Nonlinear photoluminescence from multiwalled carbon nanotubes

Visible photoluminescence from multiwalled carbon nanotubes (MWNT) was observed on excitation at 1064 nm. Strong nonlinear behavior of the photoluminescence was shown using power law dependence studies. The nonlinear response in MWNT was further investigated using degenerate four wave mixing. An ultrafast response was observed and the magnitude of the third order optical susceptibility, (chi) , was determined to be in the region of 1.2x10^-10 esu. Van Hove singularities in the density of states were identified for the first time in MWNT using optical absorption spectroscopy. Optical transitions between the singularities coincide with the spectral region of the photoluminescence. We propose that a multiphoton absorption process, followed by up conversion luminescence, is responsible for nonlinear photoluminescence in MWNT. Photoluminescence from graphitic particles (GP) was also investigated. This is shown to result mainly from thermal behavior and well-known optical centers. Blackbody radiation was observed in the near infrared region in both materials with MWNT exhibiting lower blackbody temperatures than graphite under the same irradiation conditions.