2-5 μm mid-infrared laser has important applications in directional infrared countermeasures, anti-terrorism, biological medical treatment, environmental monitoring, optical communications, strong field physics and laser fusion et al. It can also be used as pump source for 5-20 μm mid- and far-infrared laser output. The study from Nd3+ and Yb3+ at 1 μm, Er3+ at 1.5 μm, Tm3+ and Ho3+ at 2 μm, Tm3+ and Cr2+ at 2-3 μm, Er3+, Ho3+, U4+and Dy3+ at 3 μm, Fe2+and Ho3+ at 4 μm, to Dy3+, Er3+and Pr3+ beyond 4 μm, continuously expands to the mid-wave infrared direction, which is the frontier of laser crystal development. In this paper, the research of mid-infrared laser crystals in recent years is reviewed, and its future development is commented briefly. The work of some transparent ceramics was involved.

The Er3+ based near 3 μm laser is an ideal light source for precise laser surgery. In addition, it can be used as pump source for the mid- and far-infrared laser system, which is of great value for the development of long-wave lasers. Research shows that the cooperative up-conversion between Er ions can suppress the population bottleneck caused by the long lifetime of the laser lower energy level, which significantly improves the laser efficiency. However, the proposed “high concentration doping” scheme cause heavy thermal loads on the gain crystals, which limites the enhancement of laser power. This paper briefly summarizes the research results on the self-termination and cooperative up-conversion of Er3+-doped crystal lasers, and introduces the current experimental schemes and research progress to achieve high-power lasers near 3 μm. Finally, the development of high-power Er3+ based near 3 μm laser is prospected.

As one of the core gain materials of laser technology, mid-infrared laser crystal materials have very important applications in military electrooptical countermeasures, laser medicine, satellite remote sensing, molecular monitoring, basic science and other fields. Recent research progresses on crystal growth, optical performance control and laser performance in mid-infrared fluoride laser crystal materials are reviewed in this paper. Especially in the aspect of mid-infrared optical performance control, it mainly introduces the interaction based on rare earth ion energy level coupling. As a result, it can effectively suppress the self-termination effect of luminescent ions and provide an effective pump channel for luminescent ions.

3-5 μm and 8-12 μm infrared lasers are located in the atmospheric transmission window, which have broad applications in laser imaging, environmental monitoring, lidar, laser medicine, chemical remote sensing, and Infrared countermeasure. Based on the nonlinear optical crystals, optical nonlinear frequency conversion technology has obvious advantages in realization of mid- and long-ware infrared solid-state lasers output. The structure of the laser is simple, and the crystal itself are not involved in the progress of exchange of energy, so there is no quantum defect and little heat production, leading to a low generation of heat. At the same time, it has the advantages of good monochromatic, wide tuning wavelength range and high output power. This paper reviews the research progress of the applications of nonlinear optical crystals in mid-and long-wave infrared solid-state laser.

Long-wave infrared (LWIR) laser sources from 8 μm to 12 μm are promising to many fields, such as directional infrared countermeasures (DIRCM), ranging and targeting, standoff detection, trace gas monitoring, spectrum analysis and wireless communications. With the development of laser wavelength to LWIR, high-performance LWIR nonlinear crystals are urgently required．Because the performance of nonlinear optical crystals, including CdSe, GaSe, ZnGeP2, AgGaSe2, LiInSe2, CdGeAs2, BaGa4Se7, is seriously influenced by transparency range, nonlinear coefficient, thermal conductivity and wavelengths of pump, quasi-phase matching(QPM) materials with excellent performance, like OP-GaAs, OP-GaP and OP-ZnSe, can be used to produce long-wave infrared laser to overcome these defects. In this paper, the quasi-phase matching materials for LWIR generation are reviewed. Their research progresses are presented from the aspects of materials performance, preparation technology and laser application. The application potentials and technical bottlenecks are analyzed.

