Thermo-optic properties and nonlinear responses of copper nanoparticles in polysiloxane oil

Thermo-optic properties and thermal nonlinear effects of copper nanoparticles (CuNPs) in polysiloxane oil are investigated under irradiation with a continuous wave low power laser at 532 nm. The CuNPs are synthesized by nanosecond pulsed laser ablation of a copper plate in the polysiloxane oil. Far field diffraction ring patterns are observed, arising from thermally induced self-phase modulation in the laser beam as it traverses the CuNP colloids. Low power optical limiting with low limiting thresholds is obtained in the sample on the basis of thermal defocusing and self-diffraction. The nonlinear optical response is characterized by measuring the intensity dependent refractive index of the medium using the Z-scan technique. The closed Z-scan measurements of the colloidal CuNPs exhibit an asymmetric behavior at different applied laser power levels of 5-20 mW. A theoretical analysis is presented for the observed nonlinear behavior on the basis of nonlocal thermo-optic processes, without using the photothermal thin lens approximation. Excellent agreements are found for the closed Z-scan measurements of the colloidal CuNPs with the analysis presented. The values of nonlinear refractive index and thermo-optic coefficients are found to be -1.52 x 10(-7) cm(2) W-1 and -1.0 x 10(-4) K-1, respectively.