Transition metal dichalcogenides, a family of two-dimensional material with unusual electronic, optical, mechanical, and electrochemical properties, have received much research attention in recent ...years. Here we demonstrate that, another type of few-layer transition metal dichalcogenides, rhenium disulfide (ReS 2 ) nanosheets display saturable absorption property at 1.55 μm. By incorporating the ReS 2 nanosheets with the polyvinyl alcohol (PVA), a film-type ReS 2 -PVA saturable absorber is fabricated to realize Q-switching and mode locking of erbium-doped fiber lasers. The repetition rate of the Q-switched laser pulses varies from 12.6 to 19 KHz while the duration changes from 23 to 5.496 μs by tuning the pump from 45 to 120 mW. By optimizing the polarization state, the mode-locked operation is also obtained, emitting a train of pulses centered at 1558.6 nm with the duration of 1.6 ps and the fundamental repetition rate of 5.48 MHz. It is demonstrated that ReS 2 nanosheets have the similar saturable absorption property as that of MoS 2 and WS 2 , and may find potential applications in pulsed laser, optical modulators, and sensors.
Relative permeability is a critical parameter characterizing multiphase flow in porous media and it is strongly dependent on the wettability. In many situations, the porous media are nonuniformly ...wet. To investigate the effect of wettability heterogeneity on relative permeability of two‐phase flow in porous media, a multi‐relaxation‐time color‐gradient lattice Boltzmann model is adopted to simulate oil/water two‐phase flow in porous media with different oil‐wet solid fractions. For the water phase, when the water saturation is high, the relative permeability of water increases with the increase of oil‐wet solid fraction under a constant water saturation. However, as the water saturation decreases to an intermediate value (about 0.4–0.7), the relative permeability of water in fractionally wet porous media could be lower than that in purely water‐wet porous media, meaning additional flow resistance exists in the fractionally wet porous media. For the oil phase, similar phenomenon is observed. This phenomenon is mainly caused by the wettability‐related microscale fluid distribution. According to both our simulation results and theoretical analysis, it is found that the relative permeability of two‐phase flow in porous media is strongly related to three parameters: the fluid saturation, the specific interfacial length of fluid, and the fluid tortuosity in the flow direction. The relationship between the relative permeability and these parameters under different capillary numbers is explored in this paper.
Key Points
Additional flow resistance exists for two‐phase flow in fractionally wet porous media at an intermediate fluid saturation
Relative permeability of two‐phase flow is determined by fluid saturation, specific interfacial length, and fluid tortuosity
Capillary number can influence the relationship between relative permeability and microscale fluid distribution
A novel multilayer photonic structure is proposed to achieve the strong enhancement of light absorption in monolayer molybdenum disulfide (MoS
). Both numerical and analytical results illustrate that ...the absolute absorption of light in this atomically thin layer can approach as high as 96% at the visible wavelengths due to the excitation of Tamm plasmon mode. It is also found that the operating wavelength and height of sharp absorption peak are particularly dependent on the layer thicknesses and period number of dielectric grating, MoS
position in the spacer, and incident angle of light, which contribute to the tunability and selectivity of light-MoS
interaction. These results would provide a new pathway for the improvement of MoS
photoluminescence and photodetection.
•The advances and challenges of pore-scale modeling are summarized.•Porous structure imaging and computational reconstruction methods are reviewed.•Recent progresses in pore-scale numerical methods ...and schemes are introduced.•Pore-scale studies of transport processes in porous media are discussed.•Application of pore-scale modeling in geoscience and fuel cells are presented.
Porous media play important roles in a wide range of scientific and engineering problems. Recently, with their increasing application in energy conversion and storage devices, such as fuel cells, batteries and supercapacitors, it has been realized that transport processes and reactions occurring in the pores and at the interfaces of different constituents significantly affect the performance of the porous media, yet these pore-scale transport phenomena are not well described or even neglected in the conventional numerical models based on the representative element volume (REV). Pore-scale modeling is an efficient tool for the simulation of pore-scale transport and reactions in porous media because of its ability to accurately characterize these processes and to provide the distribution details of important variables which are challenging for current experimental techniques to provide either due to lack of in-situ measurement capability or due to the limited spatial and temporal resolution. In the present review, the advances and challenges of the state-of-the-art pore-scale modeling are summarized. The practical applications of pore-scale modeling in the fields of geoscience, polymer exchange membrane fuel cells (PEMFC) and solid oxide fuel cells (SOFC) are discussed. Notable results from the pore-scale modeling are presented, and the challenges facing the pore-scale model development are discussed. This in-depth review is intended to give a well-rounded introduction of critical aspects on which the pore-scale modeling can shed light in the development of relevant scientific and engineering systems.
