The year 2019 marks the 10th anniversary of the first report of ultrafast fiber laser mode-locked by graphene. This result has had an important impact on ultrafast laser optics and continues to offer new horizons. Herein, we mainly review the linear and nonlinear photonic properties of two-dimensional (2D) materials, as well as their nonlinear applications in efficient passive mode-locking devices and ultrafast fiber lasers. Initial works and significant progress in this field, as well as new insights and challenges of 2D materials for ultrafast fiber lasers, are reviewed and analyzed.

Optical modulation technique plays a crucial role in photonics technologies, and there is an ever‐increasing demand for broadband and ultrafast optical modulation in the era of artificial intelligence. All‐optical modulation is known to be able to operate in an ultrafast way and has a broadband response, showing great potential in applications for ultrafast information processing and photonic computing. Two‐dimensional (2D) materials with exotic optoelectronic properties bring tremendous new opportunities for all‐optical modulators with excellent performance, which have attracted lots of attention recently. In this review, we cover the state‐of‐art all‐optical modulation based on 2D materials, including graphene, transitional metal dichalcogenides, phosphorus, and other novel 2D materials. We present the operations mechanism of different types of all‐optical modulators with various configurations, such as fiber‐integrated and free‐space ones. We also discuss the challenges and opportunities faced by all‐optical modulation, as well as offer some future perspectives for the development of all‐optical modulation based on 2D materials.

The newly raised two-dimensional material ${\mathrm{MoS}}_2$ is regarded as an ideal candidate for saturated absorbers. Here, the open-aperture Z-scan method is used to study the saturation absorption (SA) response of monolayer and multilayer ${\mathrm{MoS}}_2$, considering laser irradiation with different pulse widths. Specifically, in cases of 10 ns and 10 ps laser pulses, the accumulative nonlinearity [e.g., free carrier absorption (FCA)] coupled with SA is found in both monolayer and multilayer ${\mathrm{MoS}}_2$. However, under a 65 fs pulse laser, the instantaneous nonlinearity [e.g., two-photon absorption (TPA)] and the SA effect turn to play a significant role. Additionally, the saturation of both TPA and FCA is observed in ${\mathrm{MoS}}_2$. Importantly, the modulation depth of ${\mathrm{MoS}}_2$ shows different change trends by adjusting the laser pulse width.

Electron-phonon scattering is the key process limiting the efficiency of modern nanoelectronic and optoelectronic devices, in which most of the incident energy is converted to lattice heat and finally dissipates into the environment. Here, we report an acoustic phonon recycling process in graphene-WS₂ heterostructures, which couples the heat generated in graphene back into the carrier distribution in WS₂. This recycling process is experimentally recorded by spectrally resolved transient absorption microscopy under a wide range of pumping energies from 1.77 to 0.48 eV and is also theoretically described using an interfacial thermal transport model. The acoustic phonon recycling process has a relatively slow characteristic time (>100 ps), which is beneficial for carrier extraction and distinct from the commonly found ultrafast hot carrier transfer (~1 ps) in graphene-WS₂ heterostructures. The combination of phonon recycling and carrier transfer makes graphene-based heterostructures highly attractive for broadband high-efficiency electronic and optoelectronic applications.