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Two-dimensional (2D) ternary transition-metal dichalcogenides (TMDCs) are of great research interest because their superior layer-dependent optical modulation properties. In this work, three different kinds of TMDC nanosheets, including hafnium diselenide (HfSe2), titanium diselenide (TiSe2) and zirconium diselenide (ZrSe2), are prepared by liquid phase exfoliation (LPE) technique. The high-quality material properties of these TMDC nanosheets are confirmed by Raman spectroscopy and X-ray diffraction analysis. Furthermore, the bandgap information of five-layer MSe2 has been investigated via utilizing density functional theory. The calculation results exhibit ultra-narrow bandgap structure (lower than 1.1 eV) for all these three materials, indicating that MSe2 is suitable for broadband photonic applications. By applying the fabricated MSe2 as saturable absorbers, high-performance Q-switched mode-locked laser operation has been realized. The laser gain media are Nd:GdVO4 cladding waveguides fabricated by femtosecond laser direct writing. As a result, the pulsed waveguide lasers are able to deliver approximately 6-GHz laser pulses with a signal-to-noise ratio of over 45 dB. The minimum pulse width is determined to be as short as 26 ps. The results demonstrated in this work exhibit the great potential of TMDCs and waveguide structures in applications of pulsed lasers with compact footprints.

Emerging from the modeling of intricate structures formed by bacterial motion in nutrient-deficient environments, the doubly degenerate nutrient taxis system ut = ∇⋅ (uv∇u) −∇⋅ (u2v∇v) + u − u2, vt = Δv − uv is considered in smoothly bounded domains Ω ⊂ ℝ2. In contrast to frameworks devoid of logistic growth where only small-data global solutions have so far been found in the literature, quadratic degradation in the logistic growth is shown to markedly amplify the overall dissipative effects to the extent that, for arbitrary smooth initial data, an associated no-flux type initial-boundary problem possesses global continuous weak solutions. Central to our findings is the identification of an advantageous dissipative structure subtly embedded in the system, the exploitation of which leads to the establishment of a positive lower bound for nutrients levels within any finite time interval, thereby ruling out the appearance of cross-degeneracies within such timeframes. [ABSTRACT FROM AUTHOR]

The CO2-responsive Pickering emulsions employed have garnered considerable attention across enhanced oil recovery agents, delivery of crude, and oilfield-produced fluid treatment. This study focuses on the design of an innovative type of Janus nanoparticle (JNP) and its application in the construction of CO2 stimuli-responsive Pickering emulsions. The emulsion stabilized by JNPs demonstrates swift phase separation subsequent to the injection of CO2. Furthermore, the mechanism of the CO2-stimulated responsiveness enabled by the JNPs was proposed. Based on the Schiff reaction, the synthesis of JNPs was achieved successfully via the Pickering emulsion template method. The properties of the JNPs were studied using various analytical techniques such as Fourier transform infrared, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, etc. The developed responsive system, composed of JNPs and sodium oleate surfactant, showed high interfacial activity at the oil/water interface. The study revealed that the responsive system-stabilized Pickering emulsions could remain stable at 70 °C and room temperature for 24 and 120 h, respectively. Furthermore, the introduction of CO2 for 1.5 min resulted in phase separation because the nanoparticles lost the amphiphilic nature, triggering a significant decrease in interfacial activity and the protonation of sodium oleate surfactant inactivation. Then, the particles started to desorb from the interface. This study proposes a convenient, simple, and cost-effective approach for preparing fast CO2-responsive Pickering emulsions. It provides novel insights into the advancement of oilfield-produced fluid treatment, transportation of crude oil, and enhanced oil recovery. [ABSTRACT FROM AUTHOR]

