Study On Growth And Properties Of Several Important Infrared Optical Crystals

3～5μm lasers have many important applications in civil and military fields. Mid-infrared nonlinear optical crystals can produce3～5μm tuning laser by nonlinear frequency conversion technology. ZnGeP2(ZGP) crystal is an excellent mid-infrared nonlinear optical crystal, which has high thermal conductivity (0.35W/cm·K), wide transparency range from about0.7to12μm, high laser damage threshold and high nonlinear optical coefficient (d36=75pm/V). It can make thermal effect low and produce high power. The pump source of ZGP optical parametric oscillator (OPO) is2μm solid-state laser, which is obtained from co-doped Tm:Ho system pumped by laser diode. Tm3+sensitized Ho3+luminescence in Y3Ga5O12YGG) host material plays a very important role in producing2μm lasers. YGG crystal has excellent thermal properties and skilled crystal growth technology. The ground state of Tm3+can be pumped directly into the H4level with an800nm source, which provides enhanced quantum efficiency through an internal cross-relaxation process and subsequently transfers energy to the5I7manifold of the Ho3+, then emits2.09μm laser by5l7→5I8-In addition,2μm lasers are covered with the absorption of several important molecules (H2O, CO2), so it has aroused great interest over a wide range of applications, including medicine, remote sensing, optical communications and military applications. But it has high quantum defect to produce2μm laser, Q-switched or mode locking technique is one of the most efficient methods to obtain high-peak-power. Cr2+:ZnSe is an excellent tunable mid-infrared laser medium at room temperature, which its emission band occurs at2～3μm with a high cross section. Recently, Cr2+:ZnSe has achieved some laser operation, including mode locking, continuous wave and Q-switched. According to the above, in order to obtain3～5μm tuning laser, ZGP-OPO can be pumped by Tm, Ho:YGG laser. Furthermore Tm, Ho: YGG laser can produce much higher peak power by a passive Q-switched Cr2+:ZnSe. In this dissertation, we report the growth of Tm, Ho:YGG, ZGP and Cr2+:ZnSe crystals and study their thermal, optical and laser properties. The outline is shown as follows:1,Study on growth and stucture of Tm, Ho:YGG, ZGP and Cr2+:ZnSe crystalsTm, Ho:YGG crystals were first obtained by the optical floating zone method, to our knowledge. The mixed raw materials are Tm2O3, HO2O3, Ga2O3, and Y2O3powder with a purity of99.99%, which are sintered and placed at the optical floating zone furnace. Finally, five Tm, Ho:YGG crystals were obtained and measured by XRD method, which belong to cubic system.ZGP single crystal was grown by using the vertical Bridgman method under the condition of spontaneous nucleation. First, ZGP polycrystalline material was successfully synthesized by means of directly mixing stoichiometric amounts of high-pure materials (Zn, Ge and P) in sealed quartz ampoule. Second, ZGP polycrystalline material was placed into the quartz crucible, which was evacuated and sealed. Third, the quartz crucible was placed at temperature gradient zone of the furnace and ZGP single crystal was successfully obtained. Thermal annealing experiments were carried out at different conditions. The rocking curves of ZGP wafers were measured and their full width at half maximum (FWHM) became small, which indicated the quality of the ZGP single crystal to be improved.Thermal diffusion experiments were performed on ZnSe wafers with99.5%CrSe powder. Quartz ampoules with samples were placed in suitable thermal zone of the furnace and quartz ampoules were evacuated and sealed off. The thermal process was carried out at different temperature (800～1000℃) for durations varing between5and12.5days. Cr2+:ZnSe crystals were measured by XRD method.2,some basic properties of Tm, Ho:YGG crystalsDensities of Tm, Ho:YGG crystals were calculated by some formulas. The compositions of Tm, Ho:YGG crystals were measured by the X-ray Fluorescence analysis method. The change of segregation coefficient of Tm3+, Ho3+in different Tm, Ho:YGG crystals was analyzed.Their specific heats, thermal expansion coefficients, thermal diffusion coefficients and thermal conductivities were also measured. Their specific heats changed little at different temperature. Their thermal expansion coefficients were improved with an increasing temperature, but their densities were reduced. Laser flash method was used to measure their thermal diffusion coefficients. According to the formula, their thermal conductivities were calculated.The absorption spectra of Tm, Ho:YGG crystals were measured and these absorption bands were assigned to different ionic transitions from the ground state to the excited states. It had a strong absorption in YGG at684and788nm, which meaned that commercially available laser diodes could be used as the pump source for this material. Using the reciprocity principle, the emission cross-section σem(λ) can be calculated via the absorption spectrum. The largest emission cross-section is located at about2086nm with σem=3.25×10-20cm2. We also speculated the energy transfer process involved in the2.0μm emission of Tm, Ho:YGG and calculated the effective gain cross-section spectrum. Meanwhile, the minimum pump intensity required to achieve transparency at the extraction wavelength was calculated. Emission spectra of Tm, Ho:YGG crystals also were measured, which were accorded with theoretical calculation basically.3,Thermal and optical properties of ZGP crystalThermo gravimetric (TG) and Differential Thermal Analysis (DTA) curves of ZGP crystal were obtained, which were analyzed by the law of element conservation and endothermic and exothermic processes. ZGP crystals were treated by annealing in vacuum, P vapor and ZGP powder environments. Then, the effective segregation coefficient of each element, Laser damage threshold (LDT), transmission spectra and Raman spectra of annealed ZGP wafers were all measured and discussed. The effective segregation coefficient of each element after annealing was different from that of unannealed crystal. The LDT value of annealed ZGP wafer was observed to increase to96.43MW/cm2, which was measured at a wavelength of1064run and10ns pulse widths. It was found that thermal annealing under different conditions could effectively diminish the content of the related point defects and disordered ZGP and make some characteristics of ZGP improve. Some dislocation and crystal boundary were observed in the eroded crystal by chemical etching method.4,Thermal and optical properties of ZnSe and Cr2+:ZnSe crystalsThe density, specific heat, mean thermal expansion coefficient and thermal conductivity of ZnSe crystal were measured to be5.2637g/cm3,0.339J/K·g,4.7859×10-6/K and16.2W/K-m, respectively. It was observed its highest transmittance to be70.4%at2.5μm by the absorption spectra of ZnSe crystal. So ZnSe crystal was an excellent mid-infrared crystal.The concentrations of Cr2+in prepared Cr2+:ZnSe crystals were measured by the X-ray Fluorescence analysis method. The absorption coefficient of Cr2+at the wavelength of1.7μm was calculated by the absorption spectra of Cr2+:ZnSe crystal. According to the formula, the density of Cr2+in Cr2+:ZnSe crystal that was prepared during5days at950℃was1.8064×1019atom/cm3. The main factors affecting diffusion process were discussed.5,Laser performance of4at%Tm,0.5at%Ho:YGG crystalThe continuous-wave (CW) laser experiments of4at%Tm,0.5at%Ho:YGG crystal were carried out by a fiber-coupled LD with a central wavelength of about795nm. The transmissivity of output coupler (OC) was5%or10%. The maximum output power is1.325W at room temperature at OC=5%with a slope efficiency (η) of13.5%and optical conversion of10.8%. When the OC is changed to10%, the maximum output power, slope efficiency and optical conversion were respectively0.97W,11.3%and8.5%. With an optical spectrum analyzer, the laser wavelength of the crystal was measured to be about2.086p.m. In the future, better laser performance can be expected from this material by optimizing doping concentrations and optical cavity.