Nonlinear refractive index of Au nanoparticle suspended in PVA solution was measured using a single beam Z-scan technique. Measurements were carried out using a green CW laser beam operated at 532 nm as excitation source. Five nanoparticle samples with different particle sizes were prepared by γ radiation method. The Au nano-fluid shows a good third order nonlinear response for particle sizes ranging from 7.0 nm to 18.7 nm. The sign of the nonlinear refractive index was found to be negative and the magnitude was in the order of 10 −8 cm 2 /W. The results show that the nonlinear effect tends to be increased linearly with the increasing of particle sizes thus could be a good candidate for nonlinear optical devices.

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A dual beam mode-mismatched thermal lens method has been employed to investigate the dependence of thermal diffusivity of gold nanofluid on nanoparticles sizes. The samples were prepared at various sizes by utilizing the gamma radiation method. In the dual beam mode-mismatched thermal lens a diode laser (532 nm) was used as an excitation beam and a He-Ne laser with the beam output at 632.8 nm was used as a probe beam. Thermal diffusivity of gold nano-fluid increased with the increasing particle sizes ranging from 10.4 to 29.6 nm.

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The thermal lens technique was utilized in silver nanofluid containing silver nanoparticles in polyvinylpyrrolidone solution to study the effect of nanoparticle size on thermal diffusivity. The different sizes of silver nanoparticles were prepared by irradiating the solution of silver nitrate in polyvinylpyrrolidone with respective dose of γ-radiation. The average sizes of particle in the prepared samples were measured using nanophox machine. The thermal lens measurement was carried out by using a diode laser of wavelength 514 nm and a He–Ne laser for the excitation source and the probe beam, respectively. The obtained result showed a decrease in the thermal diffusivity of nanofluid with the decrease in particle size.