The M² value is a general metric to characterize the beam quality of the high power fiber lasers. In this letter, we demonstrated a near single-mode fiber amplifier with 2.04 kW output. To the best of the authors' knowledge, we present the first evolution of the beam quality as a function of the signal output power in a multikilowatt class fiber laser utilizing step-index large-mode-area fiber. The M² parameter of was preserved at any power level without mode instability. What is more, the evolution of the beam quality and the mode field experimentally verifies the superior thermal-optical property of the high power fiber lasers compared with other bulk solid-state lasers because of their geometry of the gain medium.

The layered MoS₂ has recently attracted significant attention for its excellent nonlinear optical properties. Here, the ultrafast nonlinear optical (NLO) absorption and excited carrier dynamics of layered MoS₂ (monolayer, 3–4 layers, and 6–8 layers) are investigated via Z-scan and transient absorption spectra. Our experimental results reveal that NLO absorption coefficients of these MoS₂ increase from–27 × 10³ cm/GW to–11 × 10³ cm/GW with more layers at 400-nm laser excitation, while the values decrease from 2.0 × 10³ cm/GW to 0.8 × 10³ cm/GW at 800 nm. In addition, at high pump fluence, when the NLO response occurs, the results show that not only the reformation of the excitonic bands, but also the recovery time of NLO response decreases from 150 ps to 100 ps with an increasing number of layers, while the reductive energy of A excitonic band decreases from 191.7 meV to 51.1 meV. The intriguing NLO response of MoS₂ provides excellent potentials for the next-generation optoelectronic and photonic devices.

In this paper, the modified single trench fiber (MSTF) is proposed. The core of MSTF is successively surrounded by the trench and ring whose refractive index is lower than that of the inner cladding and identical with that of the core, respectively. Performances of MSTF have been numerically investigated by the finite element method at 1- μ m region. Simulations show that the MSTF can provide much larger loss of least lossy high-order-mode than the step-index fiber (SIF) and even larger than the standard single trench fiber, which is beneficial to achieve effective single-mode (ESM) operation and enhance the threshold of transverse-mode instabilities. Furthermore, the MSTF fabricated by using a conventional technique (core NA ∼ 0.06) with 25 and 30 μ m core diameter can guarantee the ESM operation, which is much superior than the SIF with the same parameters. The MSTF with the core diameter of 25 μ m is fabricated by using a conventional fabrication method. Excellent high-order-mode suppression and ESM operation based on the MSTF are experimentally verified. Finally, the MSTF is employed in a high-power fiber laser (HPFL), and the ESM operation ( M ² -factor <1.1) is obtained up to 807 W. The MSTF is a practical candidate aiming at industrial HPFLs requiring ESM.

Two-dimensional (2D) molybdenum telluride (MoTe₂) is attracting increasing attention for its potential applications in electronic, optoelectronic, photonic and catalytic fields, owing to the unique band structures of both stable 2H phase and 1T′ phase. However, the direct growth of high-quality atomically thin MoTe₂ with the controllable proportion of 2H and 1T′ phase seems hard due to easy phase transformation since the potential barrier between the two phases is extremely small. Herein, we report a strategy of the phase-controllable chemical vapor deposition (CVD) synthesis for few-layer (<3 layer) MoTe₂. Besides, a new understanding of the phase-controllable growth mechanism is presented based on a combination of experimental results and DFT calculations. The lattice distortion caused by Te vacancies or structural strain might make 1T′-MoTe₂ more stable. The conditions for 2H to 1T′ phase conversion are determined to be the following: Te monovacancies exceeding 4% or Te divacancies exceeding 8%, or lattice strain beyond 6%. In contrast, sufficient Te supply and appropriate tellurization velocity are essential to obtaining the prevailing 2H-MoTe₂. Our work provides a novel perspective on the preparation of 2D transition metal chalcogenides (TMDs) with the controllable proportion of 2H and 1T′ phase and paves the way to their subsequent potential application of these hybrid phases.

Fe ₃ O ₄ @Ag-rGO-Au composites for efficient SERS detection of different analytes.