Sesquioxides are ideal host materials for mid-infrared laser with high power and energy for their excellent thermal properties, stable physical and chemical properties, low maximum phonon energy and strong crystal field. It is very difficult to prepare large crystals with high quality due to the extremely high melting point of sesquioxides. Researchers have conducted long-term research and exploration on the crystal growth of the sesquioxides. The fast development of laser technology in recent years, which put forward urgent requires for high-quality sesquioxide single crystals, has prompted a breakthrough in crystal growth technology. In this paper, based on the brief introduction of the properties and structures of sesquioxides, the research progress on crystal growth methods and defects of sesquioxides Lu2O3, Sc2O3 and Y2O3 are reviewed in detail. Further, the laser performances of rare earth ion-doped sesquioxides in the mid-infrared band from 1 μm to 3 μm are systematically summarized. Finally, the development of the sesquioxide single crystals in the future is also prospected.

Yb∶CaGdAlO4 (abbreviated as Yb∶CALGO) crystal has partially disordered structure, excellent thermal and spectral properties, broad absorption and emission band, and is suitable to be pumped by the high-power InGaAs laser diode (LD) to achieve high power ultra fast laser. The high nonlinear refractive index coefficient of Yb∶CALGO is beneficial to the optimization of its mode-locked laser. The Yb∶CALGO crystal also has the advantages of simple energy levels, low intrinsic quantum defect and high radiation quantum efficiency etc, and thus becomes the new generation of LD pumped femtosecond laser gain medium in recent years, with the advantages of compact structure, high efficiency and low cost etc. This paper provides a brief view of the crystal structure, crystal growth, defect analysis, thermal properties and spectral properties of Yb∶CALGO crystal, and mainly focuses on the recent progress of ultra fast laser technologies based on the research results of Yb∶CALGO crystals, including the mode-locked technologies by the semiconductor saturation absorber mirror and Kerr lens mode-locked ultrafast laser, as well as the regenerative amplifier ultra fast laser technologies.

Rare earth doped fluorite halides play important roles in the fields of economy, national defense, and so on. The trivalent rare earth ions easily cluster in fluorite halides due to the special crystal structure. However, the systematic investigation into the aggregation of rare earth ions is rarely reported. In this work, the cluster structure characteristics, evolution patterns and their connections, as well as the influencing factors were systematically studied. It is found that the stabilities of 1|0|0|11 (C4v) and 1|0|0|12 (C3v) rare earth monomers vary with the ionic radius, and the trends are different in different matrix crystals, which are consistent with the analysis of lattice distortion. The coupling of lattice distortion and coulomb interactions determines the site occupations of charge compensation interstitial halide ions, i.e. the relative stabilities of C4v and C3v center. Additionally, the covalent effect makes the monomer structures of rare earth doped PbF2 and SrCl2 are different from that of CaF2, SrF2 and BaF2. The study also reveals the relationship of energy difference slopes of monomers with crystal ionicity and cell size. The structures of high-order rare earth clusters were also studied and its evolutions are intimately related with that of monomer centers. The criterion of relative stabilities in high-order clusters is proposed, which provides a guideline for the exploration, screening and development of new rare earth doped fluorite halides, and its application scope is highly expected to be extended.

Tm∶YAP/YAP composite laser crystal boule with diameter more than 40 mm is directly grown by induction heating Czochralski method through improving the thermal field, changing raw materials, and secondary welding growth. The boule shows no visible cracks, good optical homogeneity and uniform residual stress. For the cylinder part of the boule, the length of the YAP segment is 21 mm, and the length of the Tm∶YAP segment is 44 mm. Under the irradiation of green laser and He-Ne laser, there is no any scatters and bubbles observed at the composite interface. The interference fringes on both sides of the composite interface are basically continuous, and the wavefront difference of transmission is very small. Two composite Tm∶YAP/YAP crystal rods with size of 5 mm×30 mm each and YAP segment without doping at the end of 4 mm long were prepared from the boule. Both rods show excellent optical quality. The laser performances of the rods were tested on 1 940 nm solid-state laser. By single end pump of laser diode, the laser output of 42.8 W is achieved when the input pump power is 117.7 W. Compared with the non-composite laser crystal, the composite Tm∶YAP/YAP crystal shows higher output power, higher conversion efficiency and better beam quality.

