| Since the operation of the first Ruby laser device in1960, laser has been widely used in the fields of microelectronics, communication, medical treatment, military, reaserch, education and exploration, due to its unique optical characteristics, such as monochrome, orientation, coherence, which have been used to generate ultrabright and ultrashort pulses. Laser materials, especially the laser crystals, played a vital role in the development of laser techniques. Into21st century, laser and laser techniques will continue to promote the fast progress of optoelectronics industry. At the same time, people will also propose newer and higher criteria for the laser crystals, which make them become the leading frontier and hot topic both in the fields of material science and engineering development. The development of ultrafast laser gain and amplifier media suitable for LD pumping will become one of the main trends of laser crystals.Mode-locking is a technique to generate ultrashort pulses from lasers. The width of the individual mode-locked pulses is is approximately equal to the inverse of the oscillation bandwidth. Therfore, the narrower the pulse width, the larger the bandwidth required to generate the pulse. Improvement of the disorder degree of the crystal facilitates the production of short, mode-locked pulses. So disordered laser crystals have attracted a great deal of research interest. It is clearly manifested that disordered crystals have statistical structual elements or statistical occupation of similar crystallographic positions in the lattice with different calences. The crystal-field disorder at activators ions produces a large inhomogeneous broadening of the spectrum. Strong inhomogeneous broadening of the pumping band improves spectral overlap with the relatively broad emission band of a diode laser (DL). A broad fluorescence linewidth facilitates the production of short, mode-locked pulses. In addition, compared with glass, disordered crystals have large thermal conductivity, which can be applied to high-power lasers. In disordered crystals, temperature change will lead to shift of activator-ion Stark Levels. Therefore, the spectra including absorption and emission are inhomogeneously broadened, which determines its output laser wavelength to be tunable. ABC3O7crystals belong to a large disordered family with space group p42, m.Here, A=Ca, Sr, Ba; B=La, Gd; and C=Al, Ga. The ABC3O7crystal is built up from CO4layers formed in the a-b plane. Between the layers, A2+and B3+ions are distributed randomly in eight coordinated sites with Cs symmetry. The ratio is1:1. In those disordered 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. Furthermore, ABC3O7crystals have favorable properties.Since the reported melting points are higher (about1588℃for SrLaGa3O7,1600℃for SrGdGa3O7,1560℃for BaLaGa3O7) and there are no phase transitions below the melting point, these crystals could possibly be used in high temperature piezoelectric applications. In this thesis, characterization on crystal quality, thermal properties, optical properties, laser performance and high temperature piezoelectric application were carried out. The outline is shown as follows:(1) We introduce the research ideas and significance on disordered ABC3O7crystals being excellent multi-functional crystal materials, as well as the basic concepts about disorderded crystals, pulse laser generation technique, and review their recent research progresses. On this basis, we briefly introduce the research ideas and concents of this dissertation.(2) Crystal growthHigh quality single crystals with the melilite ABC3O7structure, including Nd:SrGdGa3O7, Nd:SrLaGa3O7, and Nd:BaLaGa3O7, were grown using the Czochralski technique. Influence of thermal field, crystal seed and growth process parameters (the pulling rate,the rotation rate, and cooling rate) was discussed.(3) Investigation on the crystal quality, and thermal and optical properties of Nd:ABC3O7crystals 1,X-ray powder diffraction was used for structure measurements. The space group is p421m.According to the peak2θ values in the XRPD pattern, the tetragonal unit cell parameters were calculated.2,The high-resolution X-ray diffraction method was used to check the quality of the as-grown single crystal. The full-width at half-maximum value of Nd:SrLaGa3O7was measured to be16.92". The symmetry of the diffraction peak indicates that the as-grown crystal is of high quality.3,The concentrations of elements in Nd:ABC3O7crystals were measured using x-ray fluorescence analysis. The segregation