Second harmonic generation in two-dimensional materials and its modulation methods

Two-dimensional (2D) materials exhibit numerous unique physical phenomena due to the quantum effect on the low-dimensional scale, which has aroused the research interest of many researchers. In nonlinear optics, 2D materials exhibit high second-order nonlinear polarizability and highly adjustable physical properties, making them a potential candidate for nonlinear optoelectronic devices. This study focuses on the modulation and enhancement of second harmonic generation (SHG) in 2D materials. In this study, we first briefly introduce the basic optical concept of SHG, and then discuss the different methods of symmetry breaking and resonance effect to modulate and enhance the SHG of two-dimensional materials, such as interlayer stacking, electric field, strain and exciton effect. Finally, we summarize the control methods of SHG, and look forward to the possible research directions and challenges in the future. Understanding the mechanism of SHG and various techniques of modifying and boosting SHG will be more conducive to the development of second harmonic characterization technology and the exploration of nonlinear optical devices based on 2D materials in the future.