▎ 摘　　要

We demonstrate experimental evidence for the existence of an optimal thickness that yields the maximum third-harmonic (TH) signal in thick multilayer graphene (MLG). We measure the layer-dependent TH signal from MLG on a quartz substrate for N graphene layers ranging from similar to 3 to 50. Theoretical predictions of the layer-dependent TH signal are compared with experiment, including those measured using atomic force and optical microscopy. We find the optimal TH signal for N approximate to 24, in good agreement with two theoretical models: one in which the graphene layers are assumed isolated but contribute coherently, and one in which the MLG is assumed as one continuous thin film. Our measurements reveal that for MLG, the stacking property of the linear and nonlinear conductivities are found to be remarkably preserved up to the very thick layers (N similar to 50), i.e., sigma((i))(N,g) = N sigma((i))(g) , where i = 1 and 3, respectively, and sigma((i) )(g)is the corresponding monolayer conductivity.