As one of the "four great inventions" in the 20th century, laser has played a revolutionary and irreplaceable key role in scientific research, industrial manufacturing, aerospace, biomedical, national defense and security and other fields. At present, laser technology is "ubiquitous" and has been widely used in human production and life. Laser gain medium is the core and foundation of laser, and plays a "milestone" role in the development of laser technology. Solid laser gain medium refers to single crystal, ceramics, glass and other materials doped with rare earth or transition metal ions as the activation center.

In recent years, China has also made a series of important breakthroughs in the research of solid laser gain materials: the world's largest aperture Ti: sapphire crystal successfully grown supports the establishment of a 10 PW laser system; The commercialization of self frequency doubled yellow laser crystals has become the first practical example of composite functional crystals in the world; Large size YAG crystal/ceramic and laser fiber guarantee the development of high-power laser equipment in China; The development of large size and high-performance neodymium glass and Yb: CaF2 crystal makes it possible to realize the PRF ultra intense laser technology; Wide spectrum gain glass has become a gain material guarantee for the progress of high-throughput optical communication and amplification technology.

There is no limit to exploration and application. On the one hand, laser gain materials continue to develop towards large size and super large size, promoting the continuous development of material preparation technology; On the other hand, it is developing towards multi-dimensional, including two-dimensional films, one-dimensional optical fibers, zero dimensional nanocrystals, and so on, opening up new areas of laser materials. Focusing on "solid laser gain media", this special issue has invited some domestic experts and scholars to write papers, presenting the latest research progress and achievements in visible and mid infrared laser materials, optical communication fiber laser materials, micro/nano gain media materials and micro/nano laser technology. Due to the limitation of time and space, many excellent researches were not included in this album in time. It is hoped that this album can serve as a useful reference for promoting the research and application of solid laser gain medium materials and advanced laser technology in China.