Third-order optical nonlinearities in thin films of CdS nanocrystals

The non-resonant nonlinear optical properties of CdS nanocrystals (NCs) emb edded in silica (i) and forming matrix-free (MF) almost close-packed films (ii) were studied using the Z-scan technique. Among the group (i), samples containing NCs covered with a HgS and CuS shell were investigated. The high est value of the nonlinear refractive index n(2) = 1.85 x 10(-6) cm(2)/W wa s measured for the MF films. This number exceeds by far the value typical f or bulk CdS and cannot be explained in terms of thermally induced nonlinear ity. Strong enhancement of local fluctuations of the electro-magnetic field in MF films where NCs form aggregatzs or clusters is suggested to explain this result. Within the group (ii), an enhancement of the nonlinearity was observed under pulsed irradiation for both CdS/HgS and CdS/CuS nanoparticle s. compared to CdS NCs without a shell.