Infrared nonlinear optical crystals, which can generate coherent mid- and far-infrared laser through frequency conversion technology, play an increasingly significant role in military and civil fields. Chalcogenides and pnictides have been widely regarded as excellent candidates for mid- and far-infrared nonlinear optical crystals. Over the last two decades, chalcogenide nonlinear optical materials have been widely investigated and applied, however, pnictides have been rarely studied for their nonlinear optical properties. The advances of pnictide nonlinear optical materials has been reviewed from the aspect of new material exploration. These compounds were divided into three types i.e. classical chalcopyrite, structures with homoatomic bonds and structures with tetrahedra units. The crystal structures, physical properties and structure-properties relationships were analyzed. Finally, the future development of pnictide nonlinear optical materials was discussed.

The mid infrared laser, represented by the CdSe crystal optical parametric Oscillator, has a wide range of applications in many fields such as biology, medical treatment and military. In this paper, the methods of growing CdSe single crystal include melt method, flux method and gas phase method, in which gas phase method is most common, and the HPVGF technology has been gradually adopted. The intrinsic CdSe crystal used as OPO device is applied to kinds of laser, and the output power is increasing. Doping of Cr2+ can effectively improve the pumping efficiency and realize continuous wavelength adjustment.

Mid-infrared (MIR) nonlinear optical (NLO) crystal can output MIR lasers efficiently through the typical frequency conversion technology. It is an important part of MIR laser and has been widely used in the military and civil fields. At present, most of the mid-infrared nonlinear optical materials with practical application prospects belong to tetrahedron-based chalcogenides whose structural framework is composed of tetrahedral units. This review summarizes the structure-performance relationship in MIR NLO tetrahedron-based chalcogenides by analyzing crystal structure, optical property and theoretical calculation of 21 representative tetrahedron-based chalcogenides compounds, which will be of great importance for the exploration of novel MIR NLO crystals.

The study of second-order nonlinear optical crystalline materials in the infrared region is one of the important, difficult, and hot topics in the fields of lasers and nonlinear optics. As the available ones are far from the market’s requirements, it is necessary to explore novel infrared nonlinear optical materials from the origin. Chalcogenides and halides are the most important candidates for infrared nonlinear optical materials. Chalcogenides usually present large second-harmonic generation effects and wide infrared windows, and halides usually have large optical band gaps, namely, high laser-induced damage thresholds. Combining their advantages, it is possible to obtain new infrared nonlinear optical materials with desirable properties. Based on this background, this work summarizes the recent status and progress on chalcohalide-type infrared nonlinear optical materials, including halides containing S8 unit, salt-inclusion chalcohalides, and Ba-based chalcohalides. Most of these chalcohalides powder samples exhibit nice NLO properties. The opportunity and importance of the future investigation on this topic are also proposed.

Chalcogenides have been widely used in the military and civilian infrared (IR) fields owing to their high nonlinear optical(NLO) coefficient and wide IR transparent ranges. Among them, diamond-like semiconductors(DLSs) exhibit the inherent noncentrosymmetrical(NCS) structure that can satisfy the prerequisite condition as one NLO material and I2-II-IV-VI4formula type (I=Li, Ag, Cu; II=Zn, Cd, Hg, Mg, Co, Fe; IV=Si, Ge, Sn;VI=S, Se) has become one of the important research systems. So far, 78 I2-II-IV-VI4 DL chalcogenides with NCS structures have been found in the above system, they crystallize in seven different space groups including Pmn21, Pna21, Pn, I4-, I4-2m, Cmc21, F222.This review systematically summarizes the structural characteristics and NLO properties of DLSs in different space groups and the result provides the theoretical guidance for the future exploration of IR NLO crystals with excellent performances.

Mid infrared lasers at the wavelength range of 2-5 μm have special and important applications in national defense, medical treatment, communication and so on. The gain media for direct generation of mid infrared lasers mainly include gas laser medium, semiconductor, rare earth ions or transition metal ions doped compounds. In this paper, the spectral properties of luminescent ions (including rare earth ions such as Tm3+, Ho3+, Er3+ and transition metal ions such as Cr2+, Fe2+) used in the mid infrared region are introduced firstly. Then, the fabrication and laser performance of oxides (including garnet and sesquioxide) ceramics and polycrystalline II-VI compounds (mainly ZnS/ZnSe) are mainly introduced. Finally, the problems of the two kinds of mid infrared laser ceramics are analyzed, and then their development direction is also prospected.

