Third-order optical nonlinearity of cadmium sulfide nanoparticles loaded in mesostructured silica materials

Cadmium sulfide nanoparticles have been successfully incorporated in three forms of CTAB-templated mesoporous silica materials: one is the mesoporous silica spheres suspended in ethanol solution, the other is the mesoporous silica spheres spin-coated on glass slide, and the third is the dip-coated mesoporous silica thin film. The mesostructures were characterized by XRD and TEM, respectively. Linear optical properties were investigated using UV-visible spectra, and the diameter of the incorporated CdS nanoparticles was measured to be around 3.1 nm. Z-scan technique manifested that these three composites exhibited distinct third-order optical nonlinearities due to the different preparation techniques. Reverse saturation absorption could be detected in the CdS-loaded mesoporous silica spheres suspended in solution, while those dispersed on glass slide presented saturation absorption. The difference in nonlinear absorption of the two mesoporous silica sphere samples could be attributed to defect-related transitions. On the contrary, the CdS-loaded mesoporous silica thin film showed self-defocusing behavior with no nonlinear absorption signals. Compared to that of the CdS nanoparticles with larger size previously reported, the intrinsic microscopic third-order nonlinear optical susceptibility of those incorporated in CTAB-templated mesoporous thin film was increased as predicted by the quantum theory, and the third-order optical nonlinearity was further determined to arise from intraband transitions induced by quantum confinement.