To our knowledge, this is the shortest pulse ever reported for graphene mode-locked lasers and mode-locked Yb-doped bulk lasers. Our experimental results demonstrate that graphene mode locking is a very promising practical technique to generate few-cycle optical pulses directly from a laser oscillator.
Nanostructured ferritic alloy-silicon carbide composites (NFA-SiC) were sintered at 1950°C-2100°C by spark plasma sintering (SPS). The influence of NFA addition on the sintered NFA-SiC composites has been investigated based on densification, phase, microstructure, and mechanical property. Addition of 2.5 vol% NFA can lead to full densification for the NFA-SiC composites even though it also introduces secondary carbonrich and iron-rich phases. With the increase of the sintering temperature, the Vickers hardness for the pure SiC and NFA-SiC composite samples is enhanced. However, the NFA-SiC composites have lower hardness than the pure SiC samples. The flexural strength for the pure SiC is also higher than that of the NFA-SiC composites. Nanohardness results demonstrate that the reaction products between SiC and NFA lower the mechanical properties of the NFA-SiC samples.
Highly transparent 0.5 at.% Tm:Y 2 O 3 ceramics were prepared by using solid-state reaction combined with vacuum sintering method, with ZrO 2 and Al 2 O 3 as sintering aids. Doping amount of ZrO 2 was fixed at 1 at.%, while the effect of Al 2 O 3 on densification, microstructure evolution, and transmittance of the Y 2 O 3 ceramics was carefully studied. It was found that the addition of Al 2 O 3 was very effective in improving densification of Y 2 O 3 , due to the formation of an Al-rich eutectic phase Y 4 Al 2 O 9 (YAM) during the sintering process. As the content of Al 2 O 3 was increased from 0 to 81.8 wt ppm, porosity of the ceramics was decreased and transmittance was increased. However, when the content of Al 2 O 3 was increased to 137 wt ppm, a secondary phase began to segregate at grain junctions. Further increase in the amount of Al 2 O 3 led to an increase in both amount and size of the secondary phase. At the optimized content of Al 2 O 3 with 81.8 wt ppm, the Tm:Y 2 O 3 ceramics sintered at 1860°C for 13 h exhibited an in-line transmittance of 83.0% at 2000 nm and 76.5% at 600 nm. It is expected that this finding can be readily applied to other transparent ceramics.
The crystals Gd2SiO5 and Yb‐doped Gd2SiO5 were grown by Czochralski method successfully. Dielectric properties of the two crystals were investigated in the temperature range from 100 to 1100 K and the frequency range from 20 Hz to 10 MHz. Our results revealed that these crystals show intrinsic dielectric behavior in the temperature below room temperature, whereas in above room temperature, a relaxor‐like dielectric anomaly and a metal–insulator transition were observed. It was found that the relaxor‐like anomaly contains two dielectric relaxations. Impedance analysis showed that the low‐ and high‐temperature relaxations are dipolar‐ and Maxwell–Wagner‐type relaxation, respectively. The transition was found to be strongly related to oxygen vacancies, which leads to positive temperature coefficient of resistance and an abnormal dielectric behavior contrary to the conventional thermally activated behavior at temperatures near the transition temperature.
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