All-solid-state deep ultraviolet (DUV) coherent light sources (λ<200 nm) have important applications in frontier science, high technology and many other fields. An effective and feasible technical approach is to use commercially available visible and near-infrared all-solid-state lasers as the fundamental frequency light source to generate DUV laser through cascaded frequency conversion by nonlinear optical (NLO) crystals. The basic conditions of DUV NLO crystals are described from two aspects of birefringence and dispersion, and the research progress of DUV NLO crystals and technical difficulties need to be overcome for applications are introduced. Taking KBe2BO3F2 (KBBF) and RbBe2BO3F2 (RBBF) as examples, the discovery, crystal growth, the corresponding prism-coupled device technology, their main optical properties, and their ability to generate DUV coherent light are reviewed. The research progress of DUV coherent light sources based on KBBF crystals and prism-coupled technology are discussed in detail, as well as their applications on the advanced scientific instruments, especially on ultra-high-resolution photoelectron spectrometers. At last, the perspectives for future development of DUV NLO crystals and all-solid-state DUV laser technology are given.

Titanium doped sapphire (Ti:Al2O3) laser crystals are widely used in the tunable laser, high power laser and ultra-fast laser because of their characteristics of high hardness, high thermal conductivity, broadband absorption and broadband emission. In recent years, the growth technology, theoretical analysis and device-based application of Ti:Al2O3 crystals have been greatly developed, and the research on Ti:Al2O3 crystals has been more and more intensive and extensive. The research and application of Ti:Al2O3 crystal materials are reviewed.

As the gallium volatilization is effectively inhibited by designing suitable temperature field structure and optimizing the growth atmosphere, a diameter of 3 inches Gd3Ga5O12 (GGG) crystal with high quality is successfully grown by Czochralski method with the combining of raw material pretreatment and necking technology. And the crystal structure, crystalline quality, dislocation morphology and transmission spectrum are investigated in detail. The X-ray diffraction (XRD) shows that the as-grown crystal is single phase and the lattice constant is 1.2379 nm. The X-ray rocking curve (XRC) exhibits that the crystal possesses high crystalline quality. The surface roughness of (111) crystal surface is measured to be about 0.203 nm by atomic force microscopy (AFM). The dislocation corrosion pits on the crystal surface (111) are observed and analyzed, and it shows that the dislocation density ranges from 28 /cm2 to 85 /cm2. The transmission spectrum indicates that the crystal has wide transmittance band and the Sellmeier equation coefficients are fitted. All the results indicate that the 3 inches GGG crystal can be applied as the substrate material of the magneto-optical epitaxial film and laser host, and larger-size crystal can effectively improve the utilization efficiency and homogeneity.

In order to solve the problem that it is difficult to accurately determine the crystal composition, a method for the accurate determination of crystal composition by X-ray fluorescence spectrometry with YAG crystal as the standard sample is developed. The samples are pure YAG crystal, and the doping concentration of raw materials with 1.2 at% and 2.0 at% Yb:YAG crystals. The composition of Yb: YAG crystals with different concentration is determinedby X-ray fluorescence spectrometry. Using pure YAG crystal as the standard sample, the accuracy of the detection results is greatly improved. The detection error of main components Y3+ and Al3+ is less than 1%, and that of Yb3+ is less than 5%. The results show that the segregation coefficient of Yb3+ in the crystal are 1.025 and 1.045, which are close to 1. It is conducive to the realization of high concentration doping and high quality crystal growth of Yb:YAG crystal. The doping concentration difference of 2.0 at% Yb:YAG crystal is less than 5%, which indicates that the doping concentration uniformity of equal diameter part is high.

The new long-wave infrared material PbGa6Te10 has great application potential due to its high refractive index (>3), wide infrared transmittance range (1.4～22.3μm), good stability and mechanical properties. However, PbGa6Te10 is a non-identical eutectic compound, which can easily lead to segregation of components and the formation of heterogeneous phases. For this reason, high quality PbGa6Te10 polycrystal is synthesized by the oscillation-assisted single-temperature zone method, and the single crystal with a length of 100 mm and a maximum diameter of 26 mm is grown by the vertical Bridgman method. The as-grown crystals are characterized by X-ray powder diffraction, rocking curve, thermogravimetric differential thermal analysis, infrared transmittance, et al. Results show that the grown crystal is PbGa6Te10 single crystal. It is crystallized well, and the full width at half maximum (FHWM) is 0.214°. The crystal has good thermal stability. The band gap value of PbGa6Te10 calculated from the diffuse reflection absorption curve is 1.01 eV, and the transmittance curve shows that the average transmittance from 2.5 μm to 22.3 μm is about 28%.

Deep-ultraviolet (DUV) nonlinear optical (NLO) crystal is a key material for the solid-state lasers to generate DUV coherent lights. Due to the limit of band gap, NLO coefficient and birefringence, theexisting NLO crystals are difficult to generate DUV coherent lights by phase matching, except KBe2BO3F2 crystal. It’s a new idea to solve the phase matching problem of DUV NLO crystals based on the quasi-phase matching principle. A detailed description of properties of BaMgF4 crystal, Mg3B7O13Cl crystal and LaBGeO5 crystal, and their applications on the generation of DUV coherent lights based on the quasi-phase matching principle are introduced. Finally, the suggestion and prospect on DUV quasi-phase matching crystals are presented.

