Resonant enhancement of third-order optical nonlinearities in single-walled carbon nanotubes

Third-order nonlinear optical response in single-walled carbon nanotubes (SWNTs) has been investigated by means of femtosecond pump–probe spectroscopy. The imaginary part of nonlinear susceptibility Im χ(3) measured under the nonresonant excitation conditions shows the resonant enhancement at each optical transition of the specific (n,m) tube in miscelle-suspended SWNTs. With the selective excitation of the E22 exciton transition for the (7,6) tube, the value of Im χ(3) is increased by an order of magnitude compared with the nonresonant excitation case. The nonlinear response signal at the E22 transition energy for the (8,7) tube has been also enhanced for the excitation at ∼0.22 eV above the E22 transition energy. This result is interpreted in terms of the phonon-assisted E22 transition for the (8,7) tube, indicating strong exciton–phonon coupling.