The development of semiconductors is always accompanied by the progress in controllable doping techniques. Taking AlGaN-based ultraviolet (UV) emitters as an example, despite a peak wall-plug efficiency of 15.3% at the wavelength of 275 nm, there is still a huge gap in comparison with GaN-based visible light-emitting diodes (LEDs), mainly attributed to the inefficient doping of AlGaN with increase of the Al composition. First, p-doping of Al-rich AlGaN is a long-standing challenge and the low hole concentration seriously restricts the carrier injection efficiency. Although p-GaN cladding layers are widely adopted as a compromise, the high injection barrier of holes as well as the inevitable loss of light extraction cannot be neglected. While in terms of n-doping the main issue is the degradation of the electrical property when the Al composition exceeds 80%, resulting in a low electrical efficiency in sub-250 nm UV-LEDs. This review summarizes the recent advances and outlines the major challenges in the efficient doping of Al-rich AlGaN, meanwhile the corresponding approaches pursued to overcome the doping issues are discussed in detail.

针对大型激光装置中广空间分布的甚多路高精度（一是长时间时间抖动小于5 ps，二是时间延迟微步进分辨率小于15 ps）同步触发信号的需求，设计了一种“数据流编解码光传输+高速串行收发器粗延时+宽带微带线微步进延时”的同步时序产生方案。通过数据流编解码光传输架构实现了广空间范围内时序的对齐；高速串行收发器粗延时和微带线微步进延时技术解决了同步触发信号低时间抖动和高延迟分辨的问题。通过对系统的时序逻辑和电路板的关键线路进行仿真，完成了整个系统的设计与研制，并开展了实验测试。测试结果表明：该系统可以实现广空间范围内的同步时序信号产生，同步触发信号的时间抖动精度优于3.76 ps（均方根值，8 h），39.6 ps（峰峰值，8 h），时间延迟分辨率优于15 ps；若应用于小空间范围，同步触发信号的时间精度可优于1.27 ps（均方根值，8 h），12.4 ps（峰峰值，8 h）。

研制了一种具有同步时序闭环监控的精密同步机，该同步机输出光同步信号，采用单光纤反馈光模块传输同步信号、基于内插法的时间间隔测量方法，能高精度测量反馈的光同步信号与基准信号的时间差，实现了光同步信号的时序闭环监控，达到的技术指标为同步时序闭环监控误差小于等于250 ps。采用本精密时序闭环监控同步技术一方面可以保证同步信号可靠的送达触发对象，另一方面根据测得的时间差值与设定的时序延时值进行比较，可以获得同步信号经过光传输的固有延时量，从而可以实现精密时序的精确配置要求。

The refractive-lens technique has been well developed over a long period of evolution, offering powerful imaging functionalities, such as microscopes, telescopes, and spectroscopes. Nevertheless, the ever-growing requirements continue to urge further enhanced imaging capabilities and upgraded devices that are more compact for convenience. Metamaterial as a fascinating concept has inspired unprecedented new explorations in physics, material science, and optics, not only in fundamental researches but also novel applications. Along with the imaging topic, this paper reviews the progress of the flat lens as an important branch of metamaterials, covering the early superlens with super-diffraction capability and current hot topics of metalenses including a paralleled strategy of multilevel diffractive lenses. Numerous efforts and approaches have been dedicated to areas ranging from the new fascinating physics to feasible applications. This review provides a clear picture of the flat-lens evolution from the perspective of metamaterial design, elucidating the relation and comparison between a superlens and metalens, and addressing derivative designs. Finally, application scenarios that favor the ultrathin lens technique are emphasized with respect to possible revolutionary imaging devices, followed by conclusive remarks and prospects.

Reconfigurable metasurfaces have attracted a deal of attention owing to their multifunctional and dynamic electromagnetic (EM) manipulation properties. However, most of the previous reconfigurable metasurfaces rely on manual control for function switching, which has huge limitations in practical application. Here, an intelligent metasurface with the self-adaptively EM manipulation capability is proposed. It integrates the sensing-and-feedback components to construct a closed-loop system, which can automatically adjust EM functionalities for the different incident power information. The sensing module in this metasurface can first perceive the incident EM power intensity and then provide the feedback signal to the field programmable gate array controlling platform that can send the corresponding instruction to the executing material for switching the EM functionality among transmission, reflection, and tunable absorption. Good self-adaptive reaction capability and practicability of the proposed metasurface have been demonstrated by the experiment. It has the capability of making a real-time response with adaptive EM behavior to the varying incoming wave power without the aid of human beings. Our design provides an avenue toward intelligent and cognitive metasurfaces, which has extensive application prospects in smart skin, intelligent absorber, and the related EM fields.