Ultrafast and Large Third-order Nonlinear Optical Properties of CdS   Nanocrystals in Polymeric Film
  We report the ultrafast and large third-order nonlinear optical properties of CdS nanocrystals (NCs) embedded in a polymeric film. The CdS NCs of 2-nm radius are synthesized by an ion exchange method and highly concentrated in the two layers near the surfaces of the polymeric film. The two-photon absorption coefficient and the optical Kerr coefficient are measured with laser pulses of 250-fs duration at 800-nm wavelength. The one-photon and two-photon figures of merit are determined to be 3.1 and 1.3, respectively, at irradiance of 2 GW/cm2. The observed nonlinearities have a recovery time of ~ 1 ps. The two-photon-generated free carrier effects have also been observed and discussed. These results demonstrate that CdS NCs embedded in polymeric film are a promising candidate for optical switching applications. 
J. He, W. Ji, G. H. Ma, S. H. Tang, E. S. W. Kong, S. Y. Chow, X. H.   Zhang, Z. L. Hua, J. L. Shi