Photo-induced absorption in pump probe spectroscopy of single-walled carbon nanotubes

Femtosecond pump probe spectroscopy is employed to study the photo-induced absorption feature in single-walled carbon nanotube transient spectrum. The two advantages of the experiment, a chirality enriched sample and tuning the pump wavelength to the resonance of a specific nanotube species, greatly facilitate the identification of the photo-induced absorption signal of one tube species. It is found that a photo-induced absorption feature is located at one radial breathing mode to the blue side of the E₁₁ state. This finding prompts a new explanation for the origin of the photo-induced absorption: the transition from the ground state to a phonon coupled state near the E_ii state. The explanation suggests a superposition mechanism of the photo-bleach and photo-induced absorption signals, which may serve as a key to the interpretation of the complex pump probe transient spectrum of carbon nanotubes. The finding shed some light on the understanding of the complex non-radiative relaxation process and the electronic structure of single-walled carbon nanotubes.