Growth And Spectral Properties Of Femtosecond Laser Crystals Doped With Cr³⁺

Femtosecond lasers have been widely used in physics,biology,chemically controlled reactions,optical communications,medical,aerospace fields and other fields.Meanwhile,the laser crystals with broad emission band become the key to the development of femtosecond laser systems.Cr³⁺ions have the advantages of stable valence state,the lowest excited state and so on.Especially,the broadband emission would be generated if Cr³⁺ions in weak crystal field.Therefore,Cr³⁺has become preferred activating ion for the exploration of femtosecond laser crystal.In this thesis,the growth,spectral properties and crystal fields of Cr³⁺-doped Ca₃V₂O₈ and Gd₂SrAl₂O₇crystals were studied.Cr³⁺:Ca₃V₂O₈ crystals were successfully grown by the Cz method and the substitution site of Cr³⁺ions is determined to be Ca²⁺ions.It was confirmed that the self-absorption of matrix crystals can be eliminated by annealing in air.The spectral properties and crystal field parameters of matrix crystals after annealing were studied.The polarized absorption spectral show there is a strong and broad absorption band of600-850 nm.The absorption transition cross-sections in theσandπdirections at 690 nm are respectively 3.50×10^-20 cm² and 2.24×10^-20 cm².The emission spectrum shows broadband emission around 860 nm and the emission cross sections are 3.0×10^-20cm²（σ）and 2.77×10^-20 cm²（π）,respectively;the fluorescence half-peak widths（FWHM）are178 nm（σ）and 173 nm（π）,respectively.The fluorescence lifetime is 10.5μs.The crystal field parameters were analyzed through the theory of solid spectroscopy.The spectroscopic properties of Cr³⁺:Ca₃V₂O₈ crystal reveals that the crystal is a potential femtosecond laser with LD as a pumping source.The Cr³⁺:Gd₂SrAl₂O₇ crystal was grown by the Cz method.The absorption spectrum mainly contains two absorption bands with peak positions at 400 nm and 580nm.The absorption cross sections in theσandπdirections are 0.53×10^-19 cm² and0.30×10^-19 cm² at 580 nm,respectively;and 1.58×10^-19 cm²and 2.05×10^-19 cm² at 400 nm,respectively.At room temperature and 77 K,the emission spectrum of Cr³⁺covers the range of 700-1000 nm,including a sharp emission peak and a broad emission band.Therefore,Cr³⁺:Gd₂SrAl₂O₇ undergoes both spin-forbidden ²E→⁴A₂ and spin-allowed ⁴T₂→⁴A₂ transitions,which means that the environment of Cr³⁺in Gd₂SrAl₂O₇ crystal is an intermediate crystal field.And the crystal field parameters also verify the above conclusion.The emission cross sections of Cr³⁺:Gd₂SrAl₂O₇ crystalsσ-andπ-are 4.49×10^-22 cm² and 5.52×10^-22 cm²,respectively.The fluorescence lifetime of the crystal is2.54 ms,which is beneficial to energy storage and improving the laser output power.Therefore,Cr³⁺:Gd₂SrAl₂O₇ crystal can be a candidate material for femtosecond laser crystals.By synthesizing a series of Cr³⁺:Gd₂SrAl₂O₇ polycrystalline powders with different doping concentrations,and performing spectral characterization,the optimal doping concentration is 1 at.%.Subsequently,the Sr²⁺,Al³⁺and Gd³⁺ion sites in Gd₂SrAl₂O₇were replaced by Ba²⁺,Ca²⁺,Ga³⁺,Lu³⁺ions,respectively,and a series of powders were synthesized and characterized by XRD and spectrum.The results show that the emission with stronger emission peak intensity and wider emission bandwidth were be obtained by doped with Ba²⁺and Ga³⁺ions in Cr³⁺:Gd₂SrAl₂O₇ polycrystalline powders.Specifically,the intensity of emission is 1.46 times that of the standard sample and the fluorescence half-peak width is increased from 46.24 nm to 49.51 nm in the 2 at.%Ga³⁺doped Cr³⁺:Gd₂SrAl₂O₇polycrystalline powder.And the emission intensity of 10 at.%Ba²⁺modified Cr³⁺:Gd₂SrAl₂O₇ polycrystalline powder is 1.79 times that of the sample before modification,and the half-peak width increases from 46.24 nm to 53.56 nm.It shows that the AlO₆ octahedral lattice field becomes more distorted and its crystal field strength is weakened after Ba²⁺and Ga³⁺ions are doped into Cr³⁺:Gd₂SrAl₂O₇,which makes the emission intensity enhance and the half-peak width become larger.