Coherent Phonon Anisotropy in Aligned Single-Walled Carbon Nanotubes

By time-resolved reflectivity measurements with sub-10 fs laser pulses at 395 nm, the coherent phonons of aligned bundles of single-walled carbon nanotubes are observed for various polarization directions of the pump and probe pulses. In the isotropic reflectivity measurement, we observe the radial breathing modes, G, and even D modes, while in the anisotropic reflectivity mode, only the G mode appears. A complex polarization dependence of the G band phonon amplitude in the isotropic reflectivity is explained by the superposition of G band phonons with different symmetries. Quasi-one-dimensional carbon nanotubes (CNTs), rolled-up graphene sheets, have attracted much attention because of their unique electronic and structural properties. Quantum confinement effects have significant influence on their physical properties, such as ballistic transport,1 exciton binding,2-4 and Kohn anomaly.5,6 For an isolated single-walled carbon nanotube (SWCNT) or aligned SWCNT bundle samples, the photoresponse strength in absorption,7 photoluminescence,8,9 photoconductivity,10 and polarize