Random lasers are a type of lasers that lack typical resonator structures, offering benefits such as easy integration, low cost, and low spatial coherence. These features make them popular for speckle-free imaging and random number generation. However, due to their high threshold and phase instability, the production of picosecond random lasers has still been a challenge. In this work, we have developed three dyes incorporating polymer optical fibers doped with various scattering nanoparticles to produce short-pulsed random fiber lasers. Notably, stable picosecond random laser emission lasting 600 ps is observed at a low pump energy of 50 µJ, indicating the gain-switching mechanism. Population inversion and gain undergo an abrupt surge as the intensity of the continuously pumped light nears the threshold level. When the intensity of the continuously pumped light reaches a specific value, the number of inversion populations in the “scattering cavity” surpasses the threshold rapidly. Simulation results based on a model that considers power-dependent gain saturation confirmed the above phenomenon. This research helps expand the understanding of the dynamics behind random medium-stimulated emission in random lasers and opens up possibilities for mode locking in these systems.

本文基于Φ7.5 cm×7.5 cm NaI（Tl）探测器，开展测量γ射线空气吸收剂量率方法的研究。使用蒙特卡罗方法对NaI（Tl）探测器进行仿真模拟，获取探测器50 keV~2.5 MeV能量范围的γ射线能谱图。利用G（E）函数对放射源空气吸收剂量率进行计算，求解G（E）函数时考虑K_max与优化因子M对其计算空气吸收剂量率产生的影响，对G（E）函数的求解进行优化以提高计算精度。经过优化的G（E）函数计算的空气吸收剂量率与理论标准值相对偏差S低于±1%。最后经过实验测量对比发现，G（E）函数空气吸收剂量率计算结果与剂量率仪测量结果相对偏差小于±10%，说明经过优化得到的NaI（Tl）探测器G（E）函数值可以用于空气吸收剂量率的测量应用。

Fluorescence detection is widely used in biology and medicine, while the realization of on-chip fluorescence detection is vital for the portable and point-of-care test (POCT) application. In this Letter, we propose an efficient fluorescence excitation and collection system using an integrated GaN chip consisting of a slot waveguide and a one-dimensional photonic crystal (1D PC) waveguide. The slot waveguide is used to confine the excitation light for intense light–sample interaction, and the one-trip collection efficiency at the end of slot waveguide is up to 14.65%. More interestingly, due to the introduction of the 1D PC waveguide, the fluorescence signal is directly filtered out, and the excitation light is reflected to the slot waveguide for multiple excitations. Its transmittances for the designed exciting wavelength of 520 nm and the fluorescent wavelength of 612 nm are 0.2% and 85.4%, respectively. Finally, based on numerical analysis, the total fluorescence collection efficiency in our system amounts to 15.93%. It is the first time, to our knowledge, that the concept of an all-in-one-chip fluorescence detection system has been proposed, which paves the way for on-chip fluorescence excitation and collection, and may find potential applications of miniaturized and portable devices for biomedical fluorescence detection.