Laser technology is an important tool for basic physics research, cutting-edge scientific and technological breakthroughs in multiple fields. The gain media of solid-state lasers include crystals, glass, and ceramics. Activated ions include transition metal ions and rare earth ions, etc. The matrix material must have good optical, mechanical, and thermal properties. The main characteristics of crystals are high thermal conductivity and anisotropy. The advantages of glass include convenient preparation and easy access to large-sized components, which can also be made into fibers. Ceramics have the characteristics of high thermal conductivity and easy realization of large dimensions, but are limited to cubic materials. Laser diode (LD) pumped solid-state lasers based on solid-state gain media have advantages such as compact structure, high efficiency, and long service life.

Laser crystals have milestone significance for the development of laser technology. The first laser based on ruby laser crystals was introduced in 1960s. In 1970s, neodymium doped yttrium aluminum garnet (Nd∶YAG) crystals promoted the vigorous development of solid-state lasers. In 1980s, Ti∶sapphire (Ti∶Al₂O₃) led to the rapid development of femtosecond laser technology. In 1990s, neodymium doped yttrium vanadate (Nd∶YVO₄) crystals further advanced the development of solid-state laser technology. In the 21st century, applications such as national defense, cutting-edge science, and optoelectronics have put forward new requirements for solid-state lasers. Nd∶YAG, Nd∶YVO₄, and Ti∶sapphire are three main laser crystals. They can meet the needs of most solid-state lasers. However, they still have shortcomings in some special application areas. So it is necessary to explore new laser crystals.

This paper introduces the main research progress of laser crystals in different wavelength bands at home and abroad, and focuses on the main research achievements of laser crystals in Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences, in recent years. The wavelength bands involve visible, near-infrared, and mid-infrared. The matrix crystals involved include oxides, fluorides, Ⅱ‑Ⅵ compounds, etc. In addition, research progress on the growth of polycrystalline diamonds on laser crystals is also introduced.

Visible solid-state lasers based on laser crystals are compact and light. Lasers from deep red to blue have been reported. Especially, there were a lot of reports on Pr³⁺ doped laser crystals, and continuous wave lasers around 490 nm have been achieved. Ti∶sapphire is the main gain crystal for ultrafast lasers. Superintense ultrafast lasers with peak power ranging from several hundred terawatts to ten petawatts require high-quality and large-sized Ti∶sapphire crystal. The origin of defect related optical absorption in Ti∶sapphire and the growth of large-sized high-quality crystals are two important issues that urgently need to be addressed. In recent years, we analyzed the mechanism of defect related optical absorption in Ti∶sapphire theoretically, and grew large-sized high-quality crystals through heat exchange method. The main activating ions for 1 μm laser crystals are Nd³⁺ and Yb³⁺. Nd∶YAG is the most widely used laser crystal. In recent years, we explored several new Nd³⁺ doped fluoride and oxide laser crystals, and solved the emission cross section problem of Nd∶Lu₃Al₅O₁₂. SIOM reported a new type of laser crystal Yb∶GdScO₃, of which the gain bandwidth is about 85 nm. The commonly used activation ions for 2 μm laser crystals are Tm³⁺ and Ho³⁺. Tm³⁺ can be directly pumped by laser diode. Ho³⁺ has larger stimulated emission cross section, and its emission wavelength is longer than 2 μm. We studied the growth, spectroscopy, and laser performance of Tm∶LiYF₄, Tm∶LiLuF₄, Ho∶LiYF₄, Tm,Ho∶LiYF₄, and Tm,Ho∶LiLuF₄ crystals. In addition, new laser crystals such as Tm∶PbF₂, Tm∶LaF₃, Ho∶PbF₂, Ho∶LaF₃, Ho∶CeF₃, and Tm,Ho∶LaF₃ were also explored. Part of the crystals are shown in Fig. 15. We successfully grew Tm∶LiYF₄ crystals with diameter of 3 inches using home-made raw material purification and crystal growth equipment (Fig.16). We reported a new type of laser crystal Tm∶GdScO₃. Its emission bandwidth of 2 μm band is about 269 nm, which is, to the best of our knowledge, the widest among all Tm³⁺ doped crystals. The commonly used rare earth ions in 3 μm laser crystals include Er³⁺, Ho³⁺ and Dy³⁺. We reported the deactivation effect of Pr³⁺ in Ho,Pr∶LiLuF₄ crystal. Based on this crystal, 2.95 μm continuous wave laser was achieved. The main crystals for 4 μm lasers are high concentration Ho³⁺ doped BaY₂F₈, Dy³⁺ doped sulfides and chlorides, and Fe∶ZnSe. Currently, joule level pulse laser output has been reported based on Fe∶ZnSe. Thermal effects such as thermal lenses and depolarization limit the development of high-power solid-state lasers. Diamond has extremely high thermal conductivity and is expected to be applied in the thermal management of high-power solid-state lasers. SIOM proposed a scheme for directly growing diamond on the surface of laser crystals, and successfully grew continuous, well attached, and crack-free polycrystalline diamond films on laser crystals (Figs.19 and 20).

Laser crystals have the characteristics of high thermal conductivity and anisotropy, making them suitable for high peak power, large pulse energy, and high repetition rate solid-state lasers. Although the comprehensive performance of the three basic laser crystals (Nd∶YAG, Nd∶YVO₄, and Ti∶sapphire) is excellent, the development of solid-state laser technology requires new laser crystals. SIOM has achieved several breakthroughs in new laser crystals in the visible light, near-infrared, and mid-infrared bands, effectively promoting the development of solid-state laser technology. In 2017, we developed the world’s largest Ti∶sapphire crystal (Φ235 mm), which supported the 10 PW laser output of Shanghai Superintense Ultrafast Laser Facility. The development of Yb and Tm doped GdScO₃ laser crystals with extremely wide emission spectra drives the development of laser diode pumped ultrafast solid-state lasers. With the increase in pulse energy, peak power, and repetition rate of solid-state lasers, laser crystals will develop towards larger sizes, higher crystal quality, and controllable key performance.