| The exploration of novel laser materials has always been a "hot" research area in the field of solid state lasers.As early as 1960s,rare earth niobates RENbO4(RE=Gd,La or Y)have been synthesized and proved as a series of excellent self-activated phosphor with the advantages of chemical stable,good luminescent properties,high ion conductivities and dielectric properties.Now,this series of materials have shown great application prospects in the fields of phosphors,sensors,fuel cells,photo-catalysis and scintillators,undergo about half a century development.However,up to now;the reports of this series materials used as laser matrixs are rarely to be found.But vanadates and tantalates,which have the same sub-group with niobates,have been extensively investigated and vanadates have been even commercially used in medium and low power solid state laser system.Therefore,in this paper,we want to explore the possibility of rare earth niobates as novel laser matrixs.Firstly,the band structure,density of states and optical properties of rare earth niobates are sys’temically calculated by first principles calculations on the frame of density functional theory.Secondly,four kinds of niobates crystals,including Nd:YNbO4,Nd:GdNbO4,Nd:Gd/YNbO4 and Nd:Gd/LaNbO4,have been grown successfully using Czochralski method owing to the congruent melting of the raw materials.The crystal structure,crystalline quality,segregated of Nd3+ ions,defect morphology and mechanical properties of the as-grown crystals have been systematically investigated.Besides,the absorption spectra,emission spectra,fluorescence spectra and quantum efficiencies of these four as-grown Nd3+ ions doped niobate laser crystals are also characterized and calculated in details in this work.The spectral parameters of Nd3+ions in these four crystals are obtained using J-O theoretical calculation.Furthermore,according to the measured absorption and emission spectra at 8 K temperature,we accurately assigned the energy levels of Nd3+ ions in YNbO4 and GdNbO4 crystals.Lastly,the laser properties of these four Nd3+ ions doped niobate crystals have been evaluated in details,including 1.06 μm continuous wave lasers,0.926 μm continuous wave lasers,1.06 μm passively Q-switched lasers and 1.06 μm acousto-optically Q-switched lasers.The main contents and results of this work are as following:1.The band structure,density of states,electronic structures and optical properties of rare earth niobate materials,including YNbO4,GdNbO4,LaNbO4 and LuNbO4,have been systematically calculated by first principles calculations on the frame of density functional theory.The calculated results indicating that YNbO4,GdNbO4,LaNbO4 and LuNbO4 are all belongs to the indirect band gap materials category,the large band gap(about 4 eV and close to that obtained in experimental)indicates that niobates could possess high damage threshold;Secondly,from the calculated density of state,we can conclude that there exsist strong Nb-O covalent bond in niobates,which indicates niobates should possess high laser damage threshold.Besides,the dielectric functions,absorption efficiencies,refractive index and reflectivity of YNbO4,GdNbO4,LaNbO4 and LuNbO4 are also obtained from the calculations.2.Four Nd3+ ions doped niobate crystals,including Nd:YNbO4,Nd:GdNbO4,Nd:Gd/YNbO4 and Nd:Gd/LaNbO4,have been grown successfully using Czochralski method with no bubbles and good optical qualities.XRD characterization results shown that all these Nd3+ ions doped niobate crystals belongs to M-type with monoclinic structure.The XRD data of these four Nd3+ ions doped niobate crystals are fitted using the Rietveld refinement method and the cell parameters,including band length,angle and cell volume,and so on,of these crystals are obtained.X-ray rocking curve characterization results indicating that all these as-grown niobate crystals with high crystalline quality.The Nd3+ ions concentrations in these four niobate crystals are determined using X-ray fluorescence analysis and the effective segregated of Nd3+ions in these niobate crystals are calculated.The dislocation morphology in niobate crystals was studied by chemical etching method and the etching results shown that the dislocations have deep relations with the crystal structure.The Vickers hardness of these niobate crystals were characterized and the yield strength,fracture toughness and brittleness index of these niobate crystals are also evaluated based on the Vickers hardness experiments.The basic physical properties of Nd:GdNbO4 crystal were investiagted,including thermal expansition,specific heat and density.3.The spectral properties of these four Nd3+ ions doped niobate crystals are characterized in details.Firstly,the transmission spectra of these four niobate crystals are characterized along three different crystalline orientations at room temperature and the absorption cross sections of b-cut Nd:YNbO4,Nd:GdNbO4,Nd:Gd/YNbO4 and Nd:Gd/LaNbO4 crystals at 808 nm are calculated to be 5.7×10-20 cm2,9.3×10-20 cm2,11.7×1O-20 cm2 and 10.5×10-20 cm2,respectively.The spectral transition intensity parameters Ωt(t=2,4,6)and the spectral parameters including transition oscillator strength and fluorescence lifetime of these Nd3+ doped niobate crystals have been calculated based on J-O theory.The fluorescence spectra and lifetime of Nd:YNbO4,Nd:GdNbO4,Nd:Gd/YNbO4 and Nd:Gd/LaNbO4 crystals are characterized at room temperature,the lifetime for them are fitted to be 152 μs,178 μs,156 μs and 176 μs,respectively.According to the F-L formula,the emission cross section of these crystals at 1.06 μm are calculated to be 22×10-20 cm2,24×10-2O cm2,20.5×10-20 cm2 and 18x1O-20 cm2,respectively,which are all smaller than that of Nd:YAG(28×10-20 cm2),Nd:YVO4(156×10-20 cm2)and Nd:GdTaO4(39×10-20 cm2)crystals.The small emission cross sections at 1.06 μm and long lifetime indicate that Nd3+ ions in niobates have good energy storage capacity,which is beneficial for realize Q-switch and ultrashort pulse lasers.Lastly,the crystal field energy levels of Nd3+ ions in YNbO4 and GdNbO4 crystals are assigned in details according to the measured 8 K temperature absorption and emission spectra.4.The laser properties of Nd:YNbO4,Nd:GdNbO4,Nd:Gd/YNbO4 and Nd:Gd/LaNbO4 crystals are evaluated explicitly from the continuous wave and Q-switching aspects,including 1.06 μm continuous wave lasers,0.926 μm continuous wave lasers,1.06 μm passively Q-switched lasers and 1.06 μm acousto-optically Q-switched lasers.The obtained results are as following:(a)The 1.06μm continuous wave lasers are realized successfully in Nd:YNbO4,Nd:GdNbO4,Nd:Gd/YNbO4 and Nd:Gd/LaNbO4 crystals for the first time.Recently,the laser slope efficiencies of Nd3+ ions doped niobate crystals have been enhanced over 40%.(b)Using Cr4+:YAG and two dimension WS2 nanosheet as absorber,the 1.06 μm passively Q-switched lasers are realized successfully in Nd3+ doped niobate crystals for the first time,(c)The 1.06 μm acousto-optically Q-switched lasers are realized successfully in Nd:Gd/YNbO4 crystal for the first time. |
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