Significance With the rapid development of laser technologies, the number of laser weapon equipment is increasing. At the same time, human eyes, photoelectric detection equipment, and optical systems are being exposed to strong laser environment and are vulnerable to laser attacks. It is an urgent problem to ensure that these devices have anti-attack capability based on normal operation. Consequently, it is paramount to develop a laser protection technology.

Based on the working principle, the laser protection technology can be divided into two types. One is based on the linear optical principle, such as absorption-type filter, reflection-type filter, and coherent filter. The other is based on the nonlinear optical (NLO) principle—also known as optical limiting (OL)—such as nonlinear absorption-, scattering-, and refraction-type optical limiters. In addition, there are thermally induced phase-change materials and liquid-crystal materials, etc. The OL technology can combine high transmittance to weak light and low transmittance to strong light at the same wavelength. In addition, it has obvious advantages in the protection against high-energy, continuous broadband laser spectra, and ultrafast response time. Moreover, it is one of the materials with high practical application value in the field of laser protection.

Progress Various materials, such as graphene, transition metal dichalcogenide (TMDC), black phosphorus (BP), carbon nanotube, phthalocyanine, and porphyrin, can be used to fabricate optical limiters. This study focuses on the progress of two-dimensional (2D) nonlinear optical limiting materials, such as graphene, TMDC, and BP, applied in the aspect of laser protection.

In 2009, Wang et al. first reported the OL characteristics of high-quality graphene (Fig. 3). By dispersing graphite in organic solvents, they have successfully produced large numbers of graphene mono- and multilayers. A significant NLO response of graphene dispersions to nanosecond laser pulses at 532 nm and 1064 nm was observed, thereby implying a potential broadband OL application. Nonlinear scattering arising from the formation of solvent bubbles and micro-plasmas is the principal mechanism for OL. The surface tension of solvents has a strong influence on the OL performance of graphene dispersions. The OL effect of N,N-dimethylacetamide (DMA) dispersions is better than those of N-methyl-2-pyrrolidone (NMP) and γ-butyrolactone (GBL) dispersions.

Feng et al. investigated the NLO and OL properties of graphene families, including graphene oxide nanosheets, graphene nanosheets (GNSs), graphene oxide nanoribbons (GONRs), and graphene nanoribbons (GNRs), using 532 nm and 1064 nm nanosecond lasers (Fig. 4). GNSs, GONRs, and GNRs exhibit broadband NLO and OL properties. The reduced graphene samples exhibit stronger NLO and OL responses than their corresponding oxide precursors due to their increased crystallinity and conjugation. Nonlinear scattering and two-photon absorption are found to have strong effects on the NLO and OL responses of graphene nanostructures.

Dong et al. reported the NLO properties of TMDC nanosheet dispersions, including MoS₂, MoSe₂, WS₂, and WSe₂, using nanosecond laser pulses at 1064 nm and 532 nm (Fig. 5). The results demonstrate that the TMDC dispersions exhibit a significant OL response at 1064 nm due to nonlinear scattering, contrary to the combined effect of both saturation absorption and nonlinear scattering at 532 nm. Selenium compounds exhibit a better OL performance than sulfides at near-infrared. A liquid dispersion system-based theoretical model is proposed to estimate the number density of nanosheet dispersions, the relationship between the incident laser fluence and the size of laser-generated microbubbles, and the Mie scattering-induced broadband OL behavior in the TMDC dispersions.

Li et al. synthesized mono- and multilayer MoS₂ triangular islands using a seeding method via chemical vapor deposition (Fig. 6). Distinct NLO responses demonstrate that multilayer MoS₂ exhibits the SA effect. However, monolayer MoS₂ exhibits a remarkable two-photon absorption effect for a femtosecond laser pulse at 1030 nm. Notably, they observed two-photon pumped upconverted luminescence in monolayer MoS₂, another vital third-order NLO response in 2D semiconductors.

Huang et al.investigated the wavelength- and pulse-duration-dependent SA properties of BP by a femtosecond laser pulse at 1030 nm/515 nm and a nanosecond laser pulse at 1064 nm/532 nm (Fig. 8). The results reveal that BP exhibits a better NLO response in the visible range than that in the near-infrared range and stronger SA ability in 6 ns-pulsed excitation than in 340-fs-pulsed excitation. Finally, they reported the SA-induced optical transparency and NLS induced optical limiting in BP dispersions.

Conclusion and Prospect In this paper, we have introduced the basic concept of a laser protection technology, summarized several laser protection schemes, and illustrated the mechanism of laser protection technology based on the NLO principle. Moreover, we have introduced the research progress of three types of 2D NLO materials—graphene, TMDC, and BP—in term of laser protection. In summary, these materials exhibit OL characteristics in the range of visible to near-infrared bands. For ns-pulsed laser, laser protection is mainly induced by nonlinear scattering, while for fs-pulsed laser, it is dominated by two-photon absorption. Under the same experimental conditions, the OL properties of three materials from strong to weak orders are TMDC, graphene, and BP. In addition, the covalent modification can improve the dispersion of 2D nanomaterials as well as their OL and NLO properties. However, the development of laser protection devices based on these materials is still in the research stage, and they are yet to be practicalized. For 2D nanomaterials, many problems still remain, such as: 1) the materials have a strong aggregation effect, leading to poor dispersion; 2) the problem of realizing the precise control of the layer number and size of 2D materials makes it difficult to achieve low-cost and large-scale material fabrications. In the future, we need to focus on these materials to design and fabricate high-quality covalent chemically modified 2D nanomaterials as well as develop ideal OL devices with broad protective spectral bands, low input thresholds, high linear transmittance to weak radiation, fast responses, and large damage thresholds.