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Ultrafast charge transfer in van der Waals heterostructures can effectively engineer the optical and electrical properties of two-dimensional semiconductors for designing photonic and optoelectronic devices. However, the nonlinear absorption conversion dynamics with the pump intensity and the underlying physical mechanisms in a type-II heterostructure remain largely unexplored, yet hold considerable potential for all-optical logic gates. Herein, two-dimensional ReSe2/ReS2heterostructure is designed to realize an unusual transition from reverse saturable absorption to saturable absorption (SA) with a conversion pump intensity threshold of approximately 170 GW/cm2. Such an intriguing phenomenon is attributed to the decrease of two-photon absorption (TPA) of ReS2and the increase of SA of ReSe2with the pump intensity. Based on the characterization results of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, femtosecond transient absorption spectrum, Kelvin probe force microscopy, and density functional theory calculation, a type-II charge-transfer-energy level model is proposed combined with the TPA of ReS2and SA of ReSe2processes. The results reveal the critical role of ultrafast interfacial charge transfer in tuning the unusual nonlinear absorption and improving the SA of ReSe2/ReS2under different excitation wavelengths. Our finding deepens the understanding of nonlinear absorption physical mechanisms in two-dimensional heterostructure materials, which may further diversify the nonlinear optical materials and photonic devices.

Nonlinear optical materials, especially two-dimensional materials, are anticipated to reveal broadband optical nonlinearity for future miniaturized photonic applications. Herein, we report a physical vapor deposition method to produce β-In2Se3thin film and investigate the broadband nonlinear absorption (β) and refraction (n2) characteristics. The β-In2Se3semiconductor shows an excellent optical nonlinearity with large βin 102cm GW−1scale and n2in 10−12cm2W−1scale from visible to NIR wavelengths, which are superior to those of metal carbides and nitrides (MXenes) and metal-organic frameworks. This excellent optical nonlinearity makes β-In2Se3a promising candidate for advanced nanophotonic devices and beyond.

In order to recover 3-methoxybutyl acetate from an aqueous solution containing 3-methoxybutyl acetate, four n-alkanols (C5–C8) with different carbon chain lengths were selected as solvents for the liquid–liquid equilibrium (LLE) experiment of water + 3-methoxybutyl acetate + n-alkanols and the LLE data were measured at 303.2 K, 101.3 kPa. And by calculating the distribution coefficient (D) and separation factor (S), the separation effect of different extractants was compared and discussed. The obtained LLE data were correlated by NRTL and UNIQUAC models, and the binary interaction parameters of each system were determined. The model data for each system passed the hybrid surface analysis test of the Gibbs energy topology, indicating that the obtained model data are reliable. The root-mean-square deviation (RMSD) of the two models was less than 0.72 and 0.58%, respectively. The σ-profiles and interaction energy analysis were used to investigate the effect of alkyl chain length on separation efficiency. The analysis showed that the longer the alkyl chain length of n-alkanols, the higher the extraction efficiency of n-alkanols.

Inspired by recent advancements of graphene-based ultrafast photonic devices, Ti3C2Tx-MXene, as a graphene-like two-dimensional layer-structure material, has gained extensive interest in nonlinear optical applications, especially in all-optical intensity modulation. Herein, we successfully fabricated an all-optical modulator based on Ti3C2Tx-Mxene/polyvinyl-alcohol film with nanosecond-scale response time, modulation speed of 100 kHz, and modulation depth of 10%. Furthermore, the variation of fall edges of modulated signal pulse caused by thermo-optic effect under different pump power was also observed. Considering the ease of fabrication, low cost, and ease of integration, the proposed modulator may open the door for high-speed all-optical communications and signal processing.

We demonstrate an all fiber high-repetition-rate erbium-doped ultrashort laser based on the Ti3C2Tx-Mxene saturable absorber (SA), and the 56th harmonic mode-locking with a repetition rate of 622.2 MHz, a central wavelength of 1590.7 nm, and a pulse duration of 1.31 ps is achieved. Meanwhile, dual-wavelength mode-locking pulses are also realized, with the central wavelengths of 1575.6, 1587.5 nm. The results of the work indicate that Ti3C2Tx-Mxene is an outstanding SA for generating ultrashort pulse or modulating the light and the diversity Mxene family can bring new vigor and vitality into advanced photonic devices.