Third order nonlinear optics in Ag nanocubes: local and nonlocal optical responses as a function of excitation wavelength and particle size.

Colloidal silver nanocubes in solution were analyzed by the Z-scan technique in the resonant and non-resonant regimes. Three different systems with particle sizes of 56, 99 and 215 nm, and concentrations of 0.378, 0.831 and 0.715 mg/mL, respectively, were obtained by the polyol method. Nonlinear excitation of the samples was performed with laser pulses of 26 ps, at a repetition rate of 10 Hz, and using three different wavelenghts (355, 532 and 1064 nm), exciting only the electronic part of the optical nonlinearity, and avoiding induced thermal loading of the samples. Whenever observable, samples showed saturable absorption for all wavelengths, which was dependent, in general, on concentration and incident intensity. For samples featuring sizes of 55 and 99 nm, saturable absorption could be observed for wavelengths close to their dipolar surface plasmon resonances; while, for samples with sizes of 99 and 215 nm, saturable absorption and positive nonlinear optical refraction (only for size of 215 nm) could be observed at 1064 nm. Besides, for some samples, nonlinear optical response followed the incident intensity profile at wavelengths close to resonance. On the contrary, for wavelengths out of resonance, the nonlinear optical behavior showed to be nonlocal, ie, its profile was narrower than the incident one.

Third-order nonlinear optical properties of colloidal Au nanorods systems: saturable and reverse-saturable absorption.

In this work, we present a study of the nonlinear absorption properties from different gold nanorod (NR) systems in aqueous suspension. The NRs were obtained with the bottom-up protocol by the seed-mediated growth method (SMG), using Ag(+) ions at different concentrations, and CTAB as surfactant. By using this method, aspect ratios between 2 and 5 were obtained. The transverse surface plasmons (TSP) are located between 514 - 535 nm, while the longitudinal surface plasmons (LSP) are between 639 - 921 nm, for the different samples studied. The Z-scan technique was implemented for open (OA) and closed (CA) aperture at 532 and 1064 nm, with laser pulses of 26 ps, for vertical and horizontal polarizations, with respect to the incidence plane (horizontal). At 532 nm all samples showed saturable absorption (SA), while for samples with LSP near 1064 nm, such effect was observed only at low-energy pulse experimental conditions. In the high-energy pulse regime, an apparent reverse-saturable absorption (RSA) was observed for both wavelengths. However for 532 nm, it was possible to determine that this effect results from structural changes in the samples, which are manifested through the behavior of nonlinear absorption and refraction curves. These results were used to determine the irradiances to which NRs can be modified by photodegradation.