As a kind of excellent host material for solid-state lasers, GdScO3 crystal has attracted more and more attention. In this paper, Yb,Ho∶GdScO3 crystal with the size of 30 mm×50 mm was firstly grown by Czochralski growth method. The powder diffraction data were obtained by X-ray diffraction, and the structure parameters were fitted by GSAS software. Raman spectra were measured. In the range of 100 to 700 cm-1, there are 18 Raman vibration peaks were observed. The spectral properties including transmission and emission of Yb,Ho∶GdScO3 crystal were characterized. The center of the strongest absorption band is at 975 nm, which comes from the transition of Yb3+∶2F7/2→2F5/2. The absorption cross section of Yb3+ were calculated and the values at 940, 975 nm are 0.31×10-20 and 0.42×10-20 cm2. The transition intensity parameter (Ωt) of Ho3+ was calculated by Judd-Ofelt theory, Ω4/Ω6 is 2.04. And spectral parameters such as radiation transition probability, energy level lifetime and fluorescence branching ratio were calculated. The results show that Yb,Ho∶GdScO3 is a promising candidate material for 2~3 μm laser crystals because of its excellent luminescence properties.

In this paper, rare earth ion Nd doped LiLuF4 (LLF) microcrystals were prepared by hydrothermal method. The phase analysis, morphology analysis of the crystals were performed and near infrared optical properties were studied. LLF crystals belong to the scheelite structure of tetragonal system and the space group is I41/a. The LLF microcrystalline materials prepared by hydrothermal method have good crystallinity and the particle size is about 20 μm. The oxygen content of the optimized Nd∶LLF sample is 0.001 4% (mass fraction). Under excitation of 792 nm wavelength, the main fluorescence emission peak of 3% (mole fraction) Nd-doped LiLuF4 microcrystal is located at 1 047 nm, which belongs to near infrared bands, and its fluorescence lifetime is about 0.275 1 ms. Furthermore, Nd∶LLF crystal was grown by the Czochralski method and its laser performance was evaluated. The emission cross section of Nd∶LLF crystal at 1 053 nm is 8.13×10-20 cm2. The laser performance test and analysis show that under the excitation power of 1.2 W, Nd∶LLF crystal obtains a near-infrared laser output of 0.123 W at 1 053 nm.

In this paper, 0.6%Pr∶CaxSr1-xF2 (x=0, 0.3, 0.5, 0.7, 1.0) and 0.6%Pr, 5%R∶Ca0.5Sr0.5F2 (R=Y, Lu, Gd) crystals were grown by porous crucible temperature gradient technology. The crystal structure, absorption spectra, fluorescence spectra and fluorescence lifetime were systematically analyzed and discussed. Through the analysis of spectral parameters such as absorption cross section, emission cross section, fluorescence lifetime and fluorescence full width at half maximum (FWHM), it is concluded that 0.6%Pr,5%Y∶Ca0.5Sr0.5F2 mixed crystals have the best spectral parameter compared with other doped mixed crystals. The corresponding absorption cross sections at 443 nm and red emission cross sections at 640 nm are 1.63×10-20 and 3.39×10-20 cm2, respectively. The FWHM and fluorescence lifetime are fitted to be 4.50 nm and 42.8 μs, respectively. The spectral parameters have been greatly improved compared with 0.6%Pr∶CaxSr1-xF2 (x=0, 0.3, 0.5, 0.7, 1.0) mixed crystals. All results show that 0.6%Pr,5%Y∶Ca0.5Sr0.5F2 crystals have great potential as gain media for novel Pr3+ doped broadband laser material.

The second-order nonlinear optical (NLO) crystal materials with the capability of converting the laser frequency, and modulating the amplitude and phase of the laser, are indispensable in practical applications, such as modern manufacturing, laser medical treatment, and communications, as well as in fundamental research. This article provides a brief review of the atom response theory (ART) of nonlinear optical materials developed on the basis of the partial response functional (PRF) method. ART analysis makes it possible to quantitatively evaluate the contribution of individual constituent atoms and orbitals to the second harmonic generation (SHG) response of a nonlinear optical crystal material based on the first principles density functional theory calculations. Besides, a general group division scheme based on physical performance was developed, which provides the conceptual foundation for determining the functional motifs of the SHG effect. In this review, the latest progress of atom response theory and the related new concepts and insights, such as the role of the conduction band, negative atomic contributions, the role of static dipole moment, the overwhelming contribution of non-covalent groups, linear relationships, etc. are explained and elaborated, and some examples for the design of new materials are also given.

Er3+,Yb3+∶Ba3Gd(PO4)3 crystals with different atomic fractions of Er3+ and Yb3+， i.e. 1.85%Er,23.95%Yb∶Ba3Gd(PO4)3 and 1.95%Er,55.73%Yb∶Ba3Gd(PO4)3 were successfully grown by the Czochralski method. Room-temperature absorption coefficient spectra, up-conversion fluorescence spectra, emission cross-section spectra, gain cross-section spectra and fluorscence decay curves of the crystals were measured and analyzed. Emission cross-section at peak fluorescence wavelength of 1 537 nm, fluorescence lifetime of the 4I13/2 multiplet of Er3+ and Yb3+→Er3+ energy transfer efficienecy of the 1.85%Er,23.95%Yb∶Ba3Gd(PO4)3 crystal are 0.54×10-20 cm2, 9.9 ms and 90%, respectively, while the above values of the 1.95%Er,55.73%Yb∶Ba3Gd(PO4)3 crystal are 0.58×10-20 cm2, 9.7 ms and 93%, respectively. End-pumped by 975 nm laser diode, 1 567 nm continuous-wave laser with maximum output power of 97 mW and slope efficiency of 27.1% is obtained in the 1.85%Er,23.95%Yb∶Ba3Gd(PO4)3 crystal, while 1 567 nm continuous-wave laser with maximum output power of 93 mW and slope efficiency of 17.1% is obtained in the 1.95%Er,55.73%Yb∶Ba3Gd(PO4)3 crystal.

