Pump-probe differential reflection and transmission spectroscopy is a very effective tool to study the nonequili-brium carrier dynamics of graphene. The reported sign of differential reflection from graphene is not explicitly explained and not consistent. Here, we study the differential reflection and transmission signals of graphene on a dielectric substrate. The results reveal the sign of differential reflection changes with the incident direction of the probe beam with respect to the substrate. The obtained theory can be applied to predict the differential signals of other two-dimensional materials placed on various dielectric substrates.

Due to the manifestation of fascinating physical phenomena and materials science, two-dimensional (2D) materials have recently attracted enormous research interest with respect to the fields of electronics and optoelectronics. There have been in-depth investigations of the nonlinear properties with respect to saturable absorption, and many 2D materials show potential application in optical switches for passive pulsed lasers. However, the Eigen band-gap determines the responding wavelength band and constrains the applications. In this paper, based on band-gap engineering, some different types of 2D broadband saturable absorbers are reviewed in detail, including molybdenum disulfide (MoS2), vanadium dioxide (VO2), graphene, and the Bi2Se3 topological insulator. The results suggest that the band-gap modification should play important roles in 2D broadband saturable materials and can provide some inspiration for the exploration and design of 2D nanodevices.

We propose and demonstrate a dual-wavelength single-longitudinal-mode (SLM) fiber laser with switchable wavelength spacing based on a graphene saturable absorber (GSA) and a WaveShaper. By virtue of the excellent saturable absorption ability of graphene, the linewidths of the lasing wavelengths can be effectively reduced and eventually SLM operation can be obtained. The linewidths of both wavelengths are measured to be narrower than 7.3 kHz. The obtained results suggest that the graphene would be a good candidate nonlinear optical material for applications in related photonic fields, such as SLM oscillation generation for microwave generation and optical sensing.

A compact saturable absorber mirror (SAM) based on few-layer molybdenum disulfide (MoS2) nanoplatelets was fabricated and successfully used as an efficient saturable absorber (SA) for the passively Q-switched solid-state laser at 1 μm wavelength. Pulses as short as 182 ns were obtained from a ytterbium-doped (Yb:LGGG) bulk laser Q-switched by the MoS2 SAM, which we believe to be the shortest one ever achieved from the MoS2 SAs-based Q-switched bulk lasers. A maximum average output power of 0.6 W was obtained with a slope efficiency of 24%, corresponding to single pulse energy up to 1.8 μJ. In addition, the simultaneous dual-wavelength Q-switching at 1025.2 and 1028.1 nm has been successfully achieved. The results indicate the promising potential of few-layer MoS2 nanoplatelets as nonlinear optical switches for achieving efficient pulsed bulk lasers.

Few-layer molybdenum disulfide (MoS2) is emerging as a promising quasi-two-dimensional material for photonics and optoelectronics, further extending the library of suitable layered nanomaterials with exceptional optical properties for use in saturable absorber devices that enable short-pulse generation in laser systems. In this work, we catalog and review the nonlinear optical properties of few-layer MoS2, summarize recent progress in processing and integration into saturable absorber devices, and comment on the current status and future perspectives of MoS2-based pulsed lasers.

We propose a low-threshold soliton fiber laser passively mode locked with two different types of film-like saturable absorbers, one of which is fabricated by mixing Bi2Te3 with de-ionized water, as well as polyvinyl alcohol (PVA), and then evaporating them in a Petri dish, and the other of which is prepared by directly dropping Bi2Te3 solution on the PVA film. Both Bi2Te3–PVA films exhibit outstanding features of low loss, high flexibility, and easy synthesis. By incorporating Bi2Te3–PVA films into fiber lasers, stable single-soliton emissions are obtained at a low pump power of 13 mW. Our results suggest that the Bi2Te3 can work as a promising mode locker for ultrafast lasers, while PVA is an excellent host for fabricating high-performance film-based saturable absorbers.

