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.

applications, [4,[17][18][19] and therefore have aroused enthusiasm in researchers. Due to their high crystal structure symmetry, graphene, which has been well studied, and most TMDCs exhibit an inplane isotropic feature. However, a few 2D materials with a surprisingly low lattice symmetry, such as BP, tin selenide (SnSe), [20][21][22][23] gallium telluride (GaTe), [24] and rhenium disulfide (ReS 2 ), [17,[25][26][27][28] can also show significant anisotropic in-plane optical, electrical, and thermal properties. [17,20,[23][24][25]28] For instance, the charge carrier mobility, [18,29] photoemission, [30] and thermoelectric figure of merit (ZT) [31][32][33] of BP along the armchair direction are larger than those in the zigzag direction. In addition to anisotropy, BP also has a tunable thickness-dependent direct bandgap ranging from 0.3 to 1.5 eV, [12,13] filling the space between zero-gap graphene [1,2] and large-gap (1-2.5 eV) TMDCs. [4,34] Additionally, BP has a high carrier mobility (≈10 3 cm 2 V −1 s −1 ) [18] compared with TMDCs (10-200 cm 2 V −1 s −1 ). [4] Therefore, BP has been viewed as an alluring and ideal candidate for applications in field-effect transistors (FETs), [18,35] fast-response optical switches, [36] photovoltaic devices, [37] mid-infrared polarizers, and polarization sensors, [29] owing to its distinguished physical properties. Although BP exhibits great potential for various anisotropic optical, electronic, and optoelectronic high-performance devices, it degrades within a short time when exposed to oxygen and water vapor in air, causing difficulties in practical applications. Consequently, it is highly important to explore BP-like materials with appropriate properties including a narrow bandgap, high carrier mobility, air stability, and low cost.Regarded as a promising alternative to BP, SnSe also consists of a puckered honeycomb layered crystal structure similar to that in BP, exhibiting highly anisotropic valence bands, [23] a crystal-orientation-dependent high charge carrier mobility (≈10 3 cm 2 V −1 s −1 ), [38] and linear optical absorption. [39] Additionally, due to the narrow-bandgap semiconductor nature, the indirect bandgap of SnSe is ≈0.9 eV, [39][40][41][42] whereas its direct bandgap is ≈1.3 eV, [40,42] leading to optical transitions of SnSe The deceptively simple tin selenide (SnSe) film has emerged as an appealing 2D material with a narrow bandgap, high charge carrier mobility, and significant thermoelectric figure of merit. In particular, compared with most commonly investigated 2D materials, SnSe with a puckered honeycomb structure possesses a lower lattice symmetry, resulting in prominent in-plane anisotropy. Herein, with polarization-dependent Raman spectroscopy and polarization-dependent nonlinear absorption measurements, pronounced polarization-dependent nonlinear optical properties of a SnSe flake are demonstrated originating from the anisotropic optical transition probability of SnSe, which is confirmed by ultrafast polarization-dependent pump-probe experiments. Furt...

The plasmon-mediated polarized nonlinear optical response and all-optical switching in topological insulator Bi2Se3–Au nanoantenna hybrids are studied.

Broadband transient reflectivity traces were measured for Bi 2 Se 3 thin films with various substrates via a 400 nm pump-white-light-probe setup. We have verified the existence of a second Dirac surface state in Bi 2 Se 3 and qualitatively located it by properly analyzing the traces acquired at different probe wavelengths. Referring to the band structure of Bi 2 Se 3 , the relaxation mechanisms for photo-excited electrons with different energies are also revealed and studied. Our results show a second rise of the transient reflection signal at the time scale of several picoseconds. The types of substrate can also significantly affect the dynamics of the rising signal. This phenomenon is attributed to the effect of lattice heating and coherent phonon processes. The mechanism study in this work will benefit the fabrication of high-performance photonic devices based on topological insulators.