Atomically thin Bi2O2Se has emerged as a novel two-dimensional (2D) material with an ultrabroadband nonlinear optical response, high carrier mobility and excellent air stability, showing great potential for the realization of optical modulators. Here, we demonstrate a femtosecond solid-state laser at 1.0 µm with Bi2O2Se nanoplates as a saturable absorber (SA). Upon further defect regulation in 2D Bi2O2Se, the average power of the mode-locked laser is improved from 421 mW to 665 mW, while the pulse width is decreased from 587 fs to 266 fs. Moderate Ar+ plasma treatments are employed to precisely regulate the O and Se defect states in Bi2O2Se nanoplates. Nondegenerate pump-probe measurements show that defect engineering effectively accelerates the trapping rate and defect-assisted Auger recombination rate of photocarriers. The saturation intensity is improved from 3.6 ± 0.2 to 12.8 ± 0.6 MW cm−2 after the optimized defect regulation. The enhanced saturable absorption and ultrafast carrier lifetime endow the high-performance mode-locked laser with both large output power and short pulse duration.
Dynamic characteristics of GaN HEMT grown on a native substrate were systematically investigated at 300K and 150K. Transfer and output characteristics of the GaN HEMT were measured after various off-state stressing conditions and recovery durations. In addition, a high-speed scheme was employed to finish the measurement within 75 μs, and to ensure maximum preservation of stressing/recovery consequences. The threshold voltage instability and current collapse commonly observed at room temperature were mostly diminished at 150K, which was attributed to reduced number of electrons through the metal-semiconductor contact and insufficient number of carriers overcoming the capture potential barrier. Two pulsed I-V measurements, including evaluations with various off-state quiescent bias points and "on-the-fly" on-resistance sampling, confirmed an inefficient electron capture process at 150K, with a time constant larger than dozens of seconds. The output characteristic comparison between hard switch and soft switch at 150K provided direct experimental evidence for electron capture promotion by hot carriers.
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