BaGa4Se7 (BGSe) and BaGa2GeSe(BGGSe) crystals are novel infrared nonlinear optical materials independently developed by Chinese scientists. Because of the excellent comprehensive performance and the important application value in laser industry, they have attracted much attention at home and abroad. The progress of crystal growth and laser output of BGSe and BGGSe are summarized, and the future development is prospected.

Single-crystal fiber (SCF) is a fiber-shaped monocrystalline material, which maintains the outstanding physical and chemical properties of bulk crystals and simultaneously exhibits the excellent thermal dissipation efficiency and waveguide structure of optical fibers, therefore indicating significant potential applications in solid-state lasers, scintillation detectors, high temperature sensors, etc. Recently, numerous researches have been focused on the crystal growth, the fabrication of cladding structure as well as applications of SCFs. In this paper, the characteristics and developments of the most widely applied crystal growth techniques of SCFs, including micro-pulling-down method, laser-heated pedestal growth technique and edge-defined film-fed growth technique, are introduced in detail. Recent progress of several research groups engaging in SCFs both at home and abroad are also discussed. The investigation of applications of SCFs in high-power laser systems, mid-infrared lasers as well as scintillation detectors are analyzed. The strategies of cladding fabrications is also introduced briefly.

Single-crystal fiber (SCF) is a new type of one-dimensional crystal material that combines the advantages of bulk crystal and glass fiber. It has excellent physicochemical and thermal management properties, and has huge application potential in laser, scintillation, and sensing. At present, the two main growth methods of SCFs are micro-pulling down (μ-PD) method and laser heated pedestal growth (LHPG) method. The LHPG method does not need crucible in the growth process and the SCFs with smaller diameter can be prepared to give full play to the morphological advantage of the fiber. Therefore, the LHPG SCF apparatus with independent intellectual property rights is developed, and Al2O3, YAG,LuAG and other SCFs are grown. The diameter of the YAG SCF can reach 200 μm, the length is 710 mm, and the aspect ratio is more than 3500:1. At the same time, the diameter uniformity and crystallization quality of the SCF are investigated, which shows that the quasi-one-dimensional SCF still has good physicochemical properties.

As a wide band gap semiconductor material, β-Ga2O3 has attracted wide attention from domestic and foreign scholars in recent years due to its superior optical properties, electrical properties and broad application prospects. Based on the in-depth research of our team on the two important topics of how to prepare β-Ga2O3 single crystals and how to control the electrical and optical properties of β-Ga2O3 single crystals by doping with group V ions, the preparation methods of β-Ga2O3 single crystals, the electrical and optical properties of Ta5+ and Nb5+ doped β-Ga2O3 single crystals are reviewed.

Rare earth (RE) optical crystals are one kind of advanced optical materials, which are defined as the optical crystals therein RE elements occupy one lattice site of the crystallographic structure completely. They have been widely applied in the fields of military and national defense, national economy,science and technology. The research of RE optical crystals are based on the analyses of characteristics of rare earth ions and the phase diagrams, together with the application of chemical bonding theory of single crystal growth, thus the components, thecrystal growth methods and technique parameters of the crystalline materials are determined, and finally the bulk single crystals are obtained. The luminescence theory of RE ions, analyses of phase diagrams and components of several typical RE optical crystals are introduced briefly. The latest developments of several major kinds of RE optical crystals for the applications in various fields are reviewed. The research on rare earth optical crystals are important ways to achieve high-quality development ofrare earth resources, which require the collaboration of multidisciplinary courses and interdisciplinary fields.

With the development of high repetition radiation imaging and application of time of flight in nuclear medicine imaging, the ultrafast scintillation crystals are essential and become one of the research focuses. At present, there are three kinds of applied ultrafast scintillation crystals: core-valence luminescence crystals, direct band gap semiconductors and thermal quenching scintillation crystals. These scintillation crystals show decay time of lessthan 3 ns and have been applied in the high repetition radiation imaging, particle physics experiments and nuclear experiments. However, their light yield is low and energy resolution is poor, and slow luminescence component is also observed. These disadvantages limit their applications in many fields, such as positron emission computed tomography (PET). Therefore, the innovations are needed both in scintillation mechanism and crystals preparation to obtain the scintillation crystals satisfying the requirements of the most applications.

Ce:GAGG scintillation crystals with different sizes and thicknesses (2, 4, 6, 8, 10 mm) are selected to analyze the influence of self absorption on Ce: GAGG scintillation crystals by measuring the transmittance. At the same time, the effects of different surface roughness, package and coupling mode on light output and energy resolution of Ce: GAGG scintillation crystals are investigated. The experimental results show that the optical output and energy resolution of Ce: GAGG scintillation crystal samples can be greatly improved by optimizing the surface roughness, encapsulation reflector and coupling mode. The best energy resolution of Ce: GAGG scintillation crystals is 7.0% by using 137Cs standard radioactive source.