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Thermo-Optical Properties of Colloids Enhanced by Gold Nanoparticles

This work presents a study on the thermo-optical properties of colloidal gold nanoparticles (AuNPs) under a low power laser irradiation at 532 nm. Samples of various gold volume fractions, ranging from 2.5×10-4 to 19.5×10-4%, are synthesized by nanosecond pulsed laser ablation of a pure gold plate in the distilled water. The formation of the AuNPs has been evidenced by optical absorption spectra and transmission electron microscopy. We investigate the effect of the gold nanoparticle concentration on thermo-optical properties of the colloids using the Z-scan technique. The nonlinear optical measurements exhibit a very large nonlinear refraction close to the surface plasmon resonance frequency of the nanoparticles. Our results reveal that the heat diffusion in the colloids is due to nonlocal thermal process. As the gold concentration increases, the temperature change within and around gold nanoparticles greatly enlarges the thermo-optic and thermal nonlinear refractive index coefficients of the samples. This work suggests that thermal nonlinear refraction will play an important role in development of photonic applications involving metal nanoparticles colloids.