We consider the Keller-Segel-type migration-consumption system involving signal-dependent motilities, \[ \begin{cases} u_t = \Delta \left(u\phi(v)\right), \\ v_t = \Delta v-uv, \end{cases} \] { u t = Δ (u ϕ (v)) , v t = Δ v − u v , in smoothly bounded domains $ {\Omega \subset \mathbb {R}^{n}} $ Ω ⊂ R n , $ n\ge 1 $ n ≥ 1. Under the assumption that $ \phi \in C^1([0,\infty)) $ ϕ ∈ C 1 ([ 0 , ∞)) is positive on $ [0,\infty) $ [ 0 , ∞) , and for nonnegative initial data from $ (C^0(\overline {\Omega }))^\star \times L^\infty (\Omega) $ (C 0 (Ω ¯)) ⋆ × L ∞ (Ω) , previous literature has provided results on global existence of certain very weak solutions with possibly quite poor regularity properties, and on large time stabilization toward semitrivial equilibria with respect to the topology in $ (W^{1,2}(\Omega))^\star \times L^\infty (\Omega) $ (W 1 , 2 (Ω)) ⋆ × L ∞ (Ω). The present study reveals that solutions in fact enjoy significantly stronger regularity features when $ 0 \lt \phi \in C^3([0,\infty)) $ 0 < ϕ ∈ C 3 ([ 0 , ∞)) and the initial data belong to $ (W^{1,\infty }(\Omega))^2 $ (W 1 , ∞ (Ω)) 2 : It is firstly shown, namely, that then in the case $ n\le 2 $ n ≤ 2 an associated no-flux initial-boundary value problem even admits a global classical solution, and that each of these solutions smoothly stabilizes in the sense that as $ t\to \infty $ t → ∞ we have \[ u(\cdot,t) \to \frac{1}{|\Omega|} \int_\Omega u_0 \quad \mbox{and} \quad v(\cdot,t)\to 0 \qquad (\star) \] u (⋅ , t) → 1 | Ω | ∫ Ω u 0 and v (⋅ , t) → 0 (⋆) even with respect to the norm in $ L^\infty (\Omega) $ L ∞ (Ω) in both components. In the case when $ n\ge 3 $ n ≥ 3 , secondly, some genuine weak solutions are found to exist globally, inter alia satisfying $ \nabla u\in L^{{4}/{3}}_{loc}(\overline {\Omega }\times [0,\infty);\mathbb {R}^n) $ ∇ u ∈ L l o c 4 / 3 (Ω ¯ × [ 0 , ∞) ; R n). In the particular three-dimensional setting, any such solution is seen to become eventually smooth and to satisfy (★). [ABSTRACT FROM AUTHOR]

Lead sulfide (PbS), a classical narrow‐bandgap material, has manifested itself as a reliable choice for short‐wave infrared (SWIR) detection with high sensitivity. The primary efforts of optimizing PbS‐based photodetectors rely on discovering new low‐dimensional morphology and designing delicate device architecture, which may raise problems in the synergistic improvement of response speed, sensitivity, and response region. Herein, a graphene‐PbS nanocuboids‐graphene vertical photodetector is reported, fabricated by in situ electrochemical growth of monocrystalline PbS nanocuboids on graphene using an electrochemical technique. Under illumination, the surface states of PbS nanocuboids trap photogenerated holes and form a photovoltage, which widens the conductive region in the channel. This photogating effect contributes to the photocurrent and renders the device with high responsivity. Moreover, the highly crystalline PbS nanocuboids serve as excellent vertical channels, allowing a fast speed to be achieved. At room temperature, the device shows a responsivity of ≈130 A W−1 at 2700 nm, an external quantum efficiency of ≈6000%, a detectivity of 3 × 109 cm Hz1/2 W−1 (@1 kHz), and response/recovery speeds of 15/42 ms. This vertically integrated device, composed of PbS nanocuboids and graphene, offers a promising approach to complementary metal oxide semiconductor (CMOS)‐compatible SWIR detection, thus opening up an avenue to tailor device performance by exploiting the anisotropic morphology of nanomaterials. [ABSTRACT FROM AUTHOR]

