Bridgman Growth And Spectral Properties Of Novel Laser Crystals

Bi4Ge3O12 as a laser host has been attracting much attention form materials scientists in recent year. Er3+:Bi4Ge3O12 single crystal is a novel Er3+ doped oxide laser material with a luminescence at the central wavelength of 1.5μm, which is considered to be valuable as a possible laser source in the optical fiber communication. However, there exists much difficult to grow large size Er3+:Bi4Ge3O12 crystal with high quality owing to a series of technique issues. First, the bismuth rich melt is prone to leak in the crystal growth due to its serious corrosion to the crucibles. Second, it is somewhat difficult to dope Er3+ ion into the host with high concentration due to the small segregation coefficient. Thirdly, optical scattering originates from the inclusions often occurs in the Er3+: Bi4Ge3O12 crystals because the impurities are hard to be expelled in the viscous melt. In this thesis, Er3+: Bi4Ge3O12 single crystals have been grown by vertical Bridgman method for the first time. Large size Er3+:Bi4Ge3O12 crystals with desirable transparency were grown by the growing process with appropriate conditions. Based on the characterization to the materials, the spectral property of crystal sample was investigated. The research results are summarized as follows:(1) Using Bi2O3, GeO2 and Er2O3 as the initial agents, the feed material was prepared according to the accurate stoichiometry. The polycrystalline bar with the composition of Er3+xBi(4-x)Ge3O12 was synthesized by solid state reaction at elevated temperature. By means of sintering under the sealed condition, an accurate stoichiometric composition has retained in the polycrystalline material because the volatilization in the process was suppressed. The polycrystalline material could be used to grown large size Er3+: Bi4Ge3O12 crystal with high quality.(2) In order to avoid the melting leaking effectively, the platinum crucibles with double shells were used in the Bridgman growth. In the growing process, the furnace temperature was controlled at 1150～1170℃and the temperature gradient near solid-liquid interface was adjusted around 30～50℃/cm. Applying the crucible lowering rate in the range of 0.7～1.0 mm/h, the pink transparent Er3+:Bi4Ge3O12 crystals with size of ?12×100 mm have been grown successfully by vertical Bridgman process.(3) As grown Er3+: Bi4Ge3O12 crystals were characterized by XRD, DTA/TG, absorption spectrum and fluorescence spectrum. The grown crystal shows a high optical transmittance in the visible range with desirable optical homogeneity. There exists the absorption peak attributed to Er3+ ions in the optical spectrum. Under the 980 nm pumping from laser diode, the crystal emits a strong fluorescence at 1526 nm, which was attributed to 4I13/2-4I15/2 transition in Er3+ ions. The fluorescence radiation lifetime was measured to be 4.58 ms.The fluoride crystal LiYF4 is an excellent laser host material and Ce3+ doped LiYF4 crystal is a promising ultraviolet laser material. Ce3+: LiYF4 crystal can emit a tunable radiation in the wavelength range of 305～335 nm, which is valuable to be applied in semiconductor processing and photonic chemical reaction. So far most groups have applied Czochralski method to grow Ce3+: LiYF4 crystal. However, the optical quality of grown crystals is deteriorated owing to the oxyfluorides in the medium, which are brought from the hydrolysis reaction occurred in the growth process due to the absorbed moisture. In addition, HF or CF4 atmosphere must be kept in the crystal growing system to avoid the melt oxidation in the process. In this work, the vertical Bridgman process was applied to grow Ce3+: LiYF4 crystals under nonvacuum atmosphere in order to overcome the difficulties in the crystal growth. Large size LiYF4 and Ce3+: LiYF4 crystals with high quality had been grown by the modified Bridgman process in nonvacuum atmosphere. The spectral properties of the crystals were characterized by transmission spectrum and fluorescence spectrum. The main results are summarized as follows:(1) Using high purity LiF, YF3, CeF3 as initial agents, the feed material for the crystal growth was prepared according to the molar ratio of LiF: YF3: CeF3 = 51.5: (48.5-x): x. In order to eliminate the moisture absorbed from ambient, the feed material was sintered in dried HF atmosphere. The anhydrous Ce3+: LiYF4 polycrystalline bars for crystal growth were synthesized by means of solid state reaction at elevated temperature.(2) LiYF4 and Ce3+: LiYF4 single crystals were grown by modified vertical Bridgman process in nonvacuum atmosphere. Using the platinum crucibles with double shells, it could be realized to grow the crystals in nonvacuum atmosphere by means of sealing the crucibles. To exhaust the oxygen sealed in the crucibles, a small amount of polytetrafluoroethylene power was added to the crucibles. In the Bridgman growth , the furnace temperature was controlled at 920~930oC and the temperature gradient across the solid-liquid interface was adjusted at 20~30 oC/cm. With the crucible lowering rate in the range of 0.5~0.6 mm/h, a transparent LiYF4 crystal with size of ?25×50 mm and a Ce3+: LiYF4 crystal with size of ?26×70 mm had been grown successfully by the vertical Bridgman process.(3) The grown crystals were characterized by XRD, DTA/TG, transmission spectrum and fluorescence spectrum. The transmission spectrum of LiYF4 crystal shows an optical transmittance higher than 90% in the UV-visible range with an absorption edge located at the wavelength less than 200 nm. The transmission spectrum of Ce3+: LiYF4 crystal indicates a high optical transmittance with a strong absorption at 297nm. No evident OH- absorption is observed in the IR spectrum. Under the stimulation of 297 nm radiation, the fluorescence spectrum of Ce3+: LiYF4 shows two strong emission peaks at 310 nm and 325 nm, which are attributed to 5d→F1/2 and 5d→2F5/2 transitions in Ce3+ ions respectively.