Subpicosecond coherent nonlinear optical response of isolated single-walled carbon nanotubes

We report an excitation-energy dependence of ultrafast changes of exciton absorptions in isolated semiconducting single-walled carbon nanotubes. For a photoexcitation far below the exciton transition, a blueshift of exciton absorptions originating from the optical Stark effect due to exciton-photon coupling was observed. Under near-resonant excitation conditions, a broadening of exciton absorptions was discriminated from a blueshift signal in the time and frequency domains. The broadening can be attributed to virtual exciton-real exciton scattering, which is considered to be characteristic of strongly confined excitons in nanotubes.