We propose a scheme that utilizes weak-field-induced quantum beats to investigate the electronic coherences of atoms driven by a strong attosecond extreme ultraviolet (XUV) pulse. The technique involves using a strong XUV pump pulse to excite and ionize atoms and a time-delayed weak short pulse to probe the photoelectron signal. Our theoretical analysis demonstrates that the information regarding the bound states, initiated by the strong pump pulse, can be precisely reconstructed from the weak-field-induced quantum beat spectrum. To examine this scheme, we apply it to the attosecond XUV laser-induced ionization of hydrogen atoms by solving a three-dimensional time-dependent Schrödinger equation. This work provides an essential reference for reconstructing the ultrafast dynamics of bound states induced by strong XUV attosecond pulses.

3~5 μm的中红外激光位于大气窗口，在环境监测、**、医疗、遥感等诸多领域有着重要的应用。利用纳秒量级的1064 nm调Q激光器泵浦扇形掺氧化镁周期极化铌酸锂(MgO: PPLN)，设计了一种高效率、宽调谐纳秒中红外激光输出光学参量振荡器(Optical parametric oscillator, OPO)。通过降低泵浦光的重频，有效地减小了OPO的振荡阈值，在10 kHz的泵浦重频下，OPO阈值为0.4 W。在泵浦功率为4.68 W，晶体极化周期为30.47 μm的条件下，获得了0.833 W的3.4 μm中红外激光输出，对应的光光转换效率为17.8%。实验研究了不同极化周期下的输出波长，实验结果与理论模拟值较为吻合。通过横向移动MgO: PPLN晶体改变其极化周期，在31.05~28.8 μm的调节范围内获得了1 440.7~1607.0 nm的信号光及3 171.1~4 088.1 nm的闲频光输出，其中信号光的脉宽约为8.1 ns。

Cylindrical vector beams (CVBs), with non-uniform state of polarizations, have become an indispensable tool in many areas of science and technology. However, little research has explored high power CVBs at the femtosecond regime. In this paper, we report on the generation of high quality CVBs with high peak power and femtosecond pulse duration in a fiber chirped-pulse amplification laser system. The radially (azimuthally) polarized vector beam has been obtained with a pulse duration of 440 fs (430 fs) and a maximum average output power of 20.36 W (20.12 W). The maximum output pulse energy is ∼20μJ at a repetition rate of 1 MHz, corresponding to a high peak power of ∼46MW. The comparison between simulated intensity profiles and measured experimental results suggests that the generated CVBs have a remarkable intensity distribution. The proposed configuration of our laser system provides a promising solution for high quality CVBs generation with the characteristics of high peak power, ultrashort pulse duration, and high mode purity.

We report on a mid-infrared fiber laser that uses a single-walled carbon nanotube saturable absorber mirror to realize the mode-locking operation. The laser generates 3.5 µm ultra-short pulses from an erbium-doped fluoride fiber by utilizing a dual-wavelength pumping scheme. Stable mode-locking is achieved at the 3.5 µm band with a repetition rate of 25.2 MHz. The maximum average power acquired from the laser in the mode-locking regime is 25 mW. The experimental results indicate that the carbon nanotube is an effective saturable absorber for mode-locking in the mid-infrared spectral region.

中红外波段覆盖重要的分子吸收区与多个大气透射窗口，该波段的超快激光器在多个领域具有广泛应用。基于光纤的中红外超快激光器近年来在激光发射与传输、超快脉冲产生与应用等方面发展迅速，为中红外波段超快激光开辟了新的研究手段与应用领域。综述了近十年来中红外超快光纤激光器的发展概况，介绍了近年来中红外波段的激光传输与增益手段。其中，重点回顾了近年来中红外超快脉冲产生技术的研究进展及其代表性工作，包括非线性偏振旋转、可饱和吸收体以及频移反馈锁模技术。此外，还介绍了中红外超快脉冲的压缩放大技术与超连续谱产生应用。最后讨论并总结了中红外超快光纤激光器面临的挑战与可能的发展方向。

As one of the greatest inventions in the 20th century, ultrafast lasers have offered new opportunities in the areas of basic scientific research and industrial manufacturing. Optical modulators are of great importance in ultrafast lasers, which directly affect the output laser performances. Over the past decades, significant efforts have been made in the development of compact, controllable, repeatable, as well as integratable optical modulators (i.e., saturable absorbers). In this paper, we review the fundamentals of the most widely studied saturable absorbers, including semiconductor saturable absorber mirrors and low-dimensional nanomaterials. Then, different fabrication technologies for saturable absorbers and their ultrafast laser applications in a wide wavelength range are illustrated. Furthermore, challenges and perspectives for the future development of saturable absorbers are discussed and presented. The development of ultrafast lasers together with the continuous exploration of reliable saturable absorbers will open up new directions for the mass production of the next-generation optoelectronic devices.