Locking of longitudinal modes in laser cavities is the common path to generate ultrashort pulses. In traditional multi-wavelength mode-locked lasers, the group velocities rely on lasing wavelengths ...due to the chromatic dispersion, yielding multiple trains of independently evolved pulses. Here, we show that mode-locked solitons at different wavelengths can be synchronized inside the cavity by engineering the intracavity group delay with a programmable pulse shaper. Frequency-resolved measurements fully retrieve the fine temporal structure of pulses, validating the direct generation of synchronized ultrafast lasers from two to five wavelengths with sub-pulse repetition-rate up to ~1.26 THz. Simulation results well reproduce and interpret the key experimental phenomena, and indicate that the saturable absorption effect automatically synchronize multi-wavelength solitons in despite of the small residual group delay difference. These results demonstrate an effective approach to create synchronized complex-structure solitons, and offer an effective platform to study the evolution dynamics of nonlinear wavepackets.
In this work, a numerical model for isothermal liquid–vapor phase change (evaporation) of the two-component air–water system is proposed based on the pseudopotential lattice Boltzmann method. Through ...the Chapman–Enskog multiscale analysis, we show that the model can correctly recover the macroscopic governing equations of the multicomponent multiphase system with a built-in binary diffusion mechanism. The model is verified based on the two-component Stefan problem where the measured binary diffusivity is consistent with theoretical analysis. The model is then applied to convective drying of a dual-porosity porous medium at the pore scale. The simulation captures a classical transition in the drying process of porous media, from the constant rate period (CRP, first phase) showing significant capillary pumping from large to small pores, to the falling rate period (FRP, second phase) with the liquid front receding in small pores. It is found that, in the CRP, the evaporation rate increases with the inflow Reynolds number (Re), while in the FRP, the evaporation curves almost collapse at different Res. The underlying mechanism is elucidated by introducing an effective Péclet number (Pe). It is shown that convection is dominant in the CRP and diffusion in the FRP, as evidenced by Pe > 1 and Pe < 1, respectively. We also find a log-law dependence of the average evaporation rate on the inflow Re in the CRP regime. The present work provides new insights into the drying physics of porous media and its direct modeling at the pore scale.
Molybdenum disulfide (MoS2) and tungsten disulfide (WS2), two representative transition metal dichalcogenide materials, have captured tremendous interest for their unique electronic, optical, and ...chemical properties. Compared with MoS2 and WS2, molybdenum ditelluride (MoTe2) and tungsten ditelluride (WTe2) possess similar lattice structures while having smaller bandgaps (less than 1 eV), which is particularly interesting for applications in the near‐infrared wavelength regime. Here, few‐layer MoTe2/WTe2 nanosheets are fabricated by a liquid exfoliation method using sodium deoxycholate bile salt as surfactant, and the nonlinear optical properties of the nanosheets are investigated. The results demonstrate that MoTe2/WTe2 nanosheets exhibit nonlinear saturable absorption property at 1.55 μm. Soliton mode‐locking operations are realized separately in erbium‐doped fiber lasers utilizing two types of MoTe2/WTe2‐based saturable absorbers, one of which is prepared by depositing the nanosheets on side polished fibers, while the other is fabricated by mixing the nanosheets with polyvinyl alcohol and then evaporating them on substrates. Numerous applications may benefit from the nonlinear saturable absorption features of MoTe2/WTe2 nanosheets, such as visible/near‐infrared pulsed laser, materials processing, optical sensors, and modulators.
Molybdenum ditelluride (MoTe2) and tungsten ditelluride (WTe2) are two representative transition metal dichalcogenides with near‐infrared bandgaps. Few‐layer MoTe2/WTe2, fabricated by the liquid exfoliation method, are found to exhibit nonlinear saturable absorption properties at 1.55 μm. Soliton mode locking is realized in erbium‐doped fiber lasers utilizing MoTe2/WTe2 saturable absorbers.
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