Ultrafast electron dynamics and enhanced optical nonlinearities of CdS-capped Au/BaTiO ₃ composite film

The third-order nonlinear optical responses of gold nanoparticles capped by CdS shells of different thicknesses embedded in BaTi O3 films were investigated by off-resonance femtosecond optical Kerr effect technology at 800 nm. The nonlinearities of the Au-CdS core-shell composite nanoparticles exhibit a significant enhancement, compared with their single components, and the χ(3) value apparently increases with the increase in the CdS shell thickness. The electric-field distribution in the composite nanoparticles was calculated using Neeve's theory [A. E. Neeves and M. H. Birnboim, J. Opt. Soc. Am. B 6, 787 (1989)] to explain the nonlinear enhancement mechanism, which was mainly attributed to the enhancement of the localized electric field in the CdS shell. The electron relaxation dynamics in the Au-CdS nanoparticles with a core-shell structure were found to be the same as that in pure gold nanoparticle systems using the time-resolved pump-probe technique measured at 800 nm.