Study of the solution thermal conductivity effect on nonlinear refraction of colloidal gold nanoparticles

In nanoparticle colloidal systems, the thermal nonlinearity is affected by the thermal parameters of the surrounding solution. Having a low temperature gradient rate solution may be a key factor in producing high thermal nonlinear properties in colloids. In this manuscript, the effect of the thermal conductivity of the surrounding liquid environment on the thermal nonlinear refraction of gold nanoparticles (AuNPs) synthesized by laser ablation of a gold target in different solutions is investigated. Gold nanoparticles colloids have been fabricated by the nanosecond pulsed laser ablation of a pure gold plate in different liquid environments with a thermal conductivity range of 0.14–0.60 W mK⁻¹ including cyclohexanone, castor oil, dimethyl sulfoxide, ethylene glycol, glycerin and water. The AuNPs colloids exhibit a UV-Vis absorption spectrum with a surface plasmon absorption peak at about 540  ±  20 nm. The thermal nonlinear optical responses of the gold colloids are measured using the Z-scan technique under low power CW laser irradiation at 532 nm near the surface plasmon peak of the nanoparticles. Our results show that the nonlinear refractive index of the nanoparticle colloids is considerably affected by the thermal conductivity of liquid medium. The largest nonlinear refractive index of −3.1  ×  10^–7 cm² W⁻¹ is obtained for AuNP in cyclohexanone with the lowest thermal conductivity of 0.14 W mK⁻¹ whereas the lowest one of −0.1  ×  10⁻⁷ cm² W⁻¹ is obtained for AuNP in water with the highest thermal conductivity of 0.60 W mK⁻¹. This study shows that the nonlinear refractive index value of colloids can be controlled by the thermal conductivity of the used liquid's environment. This allows us to design low threshold optical limiters by choosing a solution with low thermal conductivity for colloidal nanoparticles. (paper)