With MoS2 as saturable absorber, passive Q-switching and Q-switched mode-locking operations of a Tm-doped calcium lithium niobium gallium garnet (Tm:CLNGG) laser were experimentally demonstrated. The Q-switched laser emitted a maximum average output power of 62 mW and highest pulse energy of 0.72 μJ. Q-switched mode locking was also obtained in the experiment. The research results will open up applications of MoS2 at the mid-infrared wavelength.

Liquid-phase-exfoliation technology was utilized to prepare layered MoS2, WS2, and MoSe2 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 n2 was measured to be ～10 7 cm2·W 1, and the third-order nonlinear susceptibility χ(3) ～ 10 9 esu. The relative change of effective nonlinear refractive index Δn2e∕n2e of the MoS2, WS2, and MoSe2 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.

The paper summarizes the recent achievements in the area of ultrafast fiber lasers mode-locked with so-called lowdimensional nanomaterials: graphene, topological insulators (Bi2Te3, Bi2Se3, Sb2Te3), and transition metal sulfide semiconductors, like molybdenum disulfide (MoS2). The most important experimental achievements are described and compared. Additionally, new original results on ultrashort pulse generation at 1.94 μm wavelength using graphene are presented. The designed Tm-doped fiber laser utilizes multilayer graphene as a saturable absorber and generates 654 fs pulses at 1940 nm wavelength, which are currently the shortest pulses generated from a Tm-doped fiber laser with a graphene-based saturable absorber.

A graphene-coated microfiber (GCM)-based hybrid waveguide structure formed by wrapping monolayer graphene around a microfiber with length of several millimeters is pumped by a nanosecond laser at ～1550 nm, and multiorder cascaded four-wave-mixing (FWM) is effectively generated. By optimizing both the detuning and the pump power, such a GCM device with high nonlinearity and compact size would have potential for a wide range ofFWM applications, such as phase-sensitive amplification, multi-wavelength filter, all-optical regeneration and frequency conversion, and so on.

Two-dimensional (2D) materials have emerged as attractive mediums for fabricating versatile optoelectronic devices. Recently, few-layer molybdenum disulfide (MoS2), as a shining 2D material, has been discovered to possess both the saturable absorption effect and large nonlinear refractive index. Herein, taking advantage of the unique nonlinear optical properties of MoS2, we fabricated a highly nonlinear saturable absorption photonic device by depositing the few-layer MoS2 onto the microfiber. With the proposed MoS2 photonic device, apart from the conventional soliton patterns, the mode-locked pulses could be shaped into some new soliton patterns, namely, multiple soliton molecules, localized chaotic multipulses, and double-scale soliton clusters. Our findings indicate that the few-layer MoS2-deposited microfiber could operate as a promising highlynonlinear photonic device for the related nonlinear optics applications.

In this paper, both nonlinear saturable absorption and two-photon absorption (TPA) of few-layer molybdenum diselenide (MoSe2) were observed at 1.56 μm wavelength and further applied to mode-locked ultrafast fiber laser for the first time to our knowledge. Few-layer MoSe2 nanosheets were prepared by liquid-phase exfoliation method and characterized by x ray diffractometer, Raman spectroscopy, and atomic force microscopy. The obtained few-layer MoSe2 dispersion is further composited with a polymer material for convenient fabrication of MoSe2 thin films. Then, we investigated the nonlinear optical (NLO) absorption property of the few-layer MoSe2 film using a balanced twin-detector measurement technique. Both the saturable absorption and TPA effects of the few-layer MoSe2 film were found by increasing the input optical intensity. The saturable absorption shows a modulation depth of 0.63% and a low nonsaturable loss of ～3.5%, corresponding to the relative modulation depth of 18%. The TPA effect occurred when the input optical intensity exceeds ～260 MW/cm2. Furthermore, we experimentally exploit the saturable absorption of few-layer MoSe2 film to mode lock an all-fiber erbium-doped fiber laser. Stable soliton mode locking at 1558 nm center wavelength is achieved with pulse duration of 1.45 ps. It was also observed that the TPA process suppresses the mode-locking operation in the case of higher optical intensity. Our results indicate that layered MoSe2, as another two-dimensional nanomaterial, can provide excellent NLO properties (e.g., saturable absorption and TPA) for potential applications in ultrashort pulse generation and optical limiting.

