Effect of shell thickness on two-photon absorption and refraction of colloidal CdSe/CdS core/shell nanocrystals

Colloidal CdSe nanocrystals (NCs) are successively overcoated with CdS monolayers (MLs), and the effective two-photon absorption (TPA) coefficient and refractive index of this series of CdS/CdSe core/shell NCs are measured by a Z-scan technique at 800nm wavelength. The TPA and nonlinear refraction are enhanced dramatically with the CdS shell growth towards 3 MLs, but decreased with the further shell growth. The effects of surface states, local field, and intrinsic piezoelectric polarization are discussed to explain the optical nonlinearity of the colloidal core/shell NCs.