Third-order optical nonlinearities in thin films of CdS nanocrystals

The non-resonant nonlinear optical properties of CdS nanocrystals (NCs) embedded 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 highest value of the nonlinear refractive index n₂ = 1.85 × 10^-6 cm²/W was measured for the MF films. This number exceeds by far the value typical tor bulk CdS and cannot be explained in terms of thermally induced nonlinearity. Strong enhancemem of local fiuctuations of the electro-magnetic field in MF films where NCs form aggregates 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 nanoparticles, compared to CdS NCs without a shell.