Nonressonant nonlinear optical switching behavior of Ag monometallic and Ag@Au bimetallic investigated by femtosecond Z-Scan measurements

Abstract

Metallic nanoparticles are of great interest in technological applications, especially in photonics. Here, we describe the nonlinear optical properties of colloidal solutions of monometallic silver (AgNPs) and bimetallic nanoparticles, with Ag (core) - Au (shell) (Ag@AuNPs) structure, in the infrared region with ultrashort pulses. UV–Vis spectroscopy showed LSPR (localized surface plasmon resonance) characteristics bands. MEV measurements confirm that the nanoparticles have a spherical shape with good size distribution around 56 nm for AgNPs and 73 nm for Ag@AuNPs. The nonlinear optical properties of the nanostructures were determined by the Z-scan technique, with a laser at 1040 nm (~357 fs), for different intensities. We observed the effects of saturated absorption and reverse saturated absorption as a function of intensity for both nanoparticles with the increase of laser intensity was observed in the open-aperture (OA) measurements in nonresonant mode. Ag@AuNPs present a complete transition from saturation absorption to reverse saturation absorption. The close-aperture (CA) measurements in nonresonant mode indicate an effective negative nonlinearity for nanoparticles, corresponding to self-defocusing. Our results show that these bimetallic nanoparticles have great potential for application in photonics, especially in applications where the tuning of nonlinear optical properties is a determining factor.