Nonlinear Optical Spectroscopy of Isolated Single-Walled Carbon Nanotubes

Single-walled and multi-walled carbon nanotubes (SWNTs) and (MWNTs) are low dimensional conductors with unique electronic and electro optic properties. To probe these properties, we investigate the third-order, coherent anti-Stokes (CAS) response of individual SWNTs using a dual color, four-wave-mixing technique. Despite being nanoscale objects much smaller than the wavelength of light, the CAS response allows single SWNTs to be optically imaged with good spatial resolution using picosecond infrared laser excitation pulses and a diffraction limited, confocal microscopy technique. The absence of a pronounced Raman signature shows that the CAS response is dominated by electronic rather than vibrational dynamics. The dependence of the CAS signal on fundamental morphological properties of nanotubes is investigated and characterized. The nonlinear signal is found to depend strongly on nanotube diameter and polarization angle while the dependence on length and individual nanotube chirality is much less clear. The CAS signal is also strongly dependent on metallicity, allowing for straightforward optical discrimination between metallic and semiconducting SWNTs. However, variability of undetermined origin still exists in the CAS signal intensity from a given nanotube type. This variability may depend on electrical contact resistances, charge carrier densities, and local physiochemical effects. Furthermore, identification of the third-order CAS response in SWNTs requires proper accounting for other nonlinear luminescence mechanisms. The SWNT and MWNT optical response is found to contain contributions from competing multiphoton processes, including third-order vibrational and electronic mechanisms as well as second-order fluorescence. The relative intensity of these nonlinear processes is investigated and compared.