Direct measurement of the polarized optical absorption cross section of single-wall carbon nanotubes.

We determine optical absorption cross sections of single-wall carbon nanotubes for visible light copolarized and cross polarized with respect to the nanotube axis. The need for perfectly aligned ensembles in absorbance measurements is eliminated by using Raman scattering to measure the nematic order parameter in magnetically aligned nanotube suspensions. The absorbance data allow the first quantitative, spectral comparisons with theories of local field depolarization, and provide benchmark spectra for simple, rapid, and quantitative measurements of alignment within nanotube dispersions.