Nonlinear Optics in Two-Dimensional Materials: Fundamentals, Mechanisms, and Emerging Applications

This review surveys nonlinear optical phenomena in two-dimensional (2D) materials, including second-harmonic generation, third-order effects, high-harmonic generation, and ferroelectric control of the nonlinear response. It discusses the underlying mechanisms, such as symmetry breaking, Berry curvature effects, and exciton-enhanced responses, and clearly distinguishes thickness-normalized nonlinear susceptibility from absolute device conversion efficiency. Recent advances in waveguide integration, strain and electric-field engineering, and quantum light generation are summarized, together with a comparative table and a critical analysis of how far individual results can be generalized across 2D materials. Remaining challenges in efficiency, scalability, and thermal management are outlined. This is a preprint and has not been peer reviewed.