Femtosecond optical harmonic generation as a non‐linear spectroscopic probe for carbon nanotubes

Abstract

Abstract 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 © 2003 John Wiley & Sons, Ltd.