Gold nanoparticle (GNP) colloids stabilized with various concentrations of L -ascorbic acid were synthesized by the chemical reduction method and characterized by UV-Vis spectroscopy, XRD, FT-IR spectroscopy and TEM. XRD and TEM studies confirmed the reduction in particle size with the stabilizer concentration. UV-Vis spectra showed a blue shift in the LSPR peak. FT-IR peaks ascertained the strong encapsulation of GNPs with L -ascorbic acid functional groups. The nonlinear optical (NLO) properties of colloidal GNPs and GNP–PVP composite thin film were investigated using the Z-scan technique with CW laser excitation at 632.8 nm. The effects of stabilizer concentrations on nonlinear refractive index ( n _(2) ), nonlinear absorption coefficient ( β ) and third-order susceptibility ( χ ~((3)) ) of colloidal GNPs and GNP–PVP composite thin films were investigated. The values of the NLO parameters for the thin films were as large as n _(2) = 10 ~(?5) cm ~(2) W ~(?1) , β = 10 ~(?5) cm W ~(?1) and χ ~((3)) _(eff) = 10 ~(?5) esu. For colloidal GNPs, these parameters were n _(2) = 10 ~(?6) cm ~(2) W ~(?1) , β = 10 ~(?6) cm W ~(?1) and χ ~((3)) _(eff) = 10 ~(?7) esu. In both these cases, the NLO parameter values were found to decrease as the stabilizer concentration increased from 1 to 5 mM. The considerable enhancement in the NLO parameters may be attributed to the thermal lensing effect originating from the thermo-optic phenomenon. From the results, the influence of the concentration of the stabilizer on the NLO properties is obvious.
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