Based on the extra pressure assisted, the batch, high-purity polycrystalline materials CdSe were synthesized directly, and high-optical quality single crystals with dimensions of (50-55) mm×(80-100) mm were grown by high pressure seeded-aided Bridgman technique. Some devices with certain phase matching angles and dimensions about (6-8)2×(40-50) mm3 were fabricated. The absorption coefficients of the devices under o, e polarized lights at 2.1 μm, 2.6 μm, 10.8 μm were ≤0.02 cm-1, ≤0.02 cm-1, ≤0.01 cm-1, respectively. Through optical parametric oscillation laser experiments, the devices realize 1.05 W power output within 10.1-10.8 μm far-infrared wavelength.

Zinc germanium phosphide (ZnGeP2, ZGP) crystal is an excellent mid-infrared nonlinear optical material. In this study, high-pure polycrystalline ZGP was synthesized successfully by using a self-made two-zone tube furnace with the amount of 600 g in one run. A large ZGP single crystal with size of 55 mm×30 mm×160 mm was grown by the horizontal gradient freeze method with ultralow gradient (0.5-1 ℃/cm). The large ZGP crystal devices (12 mm×12 mm×50 mm) with low absorption coefficient of 0.03 cm-1 at 2.09 μm were successfully fabricated after orienting, cutting, post-growth thermal annealing, polishing, and high energy beam irradiation. The crystal devices are used for high energy and high average power infrared laser output.

Polycrystalline ingots were synthesized by two-temperature zone method using P, Si, and Cd as the initial materials. Single crystals CdSiP2 with the size of 12 mm×40 mm and 15 mm×50 mm were successfully grown by self-seeding and seeded vertical Bridgman technique, respectively. The full width at half-maximum of the XRD rocking curve on (004) plane is 40″. The average transmittance of CdSiP2 crystal is 57% at 2-6.5 μm, which is close to its theoretical maximum. A small amount of transition metals, such as Fe, Cr, Mn and Ti, were detected in crystal by glow discharge mass spectrometry. The presence of Fe+ and Mn2+ was determined by electron paramagnetic resonance, which may cause optical absorption of the crystal in the near infrared band.

Lithium selenoindate (LiInSe2, LISe) crystal has important application in the field of long-wave infrared lasers output. At present, it is still a challenge for the growth of large sized and high quality LISe crystals. The optical quality of crystals are improved by atmosphere annealing post-growth treatment. Large sized (40 mm×60 mm)LISe crystals were grown in batch by the vertical Bridgman method using oriented seed with precise control of seed crystal welding. To the best of our knowledge, it is the largest one for the reported LISe crystals. In addition, the annealing process of LISe crystal was systematically explored, and the results indicate that the optical quality of LISe crystal can be significantly improved by annealing at 740 ℃ for 150 h under LISe intrinsic atmosphere.

Strontium cadmium germanium selenide (SrCdGeSe4) crystal is a new infrared nonlinear optical material with excellent performance which was discovered recently. In this study, polycrystalline SrCdGeSe4 was synthesized successfully by using a self-made two-zone tube furnace with the amount of 300 g in one run. SrCdGeSe4 single crystal with size of 27 mm×80 mm was grown by vertical Bridgman-Stockbarger method for the first time. Several oriented SrCdGeSe4 plates with different sizes were obtained after orienting, cutting, and polishing processes. Measurements of rocking curve, infrared transmission spectrum, and laser damage threshold were carried out on a polished (110) plate with 8×8×2 mm3 in size. The results show that the FWHM of the (220) peak rocking curve is 44.8″; the crystal has a short-wave cutoff of 596 nm, and has highly transparent within the wide range of 1-14 μm with the transmittance over 68%, and possesses a surface laser damage threshold of 530 MW/cm2 using a Nd∶YAG laser under conditions of 5 ns pulse width, 1 Hz frequency, and D=0.15 mm spot size.

