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The Weather Research and Forecasting (WRF) model-based variational data assimilation system (WRF-Var) has been extended from three-to four-dimensional variational data assimilation (WRF 4D-Var) to meet the increasing demand for improving initial model states in multiscale numerical simulations and forecasts. The initial goals of this development include operational applications and support to the research community. The formulation of WRF 4D-Var is described in this paper. WRF 4D-Var uses the WRF model as a constraint to impose a dynamic balance on the assimilation. It is shown to implicitly evolve the background error covariance and to produce the flow-dependent nature of the analysis increments. Preliminary results from real-data 4D-Var experiments in a quasi-operational setting are presented and the potential of WRF 4D-Var in research and operational applications are demonstrated. A wider distribution of the system to the research community will further develop its capabilities and to encourage testing under different weather conditions and model configurations.

The recent progress on black phosphorus makes it a promising candidate material for broadband nanophotonic devices, especially operating in the mid-infrared spectral region. Here, the excited carrier dynamics and nonlinear optical response of unoxidized black phosphorus nanosheets and their wavelength dependence were systematically studied from 800 nm to 2.1 μm. The wavelength-dependent relaxation times of black phosphorus nanosheets are determined to be 360 fs to 1.36 ps with photon energies from 1.55 to 0.61 eV. In a comparative study with graphene, we found that black phosphorus has a faster carrier relaxation in near- and mid-infrared region. With regard to nonlinear optical absorption, the response of black phosphorus significantly increases from near- to mid-infrared, and black phosphorus is also confirmed to be better as saturable absorber to MoS2 in infrared region.

cm/GW, larger than that of conventional semiconductors by a factor of 10 3 . As a result of TPA, two-photon pumped frequency up-converted luminescence is observed directly in the monolayer MoS2. For the multilayer MoS2, the SA response is demonstrated with the ratio of the excited-state absorption cross section to ground-state cross section of ∼0.18. In addition, the laser damage threshold of the monolayer MoS2 is ∼ 97 GW/cm 2 , larger than that of the multilayer MoS2 of ∼78 GW/cm 2 .2

The demand for an ultrabroad optical material with a bandgap tunable from zero to at least 1-2 eV has been one of the driving forces for exploring new 2D materials since the emergence of graphene, transition metal dichalcogenides, and black phosphorus. As an ultra-broadband 2D material with energy bandgap ranging from 0 to 1.2 eV, PtSe 2 shows much better air stability than its analogue, black phosphorous. In this work, the superior nonlinear optical performance and ultrafast dynamics of layered PtSe 2 , and signatures of the transition from semiconductor to semimetal are systematically studied. Combining with rate equations, first-principles calculation, and electrical measurements, a comprehensive understanding about the evolution of nonlinear absorption and carrier dynamics with increasing layer thickness is provided, indicating its promising potential in nanophotonic devices such as infrared detectors, optical switches, and saturable absorbers.

Natural inspiration: A lotus‐leaf‐like hierarchical structure is formed on a polycarbonate plate under mild conditions by successive plasticization and coagulation of the polymer surface. The resultant micron‐ and nanoscale rough structure, shown in the figure allows the common plastic to be superhydrophobic with a large water contact angle and less contact angle hysteresis.

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We report on the passive-mode-locking operation of a fiber laser with a fundamental repetition rate of 463 MHz based on molybdenum disulfide (MoS(2)) saturable absorber (SA). By embedding MoS(2) into polyvinyl alcohol (PVA) thin film, MoS(2)-PVA SA was prepared with a modulation depth of 2.7% and a saturation intensity of 137  MW/cm(2). The mode-locked fiber laser-employed MoS(2)-PVA SA was achieved with center wavelength of 1556.3 nm, 3-dB bandwidth of 6.1 nm, output power of 5.9 mW, and an extinction ratio of up to 97 dB in the RF spectrum. The demonstration of mode-locking operation with high fundamental repetition rate and high spectral purity indicates that MoS(2)-PVA SA can be a good candidate for high-precision ultrafast applications.