Investigation On Growth And Optical Properties Of Rare-earth Doped BaY₂F₈Laser Crystals

Barium yttrium fluoride (BaY2F8-BaYF) is an excellent laser host crystal.Compared with other optical crystals, its advantages are as follows:(1) Very lowphonon energy (ω～360-380cm-1). The very low phonon energy can reduceoccurrence of non-radiative processes with respect to the radiative ones.(2) Lowrefractive index. The low refractive index suggests a low thermal lensing effect in theprocess of laser operation.(3) Very wide transparency spectra ranging from vacuumUV to far-infrared regions. In addition, the BaY2F8crystal also has other advantages,such as high laser damage threshold, large emission cross section, easy to dope withrare earth ions and transition metal ions. In recent years, more and more attention hasbeen paid to study on the BaY2F8crystal and Re3+: BaY2F8crystals. In this paper, wereport the growth of BaY2F8crystal and Re3+:BaY2F8(Re3+=Tm3+, Er3+) crystals bythe temperature gradient method, which is an efficient improvement ofno-seed-growth crystals. Meanwhile, we also reveal the optical properties of theTm3+:BaY2F8and Er3+:BaY2F8laser crystals via a combination of absorption,excitation, emission and up-conversion spectroscopy.Firstly, The BaY2F8, Er3+:BaY2F8and Tm3+:BaY2F8crystals were grown bytemperature gradient method. Three kinds of macro-defects are found in BaY2F8crystals during the crystal growth. These macro-defects include cracking, growthstriations and straight pipes. We investigate the reasons why these macro-defects existin the crystals and how to eliminate these macro-defects. On the basis of experimentsand theoretical analyses, the optimal growth parameters have been obtained as follows:the axial gradient temperature is6-7K/mm, the growth rate is0.25mm/h and the finalcooling rate is less than10K/h.Secondly, we have measured many optical spectra of these crystals, includingabsorption spectra, low-temperature and room-temperature emission spectra,excitation spectra, up-conversion spectra and so on. Based on Judd-Ofelt theory, thethree intensity parameters of Tm3+:BaY2F8and Er3+:BaY2F8crystals have beenobtained. Moverover, the values of radiative probablities, branding ratios, radiativelifetime and integrated emission cross-section are alse calculated. In the study oflow-temperature and room-temperature emission spectra, we focus on revealing theeffect of temperature on the optical characterization. In addition, the up-conversion spectra under the different excitation wavelength are measured and the up-conversionmechanism is analyzed in detail. More attentions are paid to investigating the rolesand influence of the ground state absorption (GSA), excited state absorption (ESA),multiphonon non-radiative transition (MPR) and Cross relaxation (CR) in theup-conversion process.