High optical quality ultraviolet nonlinear optical (NLO) crystal K3B6O10Br (KBB) with dimensions of 42 mm×20 mm×18 mm was successfully grown by the top-seeded solution growth method. According to its structural symmetry requirements, the crystal was cut and fabricated into different devices for the complete characterizations of its photoelectric properties. Additionally, the electric-elastic constants of the KBB crystal were systematically characterized, and the significant differences in piezoelectric anisotropy are observed due to its microscopic structure. The maximum piezoelectric constant d33 is measured to be 6.69 pC/N. Considering its good NLO frequency conversion, electro-optical and piezoelectrical properties, KBB crystal shows potential applications in the optoelectronics fields.

In this work, a novel electro-optic crystal KLi(HC3N3O3)·2H2O was explored by powder method. The contribution of lattice vibration was calculated by infrared reflectance spectrum and Raman spectrum. The effective nonlinear optical coefficient was estimated from the powder second harmonic generation response. Consequently, the electro-optic coefficient of KLi(HC3N3O3)·2H2O is determined to be 2.37 pm/V, which is comparable to that of commercial β-BBO. The crystal was grown by aqueous solution method. Superheating and supercooling curves were measured for different raw materials. A transparent crystal with size of 35 mm×25 mm×10 mm was obtained after optimizing the growth process. The relationship between crystal morphology and anisotropic growth rate was confirmed by means of X-ray orientation and piezoelectric coefficient measurement.

In this work, rapid sythesis strategy of Er∶YAP crystals with different doping concentrations were developed using a reverse laser-heated pedestal grwoth (LHPG) method. High quality Er∶YAP single crystal fibers of 0.8 mm×65 mm were obtained using LHPG process by optimizing growth parameters to inhibit defects such as s color centers, cracking, and diameter fluctuations during the growth process. Spectroscopic characterization of Er∶YAP crystals indicates that there is a strong energy transfer process between Er3+ when the doping concentration reaches 5% (atomic fraction), which is favorable for generating high-efficient mid-infrared lasers.

Zinc telluride (ZnTe) nonlinear crystal, a commonly used electro-optical material, can be used to generate and detect Terahertz (THz) wave, and has been widely used in the fields of THz wave detection and imaging. However, the preparation of ZnTe crystal with high quality and large size still remains a challenge. In this paper, ZnTe single crystal rod with diameter of 40 mm and length of 140 mm was successfully prepared by the pressure assisted vertical Bridgman method. The crystal structure, crystalline quality, optical and electrical properties of ZnTe crystal were characterized and analyzed. The effects of annealing process of ZnTe crystal on the crystal's properties related to Te inclusions were investigated. According to the research results, after annealed at 850 ℃ for 200 h under Zn atmosphere, the transmittance of the crystal in the visible-near infrared wave band reaches up to 62%, the resistivity of the crystal is 104 Ω·cm, and the absorption coefficient is from 5 cm-1 to 15 cm-1 at the range from 0.2 THz to 2 THz. The results provide a valuable reference for the preparation of high-quality ZnTe crystal.

In this paper, a series of Er∶CaGdAlO4 (Er∶CGA) laser crystals with different Er3+ doping concentrations (molar fraction) were grown by laser-heated pedestal growth method. The optical properties of the prepared series crystals were studied in detail. The maximum absorption coefficient, absorption cross section, full width at half maximum and radiation lifetime at the range of 780~840 and 955~1 020 nm in the absorption spectra were compared by combining the J-O theoretical calculation and optical characterization. In addition, the optical performance parameters of fluorescence emission spectrum such as emission intensity, emission cross section and energy level fluorescence lifetime were characterized and futher calculated. As a result, the optimal doping concentration of Er3+ is 5%. This work provides a certain experimental basis for obtaining a new type of laser gain medium which is expected to be used in 1.5~1.7 μm near-infrared band all-solid-state laser.