We report a simple solution-processed method for the fabrication of low-cost, flexible optical limiting materials based on graphene oxide (GO) impregnated polyvinyl alcohol (PVA) sheets. Such GO–PVA composite sheets display highly efficient broadband optical limiting activities for femtosecond laser pulses at 400, 800, and 1400 nm with very low limiting thresholds. Femtosecond pump–probe measurement results revealed that nonlinear absorption played an important role for the observed optical limiting activities. High flexibility and efficient optical limiting activities of these materials allow these composite sheets to be attached to nonplanar optical sensors in order to protect them from light-induced damage.

We report an erbium-doped fiber laser passively Q-switched by a few-layer molybdenum disulfide (MoS2) saturable absorber (SA). The few-layer MoS2 is grown by the chemical vapor deposition method and transferred onto the end-face of a fiber connector to form a fiber-compatible MoS2 SA. The laser cavity is constructed by using a three-port optical circulator and a fiber Bragg grating (FBG) as the two end-mirrors. Stable Q-switched pulses are obtained with a pulse duration of 1.92 μs at 1560.5 nm. By increasing the pump power from 42 to 204 mW, the pulse repetition rate can be widely changed from 28.6 to 114.8 kHz. Passive Q-switching operations with discrete lasing wavelengths ranging from 1529.8 to 1570.1 nm are also investigated by using FBGs with different central wavelengths. This work demonstrates that few-layer MoS2 can serve as a promising SA for wideband-tunable Q-switching laser operation.

In this paper, we reported a multiwavelength passively Q-switched Yb3+:GdAl3(BO3)4 solid-state laser with topological insulator Bi2Te3 as a saturable absorber (SA) for the first time, to the best of our knowledge. Bi2Te3 nanosheets were prepared by the facile solvothermal method. The influence of three Bi2Te3 densities on the laser operation was compared. The maximum average output power was up to 57 mW with a pulse energy of 511.7 nJ. The shortest pulsewidth was measured to be 370 ns with 110 kHz pulse repetition rate and 40 mW average power. The laser operated at three wavelengths simultaneously at 1043.7, 1045.3, and 1046.2 nm, of which the frequency differences were within the terahertz wave band. Our work suggests that solvothermal synthesized Bi2Te3 is a promising SA for simultaneously multiwavelength laser operation.

Deposition of two-dimensional (2D) MoS2 materials on the tapered fiber allows various photonic applications including saturable absorbers and four-wave mixing. Ethanol catalytic deposition (ECD) of MoS2 on the optical tapered fiber was proposed and demonstrated in this work. Different from the conventional optical driven deposition using water or organic solvent, the ECD method utilized the high volatility of the ethanol solvent, which significantly increased the movement speed of the MoS2 nanosheets and thus boosted the deposition rate and reduced the minimum power threshold to drive the deposition. We believe the ECD method should be able to be applied to other similar 2D materials such as other types of transition metal chalcogenides.

We propose and demonstrate a dual-wavelength single-longitudinal-mode (SLM) fiber laser with switchable wavelength spacing based on a graphene saturable absorber (GSA) and a WaveShaper. By virtue of the excellent saturable absorption ability of graphene, the linewidths of the lasing wavelengths can be effectively reduced and eventually SLM operation can be obtained. The linewidths of both wavelengths are measured to be narrower than 7.3 kHz. The obtained results suggest that the graphene would be a good candidate nonlinear optical material for applications in related photonic fields, such as SLM oscillation generation for microwave generation and optical sensing.