Two-dimensional (2D) nanomaterials, especially the transition metal sulfide semiconductors, have drawn great interests due to their potential applications in viable photonic and optoelectronic devices. In this work, 2D tungsten disulfide (WS₂) based saturable absorber (SA) for ultrafast photonic applications was demonstrated. WS₂ nanosheets were prepared using liquid-phase exfoliation method and embedded in polyvinyl alcohol (PVA) thin film for the practical usage. Saturable absorption was discovered in the WS₂-PVA SA at the telecommunication wavelength near 1550 nm. By incorporating WS₂-PVA SA into a fiber laser cavity, both stable mode locking operation and Q-switching operation were achieved. In the mode locking operation, the laser obtained femtosecond output pulse width and high spectral purity in the radio frequency spectrum. In the Q-switching operation, the laser had tunable repetition rate and output pulse energy of a few tens of nano joule. Our findings suggest that few-layer WS₂ nanosheets embedded in PVA thin film are promising nonlinear optical materials for ultrafast photonic applications as a mode locker or Q-switcher.

Nonlinear optical property of transition metal dichalcogenide (TMDC) nanosheet dispersions, including MoS₂, MoSe₂, WS₂ and WSe₂, was performed by using Z-scan technique with ns pulsed laser at 1064 nm and 532 nm. The results demonstrate that the TMDC dispersions exhibit significant optical limiting response at 1064 nm due to nonlinear scattering, in contrast to the combined effect of both saturable absorption and nonlinear scattering at 532 nm. Selenium compounds show better optical limiting performance than that of the sulfides in the near infrared. A liquid dispersion system based theoretical modelling is proposed to estimate the number density of the nanosheet dispersions, the relationship between incident laser fluence and the size of the laser generated micro-bubbles and hence the Mie scattering-induced broadband optical limiting behavior in the TMDC dispersions.

Liquid-phase-exfoliation technology was utilized to prepare layered ${\mathrm{MoS}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{MoSe}}_2$ nanosheets in cyclohexylpyrrolidone. The nonlinear optical response of these nanosheets in dispersions was investigated by observing spatial self-phase modulation (SSPM) using a 488 nm continuous wave laser beam. The diffraction ring patterns of SSPM were found to be distorted along the vertical direction right after the laser traversing the nanosheet dispersions. The nonlinear refractive index of the three transition metal dichalcogenides dispersions $n_2$ was measured to be $\sim {10}^{-7}{\mathrm{cm}}^2{\mathrm{W}}^{-1}$, and the third-order nonlinear susceptibility ${\chi }^{(3)}\sim {10}^{-9}\mathrm{esu}$. The relative change of effective nonlinear refractive index $\mathrm{\Delta }{n}_{2e}/n_{2e}$ of the ${\mathrm{MoS}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{MoSe}}_2$ dispersions can be modulated 0.012–0.240, 0.029–0.154, and 0.091–0.304, respectively, by changing the incident intensities. Our experimental results imply novel potential application of two-dimensional transition metal dichalcogenides in nonlinear phase modulation devices.

Two-dimensional (2D) Sn-based perovskites are a kind of non-toxic environment-friendly luminescent material. However, the research on the luminescence mechanism of this type of perovskite is still very controversial, which greatly limits the further improvement and application of the luminescence performance. At present, the focus of controversy is defects and phonon scattering rates. In this work, we combine the organic cation control engineering with temperature-dependent transient absorption spectroscopy to systematically study the interband exciton relaxation pathways in layered A₂SnI₄ (A = PEA⁺, BA⁺, HA⁺, and OA⁺) structures. It is revealed that exciton-phonon scattering and exciton-defect scattering have different effects on exciton relaxation. Our study further confirms that the deformation potential scattering by charged defects, not by the non-polar optical phonons, dominates the excitons interband relaxation, which is largely different from the Pb-based perovskites. These results enhance the understanding of the origin of the non-radiative pathway in Sn-based perovskite materials.