Optimizing Nonlinear Optical Visibility of Two-dimensional Materials

Two-dimensional materials have demonstrated interesting nonlinear optical (NLO) properties. Despite extensive researches, little attention has been paid to the relationship between the nonlinear optical strength of 2D materials and the properties of the substrates they are sitting on. In this work, we discovered that the strength of nonlinear optical signal depends significantly on the SiO₂ thickness in the SiO₂/Si substrates. A 40-fold increase of graphene nonlinear photoluminescence (NPL) signal was observed when the SiO₂ thickness was varied from 270 nm to 125 nm under 800nm excitation. Furthermore, we have extended our measurements to include transient absorption (TA) and second harmonic generation (SHG) of graphene and MoS₂, confirming that SiO₂ thickness has similar effects on all of the three major types of NLO signals. Our observations provide a critical reference for NLO researches of two-dimensional materials, and our model may serve as a quick guidance for choosing the optimum substrates to conduct nonlinear optics and optoelectronic studies.