Nonlinear optics and photonics applications of two-dimensional materials

Nonlinear optics based on two-dimensional (2D) materials, as an important branch of modern optics, has received great attention in recent years due to its significant applications in the industrial and scientific fields (You et al. [1]). Materials can interact with an electric field that is of the order of interatomic fields (105–108 V/m), thus exhibit a nonlinear optical response (Saleh and Teich [2]). In 1875 John Kerr observed that a nonlinear optical response of solids and liquids subjected to strong external DC electric fields (Kerr [3]), and Franken et al. demonstrated the generation of second-harmonic generation (SHG) in the experiment, meaning the birth of modern nonlinear optics (Franken et al. [4]). The demonstration of the first laser made this possible, which is the main development of nonlinear optics (Maiman [5]). Through field-dependent variations of optical constants, such as absorption and index of refraction, the optical nonlinearity of an optical medium manifests itself, which is usually accompanied by the generation of new optical frequencies. In this context, according to the mechanisms responsible for the variation of optical constants, the nonlinear optical response of materials can be classified into different categories, such as electronic optical nonlinearity (Varma et al. [6]), thermally induced optical nonlinearity (Wang et al. [7]), and external field-induced optical nonlinearity (An et al. [8]).