Sapphire single crystal fibers possess structural advantage of high melting point, stable physical and chemical properties of sapphire and large length-to-diameter ratio of fiber, making them widely studied in the fields of high temperature sensing and radiation detection. In this paper, high quality sapphire single crystal fibers were successfully prepared by laser-heated pedestal growth (LHPG) method with a minimum diameter of 50 μm and extremely high flexibility. On this basis, the influences of crystal orientation, crystal diameter, annealing temperature on the stress distribution and mechanical properties were systematically studied. The tensile strength of the prepared sapphire fiber exceeds 3 000 MPa, showing its excellent mechanical properties.

Nd∶CaYAlO4 single crystal fibers with different atom number fraction (0.25%, 0.50%, 0.75%, 1.00%) were grown by the micro-pull-down method. The crystal structure of the single crystal fiber was tested by X-ray diffraction (XRD), and the results show that the crystal structure is tetragonal. The polarization absorption and fluorescence spectra of Nd∶CaYAlO4 single crystal fibers were tested at room temperature, and the samples have strong absorption near 807 nm. Among them, the absorption of 1.00%Nd∶CaYAlO4absorption is the strongest, and the absorption coefficients for σ and π polarization are 4.20 and 4.06 cm-1, respectively. The strongest emission peaks of 1.00%Nd∶CaYAlO4 single crystal fiber are at 1 080 nm in both σ and π polarization. The emission bandwidths are 17.7 and 17.8 nm for σ and π polarization, respectively. The fluorescence lifetime of 0.25%, 0.50%, 0.75% and 1.00%Nd∶CaYAlO4 single crystal fibers are 129, 133, 135 and 140 μs, respectively, and no concentration quenching is observed. The results show that Nd∶CaYAlO4 single crystal fiber is a potential gain medium for ultrafast lasers.

Lithium terbium fluoride (LTF) crystal is a good kind of magneto-optical material with high Verdet constant and low absorption coefficient, which is suitable to be used in magnetic-optical devices for high-power and large energy laser systems. In this paper, large size and high optical quality LTF crystal with a diameter of 2 inch was prepared by resistance heating Czochralski method, under the conditions of a protective atmosphere mixed with Ar and CF4. During this process, high-purity TbF3 and LiF were adopted as the raw materials. The optical properties of the LTF crystal were analyzed in detail. This crystal boule exhibits a complete appearance, good optical uniformity, and low residual stress, with no visible scattering particles under He-Ne laser irradiation. The LTF crystal elements with a diameter of 10 mm were processed from this boule. The parameters of the crystal elements such as the single-pass loss coefficient, extinction ratio, wavefront distortion, and weak absorption coefficient were tested. The results show that the grown LTF crystal have good optical quality. The magneto-optical performance of the LTF crystal was also researched, and its Verdet constant is about 98% of that of the commercial TGG crystals.

In this study, a large size potassium deuterium phosphate (DKDP) single crystal with deuterium content of 70% was grown by traditional liquid phase method. The residual stress and micro-strain in the frequency tripling DKDP crystal were measured by neutron diffraction technique, and the micro-morphology of crystal in pyramidal and cylindrical faces were detected by X-ray synchrotron radiation. The distribution of macro-residual stress and micro-strain in the DKDP crystal sample along the ［100］ crystal direction were studied. The results show that the residual stress in the cylindrical region of DKDP crystal grown by traditional liquid phase method is compressive stress, while the residual stress in the pyramidal region is tensile stress. The tensile stress in the pyramidal region is easy to cause crystal cracking during growth and transportation, which is consistent with the experimental phenomenon. The residual micro-strain in the DKDP crystal grown by the traditional liquid phase method is small, however, there are still some defects in the DKDP crystal, which is one of the reasons for the existence of residual stress and strain in the crystal. The study can direct the engineering application of large size 70% DKDP crystal.

Al2O3 crystals with different Cr3+ concentrations were prepared by optical floating zone method, and cylindrical Cr∶Al2O3 crystals with different diameters (2 mm to 13 mm) and lengths greater than 100 mm were prepared. The densities of powder rods, ceramic rods and crystals obtained at different stages of Cr∶Al2O3 crystal preparation are measured to be 0.928, 2.230 and 4.051 g/cm3, respectively. XRD patterns show that Cr∶Al2O3 crystal is α-Al2O3 phase. The transmission spectra show that the transmittance of Cr∶Al2O3 crystal in visible light non-characteristic absorption band is more than 80%. The absorption spectra and photoluminescence excitation (PLE) spectra (monitoring 669 nm light) show that there are strong absorption bands at 418 and 559 nm, corresponding to the transitions of Cr3+ from 4A2 to 4T1and from 4A2 to 4T2, respectively. Under the excitation of light with a wavelength of 559 nm, the photoluminescence (PL) spectra of Cr∶Al2O3 crystals show a strong emission peak at 669 nm. At low Cr3+ concentration, the intensities of the peaks increase with Cr3+ concentration increasing, and reach the maximum value in Cr0.006 0Al1.994 0O3 crystal, and then decrease with the increase of Cr3+ concentration.