A new metal-mixed chalcogenide Ba7AgGa5S15 was successfully synthesized by high temperature flux spontaneously crystallization method. The compound crystallizes in the asymmetric space group P31c(No.159) with the cell parameters of a=0.964 53 (10) nm, c=1.805 9 (4) nm and Z=2. The structure of Ba7AgGa5S15 is regarded as an 18-member tunnel-contained three-dimensional (3D) framework formed by the［Ga4S10］ T2 supertetrahedra and the［AgS4］ tetrahedra by sharing vertexes. Isolated［Ga(2)S4］ tetrahedra locate inside the tunnels and the Ba2+ cations insert in the space of the framework. The optical properties of the title compound were systematically studied based on first-principles calculations, which includes the electronic structure, density of states, birefringence, the second order nonlinear optical (NLO) coefficients and the second-harmonic generation (SHG) density. The results show that Ba7AgGa5S15 has a direct band gap of 3.76 eV, which is mainly determined by the S 3p, Ba 5d and Ga 4s orbitals. The NLO coefficient in the d33 direction is ~0.4 times that of AgGaS2(AGS), and the main contribution of SHG is from [AgS4] and [GaS4] tetrahedrons. This work indicates that introducing alkali earth metal Ba2+ into the Ag-Ga-S system could effectively enlarge the band gap of the formed Ba7AgGa5S15 compared with classical AGS compound, which is helpful in producing higher laser damage threshold (LDT).

High quality CdSe single crystal, with the sizes of 54 mm in diameter and 25 mm in length, was successfully grown by a high pressure vertical gradient freeze (HPVGF) technique using seed. The full width at half maximum (FWHM) of (002) and (110) X-ray rocking curves of the CdSe crystal with wurtzite structure are 54.4 arcsec and 45.6 arcsec respectively. The results show that there are some small-size Se inclusions in CdSe crystal by the test of infrared microscope and SEM-EDS. The transmission spectra show that the infrared transmission is above 68% and the mean absorption coefficient is 0.037 cm-1 in the range of 2.5-20 μm. Using fabricated type II CdSe crystal with the dimensions of 10 mm×12 mm×50 mm, we demonstrated an optical parametric oscillator (OPO) pumped by a 2.09 μm acoustooptic Q-switched Ho∶YAG laser at a pulse repetition frequency of 1 kHz. Up to 389 mW output is obtained at the idler wavelength of 11.47 μm with a linewidth of 33.2 nm.

Through the design strategy of equivalent metal-ion substitution, five sodium iodates containing heavy rare-earth elements, NaRE(IO3)4(RE=Dy, Ho, Er, Yb, Lu), were synthesized. Their crystal structures were determined using single crystal X-ray diffraction. These compounds are heterogeneous isomers with the same non-centrosymmetric monoclinic space group, Cc. The linear and nonlinear optical (NLO) properties as well as thermal stability were characterized by UV-Vis absorption spectroscopy, infrared spectroscopy, thermogravimetric analysis and second harmonic generation (SHG) measurement, respectively. The second-order NLO effects of these compounds are quite different and their SHG intensities are in the range of 0.1 to 3.3 times than that of KDP with phase-maching characteristics. The energy band gaps are all over 3.85 eV and thermal decomposition temperatures are above 520 ℃.

Ge-doped β-Ga2O3 crystals were grown by the Floating Zone (FZ) method, and the doping effects on the crystal structures were investigated by XRD and Raman spectroscopy. The optical transmission spectra revealed that the optical bandgaps of Ge∶β-Ga2O3 crystals increase with the increase of Ge doping concentration. Under ultra-violet excitation of 4.67 eV, the emission intensities of Ge∶β-Ga2O3 crystals are comparable to β-Ga2O3 crystal, while the decay time is faster.

