“…Although there have been a few studies on the NLO properties of Au NPs using the z-scan approach [ 18 , 19 , 20 , 21 , 22 , 23 , 24 ], no systematic studies on the NLO properties of Au NP colloids using femtosecond laser pulses for different sizes and concentrations of Au NPs and different laser parameters, such as excitation wavelength and intensity, have been conducted to our knowledge. For instance, the authors of [ 18 , 19 , 20 , 22 , 23 ] studied the NLO properties of Au NPs at a certain excitation wavelength and for a given Au NP size.…”