coefficients of Nd ions in Nd:SrLaGa3O7, Nd:SrGdGa3O7, and Nd:BaLaGa3O7crystals were calculated to be0.99,1.36and1.12, respectively.4,The melting pint of Nd:SrLaGa3O7was determined to be1588oC using a TG/DTA thermal analyzer (SETSYS-2400CS Evolution, France). The variation of thermal expansion, specific heat, thermal diffusion, and thermal conductivity with temperature were determined. The expansion coefficients were measured to be on the order of10-6/K by the thermal-mechanical analyzer. The specific heat of Nd:SrLaGa3O7, Nd:SrGdGa3O7, and Nd:BaLaGa3O7crystals were determined to be0.401J/gK,0.419J/gK and0.365J/gK, respectively, using a differential scanning calorimeter (Diamond model DSC-ZC). The thermal diffusivity components of the crystal were measured by the laser pulse method using a Netzsch Nanoflash model LFA447apparatus along the crystallographic axes. For Nd:SrLaGa3O7, λ11=0.93mm2/s, λ33=0.81mm2/s. For Nd:SrGdGa3O7, λ11=0.67mm2/s, λ33=0.59mm2/s. For Nd:BaLaGa3O7, λ11=0.97mm2/s, λ33=0.85mm2/s. Thermal conductivity was calculated with the measured data of specific heat, thermal diffusion coefficient and density. For Nd:SrLaGa3O7, κ11=1.95Wm-1K-1,κ33=1.70Wm-1K-1. For Nd:SrGdGa3O7, κ11=1.59Wm-1K-1, κ33=1.40Wm-1K-1. For Nd:BaLaGa3O7, κ11=1.96Wm-1K-1, κ33=1.72Wm-1K-1. All the values were higher than that of glass, making Nd: ABC3O7crystals suitable for such applications in medium and high laser system. In addtion, it was found that the thermal conductivity increases with increasing temperature, which is similar to the behavior of glass.5,The refractive indices of Nd:SrGdGa3O7were measured by the minimum-deviation method at13different wavelengths ranging from253to2325nm. Sellmeier's equations fitted by the least-squares method. The results showed that The crystal is positive uniaxial with a relatively small birefringence. We measured the polarized absorption and emission spectra of Nd3+, and calculated the spectral parameters based on the J-O theory. Thee σ-polarization absorption is stronger than π-polarization absorption. The absorption and emission spectra of Nd3+are inhomogeneously broadened, which can be associated with the disordered melilite structure. The smaller emission cross-section and longer fluorescence lifetime indicate that Nd:ABC3O7should have a higher energy storage capacity and excellent Q-switched laser properties.(4) Laser experiments1,The continuous-wave laser performance of Nd:SrLaGa3O7, Nd:SrGdGa3O7, and Nd:BaLaGa3O7crystals have been demonstrated, and the maximum output power was obtained to be3.88W,1.71W and0.57W at1.06μm. For the first time, a tunable laser with a disordered Nd:SrLaGa3O7crystal by temperatures was demonstrated. The central wavelength of the laser emission was observed to shift to longer with increasing the output couplings, which was resulted from heat-induced redistribution of the Nd-ion population on the energy levels. And the varation of the laser wavelength with the pump power was investigated for the first time.2,With Cr4+:YAG as saturable absorber, the passive Q-switching performance of Nd:SrLaGa3O7and Nd:SrGdGa3O7was demonstrated fo rthe first time. With Nd:SrLaGa3O7, the maximum pulse energy, shortest pulse width, and highest peak power were measured to be60.3μJ,10.6ns,and5.3kW, respectively. With Nd:SrGdGa3O7, the maximum pulse energy, shortest pulse width, and highest peak power were measured to be89.1μJ,15.8ns,and5.6kW, respectively,3,We explore the operation of spontaneous mode locking in a diode-pumped Nd:SrGdGa3O7disordered crystal laser. The first-and second-order autocorrelations are simultaneously performed to evaluate the temporal characteristics. An80GHz pulse train with a pulse duration as short as616fs is observed. The maximumoutput power is415mWat a pump power of6.1W.(5) Piezoelectric propertiesAll the piezoelectric, elastic, and dielectric constants were determined and analyzed, exhibiting excellent piezoelectric properties compared to SiO2, La3Ga5SiO14, and YCa4O(BO3)3. The piezoelectric constant d14and electromechanical coupling coefficient k'12(xyt)45°of the crystals were found to be on the order of12-15pC/N and16-19%, respectively. The piezoelectric parameters showed much better temperature-independent properties over the range of-50to120℃than the comparison materials. Together with a broad useful temperature range (theoretically up to the melting temperature at～1600℃), these properties make ABC3O7crystals promising candidates for sensor applications at ultra high temperatures. |
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