Growth And Pulse Energy Enhancement Of Novel Vanadate Crystals

Laser is a simplified word of light amplification by stimulated emission of radiation. Actually, the laser equipment is just a cavity which makes the light oscillating and coherent by stimulated emission of radiation. It is monochromatic, directional, and coherent, and has become one of the important parts of military, medicine, scientific research, and our daily life. Recently, as the development of laser diodes, the diode pumped solid state lasers (DPSSLs) have taken a revolution in the laser field and have been applied in many fields, such as material processing, medicine, optical communications, lasers display and laser nuclear fusion, etc.As the most important part of lasers, laser gains ultimately determine the output performance and applications of the lasers. Among the laser materials, neodymium doped vanadate crystals, such as Nd:YVO₄, Nd:GdVO₄ and Nd:LuVO₄, have been identified to be excellent gains due to their broad absorption bandwidths, large absorption and emission cross-sections, polarized emission and high chemical stability. As the development of pulsed lasers, the gain with broad emission linewidth has attracted more and more attention. Because of same ZrSiO₄ structure, with a tetragonal space group I4₁/amd, and close melting points, nearing 1800℃, of the three vanadate crystals, the mixed Nd:Y_xGd_1-xVO₄, Nd:Lu_xGd_1-xVO₄ and Nd:Y_xGd_1-xVO₄ (0≤x≤1) can be grown as single crystal. In those mixed crystals, due to the variation of the local crystal fields surrounding the Nd ions, the spectra, including absorption and fluorescence lines, are inhomogeneous broadened and the emission cross-sections are reduced. In the laser field, smaller emission cross-sections and broadened fluorescence spectra are favorable by the Q-switched and mode-locked lasers, respectively. Besides the laser crystals, the saturable absorber is also an important part in the passive Q-switching and even mode-locking. In the passive Q-switching regime, chromium (Cr⁴⁺) doped YAG crystals have domained. However, due to the substitution of Cr⁴⁺ ions for a fraction of Al³⁺ ions at Al-ion sites in this crystal, Ca²⁺ or Mg²⁺ ions should also be co-doped to keep the balance of the charge, which brings some complexities in the crystal growth and problems in the applications. In this work, we studied the characteristics of Nd:Lu_xGd_1-xVO₄ and Nd:Y_xGd_1-xVO₄ from three aspects, including preparations, basic properties measurements, and laser applications. Based on the growth of the vanadates, we also primarily investigated the saturable absorber Cr⁵⁺:GdVO₄ and self-Q-switched laser materials Nd:Cr:GdVO₄, in order to overcome the drawbacks of conventional Cr⁴⁺:YAG in the preparations and applications.The contents of this dissertation is outlined as following:1. Nd:Lu_xGd_1-xVO₄ and Nd:Y_xGd_1-xVO₄ (0≤x≤1) with different components were grown by the Czochralski method. In order to apply the crystals in the high pumped power level, Nd concentrations in the crystals are determined to be as low as 0.5at.%. During the preparation, different segregation coefficients of Nd ion in YVO₄, LuVO₄ and GdVO₄ were considered. By the X-ray fluorescence method, Nd concentrations in all crystals are determined to be about 0.5at% and x ranges from 0 to 1, as expected.2. Thermal properties of the crystals are measured. It has been found that the thermalproperties, including average linear thermal expansion coefficients, thermal diffusion coefficients, specific heats and thermal conductivities show obviously variations as x approaching 0.5 for Nd:Lu_xGd_1-xVO₄, and 0.37 for Nd:Y_xGd_1-xVO₄. By considering the movement of phonons in crystals, the reduction of thermal conductivities were explained. The thermal fracture limits have been estimated and found to be comparable with Nd:YVO₄, which means that the mixed crystals can be used in the high pump power level.3. The absorption and fluorescence spectra, and fluorescence lifetimes of the crystalshave been measured. With J-O theory and absorption spectra, fluorescence lifetimes of Nd:Lu_xGd_1-xVO₄ have been calculated in theory, which agree well with the experimental results. It was found that due to the variation of the crystal field neighboring the Nd ions, the spectra are inhomogenously broadened, and the energy storability are improved as x approaching 0.5 for Nd:Lu_xGd_1-xVO₄, and 0.37 for Nd:Y_xGd_1-xVO₄. The results indicated that the mixed crystals should have promising properties in the pulsed lasers.4. The continuous-wave laser performance of Nd:Y_xGd_1-xVO₄ and Nd:Lu_xGd_1-xVO₄ hasbeen demonstrated. With Nd:Lu_xGd_1-xVO₄, and Nd:Y_xGd_1-xVO₄, the maximum output power of 7.4 W was obtained at 1.06μm, and 2.56 W at 1.34μmn. 5. With Cr⁴⁺:YAG as saturable absorber, the passive Q-switching performance of a-cutNd:Lu_xGd_1-xVO₄, and c-cut Nd:Y_xGd_1-xVO₄ and Nd:LuVO₄ was demonstrated. Compared with Nd:GdVO₄, the energy of a-cut Nd:Lu_xGd_1-xVO₄ was found to be enhanced in the same conditions. The maximum pulse energy was obtained to be 192.5μJ with highest peak power of 31.06 kW and shortest pulse width of 6.2 ns. With c-cut Nd:Y_0.37Gd_0.63VO₄, the maximum pulse energy of 201.7μJ was obtained with peak power of 30.6 kW and pulse width of 6.6 ns. With c-cut Nd:LuVO₄, the maximum pulse energy, shortest pulse width, and highest peak power were measured to be 84.93μJ, 12 ns,and 7.02 kW, respectively,6. The passive mode-locking performance of a-cut Nd:Lu_xGd_1-xVO₄ for x=0.5 and 1,was demonstrated with SESAM and GaAs as saturable absorbers. The pulse width of shorter than 10 ps was obtained. Compared with Nd:LuVO₄, the pulse width was found to be shortened with Nd:Lu_0.5Gd_0.5VO₄.7. For the first time, the saturable absorber of Cr⁵⁺:GdVO₄ and self-Q-switchingmaterial Nd:Cr:GdVO₄ were grown. Their passive Q-switching performance wasprimarily investigated.Based on the above research, the proming applications of the mixed crystals have been proved. It has also been found that the energy storability can be improved as x approaching 0.5 and 0.37 for Nd:Lu_xGd_1-xVO₄ and Nd:Y_xGd_1-xVO₄, respectively. In the mode-locking, the Nd:Lu_0.5Gd_0.5VO₄ was found to have pulse width shortening property, compared with Nd:LuVO₄. The primarly investigation on the Cr⁵⁺:GdVO₄ and Nd:Cr:GdVO₄ has proved their saturable properties and promising applications in the Q-swithched lasers.