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...

of the photogenerated electrons and holes seriously restrains the photocatalytic efficiency. [2] To reduce the recombination rate of photogenerated carriers, a variety of strategies have been developed, including defect engineering, [3] cocatalyst decoration, [4] formation of heterojunction or Z-scheme, [5] etc. Among them, the creation of built-in electric field as a driving force for charge separation has been recognized to be an effective strategy. [6] Under mechanical stretch or strain along asymmetry direction, the piezoelectric material deforms and the center of positive/negative charges in the unit-cell displace, leading to the spontaneous polarization. Accordingly, the positive and negative charges are generated on two opposite surfaces, leading to the generation of the built-in electric field. Moreover, the generated built-in electric field also induces the band bending at the solid-liquid interface, which would be further favorable to the enhancement of catalytic activity. [7] Therefore, piezo-photocatalysis, in which the piezoelectric materials are subjected to simultaneous light irradiation and mechanical stress, has become a research focus. [8] To apply mechanical stress to piezo-(photo)catalysts, ultrasonic irradiation and mechanical stirring are the most common methods. [9] However, both methods are able to accelerate the The built-in electric field can be generated in the piezoelectric materials under mechanical stress. The resulting piezoelectric effect is beneficial to charge separation in photocatalysis. Meanwhile, the mechanical stress usually gives rise to accelerated mass transfer and enhanced catalytic activity. Unfortunately, it remains a challenge to differentiate the contribution of these two factors to catalytic performance. Herein, for the first time, isostructural metal-organic frameworks (MOFs), i.e., UiO-66-NH 2 (Zr) and UiO-66-NH 2 (Hf ), are adopted for piezo-photocatalysis. Both MOFs, featuring the same structures except for diverse Zr/Hf-oxo clusters, possess distinctly different piezoelectric properties. Strikingly, UiO-66-NH 2 (Hf ) exhibits ≈2.2 times of activity compared with that of UiO-66-NH 2 (Zr) under simultaneous light and ultrasonic irradiation, though both MOFs display similar activity in the photocatalytic H 2 production without ultrasonic irradiation. Given their similar pore features and mass transfer behaviors, the activity difference is unambiguously assignable to the piezoelectric effect. As a result, the contributions of the piezoelectric effect to the piezo-photocatalysis can be clearly distinguished owing to the stronger piezoelectric property of UiO-66-NH 2 (Hf ).

On the basis of the carboxylic structure of trimesic acid (TMA) and 5-borono-1,3-benzenedicarboxylic acid (B-BDC), here two hydrogen-bonded organic frameworks (HOFs), HOF 1 (MA-TMA) and HOF 2 (MA-B-BDC), were prepared by reacting melamine (MA) with trimesic acid and 5-borono-1,3-benzenedicarboxylic acid, respectively. The as-prepared HOF 1 and HOF 2 with 3D supramolecular structure exhibit excellent performance in proton transport. AC impedance test shows that the proton conductivities of HOF 1 and HOF 2 reach 3.11 × 10 −4 S•cm −1 (343 K, 98% RH) and 4.32 × 10 −4 S•cm −1 (323 K, 98% RH), respectively. To increase the proton conductivity, Im@HOF 1 and Im@HOF 2 were obtained by introducing imidazole (Im), which acts as a jumping site for proton transfer. The proton conductivities of Im@HOF 1 and Im@HOF 2 reached 4.12 × 10 −4 S•cm −1 (333 K, 98% RH) and 1.20 × 10 −3 S•cm −1 (353 K, 98% RH), respectively. This is the first time that guest molecules were introduced into HOFs. The stability and proton conductivity of the HOFs are improved due to the interaction of imidazole with carboxyl groups and water molecules.

A new type of metal−organic framework, [Cd 2 (pdc)(H 2 O)(DMA) 2 ] n (pdc = 3,5-pyrazoledicarboxylic acid; DMA = dimethylamine), named Cd-MOF, was synthesized and characterized. There are regular rectangular pore channels containing a large number of dimethylamine cations in the crystal structure. AC impedance test results show the proton conductivity of Cd-MOF reaches 1.15 × 10 −3 S cm −1 at 363 K and 98% RH. In order for its application in fuel cells, the Cd-MOF was introduced into a sulfonated polyphenylene oxide matrix to prepare a hybrid membrane, and the proton conductivity of the hybrid membrane has a high value of 2.64 × 10 −1 S cm −1 at 343 K and 98% RH, which is higher than those of most MOF polymer hybrid membranes. The proton conductivity of the hybrid membrane of the SPPO polymer still maintains a certain degree of stability in a wide temperature range. To the best of our knowledge, it is the first proton exchange membrane that combines pyrazolecarboxylate cadmium MOFs and an SPPO polymer with high proton conductivity and good stability. This research may help to further develop the application of MOFs in the field of proton exchange membrane fuel cells.

Due to the unique anisotropic chemical and physical properties, two-dimensional (2D) layered materials, such as IV-VI monochalcogenides with puckered honeycomb structure, have received considerable interest recently. Among the IV-VI layered MX (M = Ge, Sn; X = Se, S) compounds, germanium sulfide (GeS) stands out for its strongest anisotropic thermal conductivities and figure-of-merit values. Additionally, the layer-independent direct energy bands (E g~1 .6 eV, E 1~2 .1 eV) of GeS flake provide excellent insights into further applications as visible photodetectors. Herein, the polarization-tunable nonlinear absorption (NA) patterns of GeS flake have been systematically investigated. Specifically, both the polarization-dependent Raman spectroscopy and the linear absorption (LA) spectroscopy were employed to characterize the lattice orientation and absorption edges of the 251-nm GeS flake. Considering the low damage threshold of GeS flake, the GeS/graphene heterostructure was fabricated to increase the threshold without changing the nonlinear properties of GeS. Our NA results demonstrated that a 600-nm femtosecond laser with different polarizations would excite the saturated-absorption (SA) effect along armchair and reversesaturated-absorption (RSA) effect along zigzag in the GeS/graphene heterostructure. Moreover, the function of the polarization-based GeS/graphene heterostructure all-optical switch was experimentally verified. Notably, thanks to the polarization-dependent NA patterns (SA/RSA) of GeS, the "ON" and "OFF" states of the all-optical switch can be accomplished by high and low transmittance states of continuous-wave laser (532 nm, 80 nW), whose state can be controlled by the polarization of femtosecond switching laser (600 nm, 35 fs, 500 Hz, 12 GW cm −2). The ON/OFF ratio can achieve up to 17% by changing polarization, compared with the ratios of 3.0% by increasing the incident power of switching light in our experiment. The polarization-tunable absorption patterns introduced in this work open up real perspectives for the next-generation optoelectronic devices based on GeS/graphene heterostructure.

A new metal-organic framework (MOF), [Eu2(HBDPP)2(H2O)2(DMF)2](H2O)2 (H4BDPP = 3,5-bis(3,5-dicarboxylphenyl) pyridine; DMF = N,N-dimethylformamide) (Eu-MOF), has been successfully synthesized under solvothermal condition. The 1D chain is formed by the adjacent Eu2(COO)24+...

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