotube (MWCNT) powder in chloroform solution under CO(2) infrared laser irradiations is reported for, to our knowledge, the first time. Here we demonstrate the pure thermal origin of the optical limiting effect in the 160 ns time scale studied. The Z-scan technique appears to be an excellent tool for experimental evaluation of the nonlinear refractive index. An experimental model for the optical limiting behavior of MWCNT suspensions in chloroform is presented. The occurrence of a laser-induced thermal lens through the absorption of energy by the MWCNTs and subsequent heat transfer to the solvent, followed by solvent vapor bubble growth, is the main factor governing the observed drop in transmittance. Pump-probe experiments have been performed to obtain some quantitative estimation of both the rise and decay times of the thermal lensing phenomenon. It was found that the early probe signal decay, tau(1)=149 ns, was of the same order of magnitude as the rise time of the thermal lens, tau(r)=121 ns. When the nonlinear scattering was considered, a total decay time of tau=1.8 micros was obtained. A recovery time for the thermal lens of several tens of milliseconds has been experimentally determined, which is in good accordance with the theoretical value.