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.

Monitoring the gold nanoshell growth mechanism: stabilizing and destabilizing effects of PEG-SH molecules.

Plasmonic nanoshells have attracted significant interest due to their resonant optical properties providing excellent spectral tunability, promising for various biophotonic applications. In this work we discuss our experimental and theoretical results related to the synthesis and optical characterization of surface-modified gold nanoshells. The nanoshell growth mechanism is monitored by IR spectroscopy, and the effects of modification of the gold nanoshell surface by PEG-SH ((11-mercaptoundecyl)tetra(ethylene glycol)) molecules are studied using TEM and optical methods. A red shift of localized surface plasmon resonance is observed upon formation of a layer of PEG-SH molecules on the completed gold nanoshells. Uncompleted gold shells show tendency to detach from the spherical silica cores, and the underlying destabilizing mechanism is discussed. The experimentally measured optical extinction properties are in good agreement with the results of numerical simulations, which additionally shed light on the localized plasmon modes contributing to the extinction, as well as on the effects of nanoshell surface nonuniformity on the resonant plasmonic properties and local field enhancements.