2-inch sapphire-based aluminum nitride templates with different thickness of 200 nm, 400 nm and 800 nm were fabricated by DC reactive magnetron sputtering and high temperature annealing. The characterization results of AlN templates with different thicknesses before and after high-temperature annealing were compared. The results show that the quality of AlN templates with a thickness of 200 nm fabricated by DC magnetron sputtering were significantly improved after high-temperature annealing: the full widths at half maximum (FWHM) of the rocking curves measured by high-resolution X-ray diffraction(HRXRD) before and after high-temperature annealing for (0002) were 632-658 arcsec and 70.9-84.5 arcsec respectively, while the ones before and after high-temperature annealing for (10-12) were 2 580-2 734 arcsec and 273.6-341.6 arcsec, respectively. The root mean square roughness of a 5 μm×5 μm surface area is less than 1 nm. The absorption coefficient revealed by optical transmission spectra in the UV range 260-280 nm is 14-20 cm-1. The FWHM of E2(high) phonon mode evaluated by Raman spectra decrease from 13.5 cm-1to 5.2 cm-1 and the peak shifts from 656.6 cm-1 to 657.6 cm-1, which indicate that the tensile stress in AlN templates is relaxed and became stress-free after high-temperature annealing.

The crystal of 0.6at%Eu∶CaF2 and 0.6at%Eu, 6.4at%Na∶CaF2, 0.6at%Eu, 6.4at%Gd∶CaF2 were grown by porous crucible temperature gradient method. XRD results indicate that these crystals are still keeping a pure CaF2 cubic phase, and the absorption intensity decreases after co-doping. The fluorescence spectrum of the crystal material excited by 398 nm xenon lamp shows that the luminescence intensity of Eu2+ at 424 nm could significantly increases after the co-mixing of ions, while the characteristic emission of Eu3+ 5D0→7FJ(J=1,2,3,4) shows no obvious change either in peak position or intensity except for a broadening of full width at half maximum (FWHM). The emissions of 5D2→7F3 and 5D0→7F0 transitions in 0.6at%Eu, 6.4at%Gd∶CaF2 are mainly due to the fact that the inversion center lattice is broken after co-doped Gd3+, and the R value (electric and magnetic dipole transition ratio) significantly reduces, which improves the symmetry of crystal lattice structure. After co-doping Na+, the coordinates of CIE color gamut changes from (0.303 7, 0.142 5) to (0.204 3, 0.062 6), corresponding to the gamut regulation from purple to blue.

As an important Light Emitting Diode(LED) substrate material, GaAs single crystal has been widely used in optoelectronic devices, but the defects such as uneven distribution of carrier concentration (C.C.) and high concentration of impurities seriously affect the performance of corresponding devices. In order to prepare a Si-doped HB-GaAs single crystal with uniform longitudinal distribution of C.C., the influence of the melting zone length on the longitudinal distribution of C.C. during crystal growth was investigated. Using high-purity GaAs polycrystals as raw materials, setting different crystal pull temperature curves, and using narrow-melt zone technique for the crystal growth research, finally, N-type Si-doped GaAs single crystal having a more uniform distribution of C.C. and lower dislocation density (EPD≤10 000 cm-2) was obtained. The crystal was characterized by glow discharge mass spectrometry (GDMS) and Vanderbilt method. The purity of the single crystal reaches 5N which stained without boron impurities.

The electronic structure of α2-Ti3Al (0001) surface was calculated by the first principle based on density functional theory. The results show that: (1)the surface energy of the α2-Ti3Al (0001) surface is 2.03 J/m2, and the surface work function is 4.265 eV; (2)the total density of states on the surface reaches a maximum value at the Fermi level, and the system exhibits a metallic property, which is significantly different from the semimetal of the α2-Ti3Al bulk, Ti-s, Ti-p and Al-s orbitals are less affected by the number of layers, while Ti-d and Al-p orbitals are more affected by the number of layers, and all of them have maximum values at the Fermi level; (3)the nodal line ring is not present in the band structure of the surface, but there is a three electronlike bands crossing point at Γ point under the Fermi level, and there is a two holelike bands crossing point at Γ point above the Fermi level.