Ultrathin two-dimensional (2D) nanomaterials are a new type of nanomaterials. They have a sheet-like structure with a lateral dimension that exceeds 100 nm or as high as several microns or even larger, but the thickness is only one or several atoms. Because electrons are confined in a 2D space, the characteristics of 2D materials are unique, as they exhibit unprecedented physical, electronic, and chemical characteristics. Hence, they have attracted significant attention.

The study of ultrafast nonlinear optical (NLO) properties and ultrafast carrier dynamics is crucial for the development of photonics and optoelectronic devices. It is necessary to extensively investigate the NLO characteristics of 2D materials. Under the action of intense lasers, different 2D materials show different NLO responses. With the changes in preparation methods, test environment, and material customization, the NLO characteristics of materials are more colorful. 2D materials are widely used in laser protection, Q-switching or mode-locking, optical modulation, and various miniaturized all-optical devices because of their unique optical characteristics, such as ultrafast optical response, significant optical nonlinearity, and strong exciton effect.

Different types of 2D materials, such as monoelemental materials and 2D metal sulfides, exhibit distinct NLO properties because of their unique structures. Clarification of the NLO mechanism of 2D materials and reasonable selection will help further expand the application field of 2D materials.

First, this study introduces several methods for preparing 2D materials, the basic principle of NLO, and the experimental devices for common NLO and ultrafast carrier dynamics testing. The experimental devices include the Z/I-scan system, second-harmonic test device, and pump-probe detection system. Next, the NLO properties and principles of different types of 2D materials are comprehensively summarized based on previous studies in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. Cheng et al. investigated the influences of different dispersion solvents on graphene dispersion. They observed that the NLO performance of graphene dispersion originated from the solvent and carbon-vapor bubble-induced nonlinear scattering (NLS). They also showed that the NLO performance of graphene could be improved by reducing the air pressure, as shown in Fig.4(b). Many studies have shown that the NLO properties of materials can be significantly improved via complex or covalent modification. For example, Tong et al. reported that graphene/ZnO composites can change the multiphoton absorption (MPA) by controlling the incident laser intensity, and can also transform the saturated absorption (SA) into the reverse saturated absorption (RSA), as shown in Fig.4(d). Huang et al. investigated the NLO characteristics of black phosphorus (BP) and found that BP is more easily saturated under long laser pulses and exhibits a stronger SA response in the visible light band than in the infrared band. The NLO response of BP dispersion under different laser intensities is attributed to the combined effect of SA and NLS. The problem of material oxidation in air can be minimized by embedding the material into the polymer body as an inclusion to form a composite material. For example, Shi et al. synthesized a mixture of BP∶C₆₀ and embedded it in polymethyl methacrylate(PMMA). After annealing, the heat-induced intermolecular charge transfer effect between BP and C₆₀ was strengthened; hence it exhibited improved NLO characteristics, as shown in Fig.5(c). Dong et al. investigated the NLO properties of transition metal dichalcogenides (TMDs) (MoS₂, MoSe₂, WS₂, WSe₂) dispersion. They found that TMDs showed optical limiting response under nanosecond pulses with different wavelengths, and selenide exhibited better optical limiting performance than sulfide in the near-infrared region. In addition, TMDs with different thicknesses may have different energy band structures, which improves the NLO characteristics. More new 2D materials exhibit fascinating NLO properties because of their unique structures, and their NLO properties can be regulated by controlling the preparation environment and modifying the materials. Finally, according to previous research, the application of NLO properties of 2D materials is summarized. 2D materials can be used as saturable absorbers for Q-switched or mode-locked lasers because of their excellent SA properties. In addition, the excellent optical limiting performance also makes 2D materials candidates as laser protection materials. Different materials exhibit different NLO characteristics, and they can be used in various applications, such as optical modulators, optical nonlinear activators, and visible light thresholders.

2D materials exhibit fascinating optical properties because of their unique structures. Materials exhibit different NLO properties because of the different preparation methods, test environment, and other factors. Composite materials and covalent modification of materials prevent material oxidation and enhance the charge transfer, thus significantly enriching and improving the NLO properties of the materials. These excellent NLO characteristics of 2D materials can be applied to different fields, such as laser mode-locking or Q-switching, laser protection, and optical modulators. However, 2D materials face several challenges, and the problem of how to obtain large-area and high-quality ultrathin 2D materials must be solved urgently. In the future, more new 2D materials and structures with excellent properties can be developed and designed to satisfy application needs in different scenarios.