Photodetectors that can simultaneously exhibit high gain, fast response, and low dark current are of significant value in modern optoelectronic systems. However, the recently reported photovoltage‐driven phototransistors are encountering certain challenges to meet the above requirement. In this work, a "photo‐driven" semi‐metal–semiconductor field‐effect transistor (photo‐sMESFET), based on graphene/silicon‐on‐insulator hybrid structure, is proposed and demonstrated to realize the synchronous optimization of gain, response speed, and dark current. Taking advantage of the unique semimetal characteristics of graphene, a high‐quality Schottky junction between graphene and silicon is produced, which can greatly deplete the silicon channel and suppress dark current. After illumination, the photovoltage at graphene–silicon interface efficiently shrinks the depletion layer and tremendously enhances the channel conductance. Combined with the high transconductance of the silicon channel, the photo‐sMESFET exhibits a gain of more than 104, a fast response time (τ) of 3 µs, and a low dark current (Jd) of 2.5 µA. Furthermore, the figure of merit, GTD=(Gain×τ)/Jd\[{\rm{GTD}} = ({\rm{Gain}}{\bm{ \times }}\tau)/\sqrt {{J_d}} \], evaluating the comprehensive performance on photodetectors, is up to 8 × 108 A1/2 cm s−1, which is superior to the counterparts of graphene‐based photodetectors. The results presented leverage a new detection strategy based on the semimetal–semiconductor photovoltaic effect. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) carbides of transition metals, so called "MXenes", maintain excellent optical properties and unique layer stacking form via hydrogen bonds in contrast to other low-dimensional materials. These promising features have attracted increasingly research interest in the field of ultrafast photonics. In this work, titanium carbide (Ti3C2Tx) and tantalum carbide (Ta4C3Tx), as two promising MXene materials for photonic applications, are experimentally synthesized by the ultrasound-assisted liquid phase exfoliation technique. The morphology of prepared few-layer Ti3C2Tx and Ta4C3Tx has been systematically characterized by transmission electron microscopy and X-ray diffraction analysis. Density functional theory is performed to obtain their electronic band structures, demonstrating metallic properties and verifying their optical absorption at 1 μm. Their tempting optical modulation properties for multi-gigahertz pulsed laser emission are demonstrated by using them as saturable absorbers in a waveguide laser cavity. Particularly, high-performance Q-switched mode-locked lasers with/without laser mirrors are realized based on the hybrid waveguide laser configuration, delivering 1 μm laser pulses with durations as short as 30 ps. The results presented in this work show the great potential of metallic MXenes and waveguide structures for applications in functional photonic devices. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) ternary transition-metal dichalcogenides (TMDCs) are of great research interest because their superior layer-dependent optical modulation properties. In this work, three different kinds of TMDC nanosheets, including hafnium diselenide (HfSe2), titanium diselenide (TiSe2) and zirconium diselenide (ZrSe2), are prepared by liquid phase exfoliation (LPE) technique. The high-quality material properties of these TMDC nanosheets are confirmed by Raman spectroscopy and X-ray diffraction analysis. Furthermore, the bandgap information of five-layer MSe2 has been investigated via utilizing density functional theory. The calculation results exhibit ultra-narrow bandgap structure (lower than 1.1 eV) for all these three materials, indicating that MSe2 is suitable for broadband photonic applications. By applying the fabricated MSe2 as saturable absorbers, high-performance Q-switched mode-locked laser operation has been realized. The laser gain media are Nd:GdVO4 cladding waveguides fabricated by femtosecond laser direct writing. As a result, the pulsed waveguide lasers are able to deliver approximately 6-GHz laser pulses with a signal-to-noise ratio of over 45 dB. The minimum pulse width is determined to be as short as 26 ps. The results demonstrated in this work exhibit the great potential of TMDCs and waveguide structures in applications of pulsed lasers with compact footprints. [ABSTRACT FROM AUTHOR]

Second‐harmonic generation (SHG) of green lasers generated in KTiOAsO4 (KTA) cladding waveguides fabricated by femtosecond laser direct writing (FsLDW) is reported. The minimum propagation loss of the fabricated waveguides is determined to be 2.39 ± 0.24 dB cm−1 at 1064 nm. The SHG performance is characterized by utilizing a 1064 nm pulsed laser. The maximum SHG conversion efficiency reaches 13.67%. This work suggests potential applications of KTA crystals and FsLDW technology in definition of compact waveguide devices for integrated nonlinear optics. [ABSTRACT FROM AUTHOR]

In a bounded domain Ω ⊂ Rn with smooth boundary, this work considers the indirect pursuit-evasion model {ut = Δu − χ∇ ⋅ (u∇w) + u(λ − u + av), vt = Δv + ξ∇ ⋅ (v∇z) + v(μ − v − bu), 0= Δw − w + v, 0 = Δz − z + u, with positive parameters χ, ξ, λ, μ, a and b. It is firstly asserted that when n ≤ 3, for any given suitably regular initial data the corresponding homogeneous Neumann initial-boundary problem admits a global and bounded smooth solution. Moreover, it is shown that if bλ < μ and under some explicit smallness conditions on χ and ξ, any nontrival bounded classical solution converges to the spatially homogeneous coexistence state in the large time limit; if bλ > μ, however, then under an explicit smallness assumption on χ but without any restriction on ξ, any bounded classical solution (u,v) with u &#8802 0 stabilizes to (λ,0) as t → ∞. [ABSTRACT FROM AUTHOR]

The van der Waals (vdWs) heterostructures, with vertical layer stacking structure of various two-dimensional (2D) materials, maintain the reliable photonic characteristics while compensating the shortcomings of the participating individual components. In this work, we combine the less-studied multilayer tin selenide (SnSe 2 ) thin film with one of the traditional 2D materials, graphene, to fabricate the graphene-based vdWs optical switching element (Gr-SnSe 2 ) with superior broadband nonlinear optical response. The transient absorption spectroscopy (TAS) measurement results verify that graphene acts as the recombination channel for the photogenerated carrier in the Gr-SnSe 2 sample, and the fast recovery time can be reduced to hundreds of femtoseconds which is beneficial for the optical modulation process. The optical switching properties are characterized by the I-scan measurements, exhibiting a saturable energy intensity of 2.82 mJ·cm -2 (0.425 µJ·cm -2 ) and a modulation depth of 15.6% (22.5%) at the wavelength of 1030 nm (1980nm). Through integrating Gr-SnSe 2 with a cladding waveguide, high-performance picosecond Q-switched operation in the near-infrared (NIR) and mid-infrared (MIR) spectral regions are both achieved. This work experimentally demonstrates the great potential of graphene-based vdWs heterostructures for applications in broadband ultrafast photonics.