Femtosecond pulses of a Cr:forsterite laser are used to study second-and third-harmonic generation in a layer of single-wall carbon nanotubes produced by low-velocity spraying. The harmonic amplitude in our experiments scales as (Ip)n as a function of the pump intensity Ip, with n=2 and 3 for the second and third harmonics, respectively. This scaling law holds up to pump intensities on the order of 1012W/cm2. The ratio of the maximum signal to the averaged background in the spectra of the second and third harmonics is estimated as 50 and 30, respectively. The second and third harmonics produced by a linearly polarized pump field are also linearly polarized, with their polarization vectors oriented along the polarization direction of the pump field. The capabilities of nonlinear-optical methods for structural and morphological analysis of carbon nanotubes are discussed, as well as ways to create solid-state carbon-nanotube generators of optical harmonic.

The growth of carbon nanotubes usually requires a catalyst (typically Ni, Co or Fe). Although these catalysts promote the growth of nanotubes, they also require complex purifying treatments to remove them, which damage and dope the final product. Alternative catalyst-free growth of nanotubes films, as needed for electronic applications, was achieved by laser-annealing of amorphous SiC in vacuum at a relatively low temperature. In this work, we present the main results obtained and give some relevant examples of such carbon systems. The formation mechanism of carbon nanotube films is also discussed.

The role played by symmetry properties of a carbon nanotube in the generation of second harmonic radiation is studied by using the formalism of irreducible representation of the symmetry point group. Experimental results of second harmonic generation in a carbon nanotube sample are discusses in view of the theoretical prediction obtained.

Second- and third-harmonic generation in single-walled carbon nanotube films is experimentally investigated with the fundamental 1064nm radiation from a Q-switched Nd:YAG laser. Measurements were performed both on commercially available carbon nanotubes and on samples of carbon nanotubes grown with a catalyst-free method. Third-harmonic generation was observed in both samples while only the sample grown with a catalyst-free method generated a second-harmonic signal. The quantum yield of second- and third-harmonic signal was proven to scale, respectively, with the second and third power of the pump pulse energy up to intensities of 109W∕cm2.

Non-linear optical response of single-wall carbon nanotubes, produced without catalyst, was investigated. Third-order susceptibility Degenerate Four Wave Mixing measurements have been performed on thick film of nanotubes deposited onto quartz substrate. Results are presented as measured in the nanosecond time scale at 532 nm. The second-order hyperpolarizability γ was estimated to be as large as 1.6×10−29 e.s.u.

We have investigated the third order nonlinear optical properties of carbon nanotubes thick films. Nanosecond Degenerate Four Wave Mixing (DFWM) measurements have been performed at 532 and 355 nm yielding effective χ(3) values of the order of 10−11 esu. and indicating the resonant nature of the excitation involved. The determination of the CNTs sample density by means of Scanning Electronic Microscope technique allowed to relate sample bulk properties like χ(3) to the single CNT second order polarizability coefficient. Copyright © 2003 John Wiley & Sons, Ltd.

Second- and third-harmonic generation in single-wall carbon nanotube layers produced by low-velocity spraying was studied experimentally with the use of amplified 75 fs pulses of Cr:forsterite laser radiation within the range of pump pulse intensities up to the breakdown threshold. Harmonic-generation processes are shown to be ideally suited for the non-linear spectroscopy and structure analysis of carbon nanotubes, in many ways supplementary to Raman spectroscopy. The second- and third-harmonic yields scale as quadratic and cubic functions, respectively, of the pump pulse energy up to pump intensities of 10(12) W cm(-2), indicating the perturbative regime of non-linear-optical interactions and suggesting a convenient calibration for the second and third harmonics employed as spectroscopic probes. Comparison of harmonic-generation data with absorption spectra of carbon-nanotube layers reveals an important role of quantum confinement effects in the non-linear coherent scattering of light from carbon nanotubes. Femtosecond pulses of Cr: forsterite laser radiation also generate the fifth optical harmonic at 251 nm in the glass substrate of carbon-nanotube samples, which can be employed for UV spectroscopy and subwavelength-resolution non-linear microscopy of carbon nanotubes. Copyright (C) 2003 John Wiley Sons, Ltd.