Exciton dynamics and biexciton formation in single-walled carbon nanotubes studied with femtosecond transient absorption spectroscopy

We used femtosecond transient absorption (TA) spectroscopy to examine the excited state dynamics of single-walled carbon nanotube (SWNT) bundles embedded in polymer matrices. The SWNTs were excited by a femtosecond pump pulse centered at either 1800, 900, or 550 nm and probed using a white-light continuum extending from 400 to 750 nm. We observed a structured TA spectrum consisting of a series of narrow induced transmission (IT) and induced absorption (IA) bands. The TA spectrum, which is independent of excitation wavelength, appeared on a time scale shorter than our instrument response (200 fs) and persisted for up to 100 ps. TA spectra obtained at a series of pump-probe delay times provided a window through which to monitor the exciton dynamics. We observed three distinct spectral signatures in the time-dependent data: (1) the decay of a broad photobleach, (2) the biphasic decay of narrow IT and IA features, and (3) a dynamical spectral shift of IA bands. These processes were attributed to plasmon relaxation, electron-hole recombination, and lattice relaxation associated with exciton self-trapping, respectively. Analysis of the transient spectrum suggested that it arose from a nonlinear optical response of the SWNT, where excitons produced by the pump pulse modified the transition frequencies of subsequent carrier excitations. The result was a series of IT bands (bleaches) at the ground state absorption frequencies, and associated with each was a corresponding red-shifted absorption band. These induced absorptions were attributed to the formation of biexcitons, four-particle excitations that are produced through the sequential excitation of two closely spaced electron-hole pairs.