Exciton dynamics in single-walled nanotubes: transient photoinduced dichroism and polarized emission

Ultrafast relaxation of photoexcitations in semiconducting single-walled carbon nanotubes (S-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed thin NT films and D2O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. The PA and PB decay dynamics as a function of time, t, follow a power law, std−a with a in the range of 0.7 to 1. Whereas the PA bands in S-NTs in solution uniformly decay, the PB bands, in contrast, have different decay dynamics across the spectrum, which originates from an ultrafast spectral shift. Nevertheless the dynamics of the PA and PB bands for NTs in solution are the same when the spectral shift is accounted for, indicating a common origin. In addition S-NTs in D2O solution show polarized PL emission bands in the mid infrared spectral range that follow almost exactly the infrared absorption peaks of the isolated NTs, as well as their transient PB spectrum.