Atomically thin Bi₂O₂Se has emerged as a novel two-dimensional (2D) material with an ultrabroadband nonlinear optical response, high carrier mobility and excellent air stability, showing great potential for the realization of optical modulators. Here, we demonstrate a femtosecond solid-state laser at 1.0 µm with Bi₂O₂Se nanoplates as a saturable absorber (SA). Upon further defect regulation in 2D Bi₂O₂Se, the average power of the mode-locked laser is improved from 421 mW to 665 mW, while the pulse width is decreased from 587 fs to 266 fs. Moderate Ar⁺ plasma treatments are employed to precisely regulate the O and Se defect states in Bi₂O₂Se nanoplates. Nondegenerate pump-probe measurements show that defect engineering effectively accelerates the trapping rate and defect-assisted Auger recombination rate of photocarriers. The saturation intensity is improved from 3.6 ± 0.2 to 12.8 ± 0.6 MW cm⁻² after the optimized defect regulation. The enhanced saturable absorption and ultrafast carrier lifetime endow the high-performance mode-locked laser with both large output power and short pulse duration.

Few-layered titanium carbide (${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{T}}_x$), a novel two-dimensional (2D) Van der Waals material in the MXene family, was fabricated with a liquid-phase method and applied as a saturable absorber for a continuous-wave mode-locked femtosecond bulk laser. Pulses as short as 316 fs with a repetition rate of 64.06 MHz and maximum output power of 0.77 W were achieved at the central wavelength of 1053.2 nm, demonstrating the first known, to the best of our knowledge, application of MXene in an all-solid-state laser. Considering the flexible band gap for different surface functional groups of ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{T}}_x$, these results may promote the development of ultrafast photonics and further applications of 2D optoelectronic layered materials in the infrared and mid-infrared regions.

Rhenium disulfide (${\mathrm{ReS}}_2$), a member of group VII transition metal dichalcogenides (TMDs), has attracted increasing attention because of its unique distorted 1T structure and electronic and optical properties, which are much different from those of group VI TMDs (${\mathrm{MoS}}_2$, ${\mathrm{WS}}_2$, ${\mathrm{MoSe}}_2$, ${\mathrm{WSe}}_2$, etc.). It has been proved that bulk ${\mathrm{ReS}}_2$ behaves as a stack of electronically and vibrationally decoupled monolayers, which offers remarkable possibilities to prepare a monolayer ${\mathrm{ReS}}_2$ facilely and offers a novel platform to study photonic properties of TMDs. However, due to the large and layer-independent bandgap, the nonlinear optical properties of ${\mathrm{ReS}}_2$ from the visible to mid-infrared spectral range have not yet been investigated. Here, the band structure of ${\mathrm{ReS}}_2$ with the introduction of defects is simulated by the ab initio method, and the results indicate that the bandgap can be reduced from 1.38 to 0.54 eV with the introduction of defects in a suitable range. In the experiment, using a bulk ${\mathrm{ReS}}_2$ with suitable defects as the raw material, a few-layered broadband ${\mathrm{ReS}}_2$ saturable absorber (SA) is prepared by the liquid phase exfoliation method. Using the as-prepared ${\mathrm{ReS}}_2$ SA, passively $Q$-switched solid-state lasers at wavelengths of 0.64, 1.064, and 1.991 μm are investigated systematically. Moreover, with cavity design, a femtosecond passively mode-locked laser at 1.06 μm is successfully realized based on the as-prepared ${\mathrm{ReS}}_2$ SA for the first time. The results present a promising alternative for a rare broadband optical modulator and indicate the potential of ${\mathrm{ReS}}_2$ in generating $Q$-switched and mode-locked pulsed lasers. It is further anticipated that this work may be helpful for the design of 2D optoelectronic devices with variable bandgaps.

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A novel two-dimensional (2D) material member in the transition metal dichalcogenides family, few-layered rhenium disulfide (${\mathrm{ReS}}_2$) was prepared by liquid phase method successfully. By using the open-aperture Z-scan method, the saturable absorption properties at 2.8 μm were characterized with a saturable fluence of $22.6\mathrm{\mu J}/{\mathrm{cm}}^2$ and a modulation depth of 9.7%. A passively $Q$-switched solid-state laser at 2.8 μm was demonstrated by using the as-prepared ${\mathrm{ReS}}_2$ saturable absorber successfully. Under an absorbed pump power of 920 mW, a maximum output power of 104 mW was obtained with a pulse width of 324 ns and a repetition rate of 126 kHz. To the best of our knowledge, this is the first demonstration of applying ${\mathrm{ReS}}_2$ in an all-solid-state laser. Moreover, this represents the shortest pulses in $Q$-switched MIR lasers based on a 2D material as the saturable absorber, which demonstrated the superiority of ${\mathrm{ReS}}_2$ acting as an optical modulator for generating short-pulsed lasers. The results well prove that 2D ${\mathrm{ReS}}_2$ is a reliable optical modulator for MIR solid-state lasers.