Fig. 1. Structure diagram of article

Fig. 2. Schematics of sample preparation. (a) Gold-assisted mechanical exfoliation of two-dimensional materials^[1]; (b) preparation process by sonication-assisted liquid-phase exfoliation^[2]; (c) schematics of CVD device and main growth process^[3]; (d) epitaxial growth of uniform monolayer MoS₂ single crystal on sapphire^[4]; (e) bilayer MoS₂ grown by CVD method has different stacking methods (scale of 100 μm)^[5]; (f) optical images (scale of 20 μm) and atomic resolution STEM-ADF images (scale of 5 nm) of tetragonal (left) and hexagonal (right) FeTe nanosheets^[6]

Fig. 3. Schematics of experimental system for studying NLO characteristics and ultrafast carrier dynamics. (a) Z-scan system^[20]; (b) μ-Z/I-scan system^[21]; (c) reflective SHG test system^[22]; (d) degenerate pump detection system^[23]

Fig. 4. NLO properties of graphene and its composites. (a) Nonlinear transmissivity of three graphene absorbers versus input laser intensity under action of 1030 nm femtosecond laser^[24]; (b) normalized transmissivity and ratio of non-linear extinction cross-section to linear extinction cross-section versus incident intensity for graphene dispersions^[25]; (c) relationship between relative change of local nonlinear refractive index of graphene NMP dispersion and incident light intensity under action of 633 nm laser^[26]; (d) energy level diagram of GZO nanocomposites^[27]; (e) Z-scan results of graphene and its CuO nanocomposites under action of 1030 nm laser^[28]; (f) nonlinear transmissivity (solid symbol) and scattering (hollow symbol) of GO-PcZn, GO dispersion and PCZn solution at 532 nm^[29]

Fig. 5. NLO characteristics of Xene. (a) Relationship between normalized differential absorptivity of BP and pulse fluence^[37]; (b) modulation depths of BP dispersions under different pump fluences^[38]; (c) normalized transmissivity of PMMA-based films versus input laser intensity at 532 nm^[39]; (d) competition mechanisms between SA and NLS under excitations of weak pump, medium pump, and strong pump^[38]; (e) 2D SHG mapping of Te nanosheet^[40]; (f) SHG spectra of Te nanosheet under different excitation wavelengths^[40]; (g) polarization angle dependence of SHG intensity^[40]

Fig. 6. NLO characteristics of MoX₂ and WX₂ (X=S, Se, Te). (a) Schematics of S vacancy defect levels within MoS₂ bandgaps, resulting in SA^[50]; (b) high harmonic spectrum measured from monolayer MoS₂^[51]; (c) optical modulation depths of MoS₂, MoSe₂, and graphene dispersions under different pump fluences with 632.8 nm CW probe light^[52]; normalized transmissivity (solid circles) and scattering response (hollow circles) of different nanosheet dispersions at (d) 1064 nm and (e) 532 nm^[53]

Fig. 7. NLO characteristics of two-dimensional metal sulfides. (a) Schematic of surface and inner bulk recombination of 2D PtS^[59]; (b) ultrafast carrier dynamic of PtS thin films with thickness of 14.1 nm^[59]; (c) open-aperture Z-scan results of 4, 7, 17, and 55 layer PtSe₂ films under 1030 nm femtosecond pulse excitation^[60]; (d) carrier relaxation processes of InSe dispersions with different pump intensities^[61]; (e) schematics of recombination processes of InSe under low carrier density and high carrier density excitations^[61]

Fig. 8. NLO characteristics of two-dimensional materials. (a) Normalized transmissivity of BiOCl {001} and BiOCl{010} versus incident laser intensity^[70]; (b) experimental result of time-resolved absorption of MXene monolayers with large flakes^[71]; (c) temperature dependence of SHG signal of two-dimensional perovskite ferroelectrics, revealing symmetry breaking phase transition^[72]; (d) photoluminescence spectrum of two-dimensional perovskite ferroelectrics excited at 800 nm with schematic of TPA and photoluminescence process shown in inset^[72]; Z-scan results of COF-Pors under excitations of (e) 532 nm and (f) 1064 nm nanosecond pulses^[73]

Fig. 9. Two-dimensional materials are used as saturable absorbers for Q switching or mode locking. (a) Relationship between transmissivity of graphene and incident energy density at 1053 nm^[76]; (b) experimental setup of graphene mode-locked Raman fiber laser^[76]; (c) mode-locked pulse sequence with repetition rate of 0.4 MHz^[76]; (d) output radio-frequency spectrum of mode-locked laser^[76]; (e) schematic of passively Q-switched Tm∶GdScO₃ laser using SnS₂ saturable absorber^[77]; (f) typical Q-switching pulse train and temporal pulse profile^[77]

Fig. 10. Schematics of laser protection. (a) Z-scan data of typical openings of graphene dispersion in NMP at 1064 nm under different pressures^[25]; (b) Z-scan curves of GO, TiO₂,TBT, and P25 samples under 1064 nm nanosecond pulse^[85]; (c) degenerate TPA process of monolayer layer MoS₂^[86]; (d) SA process of multilayer MoS₂^[86]; (e) schematic of all-optical modulation process, and SA and NLS mechanism diagram in nanosheet dispersion^[53]

Fig. 11. Applications of NLO characteristics of two-dimensional materials. (a) Schematic of simulation process of visible light thresholder^[87]; (b) combined signals before and after three layers of PdSe₂ in experiment^[87]; (c) architecture of fully connected network for MNIST and Fashion-MNIST classification tasks^[88]; (d) low-resolution microtubule images are reconstructed into high-resolution images using three-layer convolutional neural network^[88]; (e) nonlinear energy-dependent transmission curve of microfiber with deposited Nb₄C₃T_y film^[89]; (f) schematic of all-optical modulator^[89]; (g) pulse modulation signal with frequency of 5 MHz and power of 62.82 mW^[89]