Observation of Interband Two-Photon Absorption Saturation in CdS Nanocrystals

We report the observation of interband two-photon absorption (TPA) saturation in cadmium sulfide nanocrystals (CdS NCs) under intense femtosecond laser excitation with 1.6-eV photon energy. The observation has been compared to interband TPA saturation in bulk CdS under the same experimental conditions. By using both Z-scan technique and transient absorption measurement, the saturation intensity has been determined to be 190 GW/cm^2 for CdS NCs of 4-nm diameter, which shows two orders of magnitude greater than that for CdS bulk crystal. The results are in agreement with an inhomogeneously-broadened, saturated TPA model.