Optical third-harmonic generation from an array of aligned carbon nanotubes with randomly distributed diameters

The optical susceptibility for third-harmonic generation (THG) from an array of aligned carbon nanotubes (CNs) is theoretically studied, taking into account the diameter distribution of CNs. The average THG susceptibility is calculated for three different types of distribution of CNs: Gaussian, rectangular and triangular ones. The results clearly demonstrate a strong broadening, intensity decrease, and red shift of the main three-photon resonant peak in the THG spectrum. It is shown that, in spite of the overall suppression of the average THG intensity, the amplitude of the peak strongly enhances with an increase of the average radius (R-av) of CNs in a sample achieving record values up to similar to 10(-4) e.s.u. at R-av = 5.2 nm for a typically occurring normal distribution of CNs. The enhancement of the height of the peak is accompanied by the shift of the top of the peak towards lower frequencies. Both the resonance frequency and magnitude of enhancement are found to be dependent on the type of distribution of CNs. Our results thus indicate the possibility of using THG measurement as an experimental tool to extract information about the prevailing type of CN distribution in a sample. (C) 2001 Elsevier Science B.V. All rights reserved.