Third-Order Nonlinear Optical Properties of Aqueous Silver Sulfide Quantum Dots

Wide spectral wavelength range (500-1600 nm) measurements of nonlinear optical properties of silver sulfide (Ag₂S, with 2- or 3-mercaptopropionic acid, 2 or 3MPA ligands) quantum dots (QDs) in aqueous colloidal solutions were performed using the Z-scan technique with tunable ∼55 fs laser pulses at 1 kHz. We have identified regions of the occurrence of various NLO effects including two-photon absorption, nonlinear refraction, as well as saturation of one-photon absorption. At the same time, we evaluated the relationship between the properties of the QDs and the variation of the material that covers their surface. The peak two-photon absorption cross section (σ₂) values were determined to be 632 ± 271 GM (at 850 nm) for Ag₂S-2MPA QDs and 772 ± 100 GM (at 875 nm) for Ag₂S-3MPA QDs. The physicochemical factors influencing the three-dimensional self-organization of Ag₂S QDs in water as well as their impact on spectroscopic properties were also investigated.