Research On Growth Mechanism And Properties Of Plate Nd:YAG Crystal Grown By HDS Method

Neodymium-doped yttrium aluminum garnet （Nd:YAG） crystal has become themost suitable and dominant solid-state laser material since the1960’s. It is primarilyattributed to its excellent laser properties and outstanding thermal and mechanicalproperties. Recently, along with the development of pump technology of lasercrystal using laser diode, Nd:YAG single crystal has been widely applied in thefields of communication, material processing and laser medical treatment.Especially with the prosperous of the pumping source of the inertial confinementfusion （ICF） using laser diode, the field of military application has been moreprominent. The Nd:YAG crystal has become one of the "three basic laser crystals"and the main pillar of medium and high power laser applications. The increasingdemands for high-energy and high-power solid laser technology in those areas causethe Nd:AYG crystal struggles to satisfy the clients’ demands, however, the centralcores, lateral core and Nd³⁺concentration gradient of Nd:YAG crystal grown byexisting technologies cannot settle the problem. This article addresses the aboveproblems by using the horizontal directional solidification （HDS） method to growNd:YAG single crystal. The mode of solid-melt interface and mechanism of meltflow patterns were computed, Nd³⁺ion distribution and the relation with meltconvection were discussed. The quality and morphology of the structure,constituents and formation reasons of defects in crystal were analysised. Finally, theJudd-Ofelt theory was introduced, and absorption cross section, radiative life timeand emission cross section were estimated based on the absorption and fluorescencespectra, in order to predict the laser output property.The large-plate Nd:YAG laser crystal with a size of220mm×140mm×25mmwas successfully grown by HDS method based on reasonable process parameters.The analytical tests of phase, X-ray rocking curves and TEM shown that thestructural integrity of Nd:YAG laser crystal was excellent. The results ofinterference pattern and extinction ratio indicated that the quality of crystal withoutcentral cores and lateral cores was excellent, and the relations between structuralintegrity and anneal were studied as well. Meanwhile, analysis of thermal andmechanical properties also shown that the basic physical performances of crystalgrown by HDS method were good.For the change of heat mass transfer of solid-melt interface in the Nd:YAGcrystal growth process, which resulted in the inconformity of the crystallizationfront velocity and crucible pulling rate, the formula of crystallization front velocityin Nd:YAG crystal growth process by HDS method was derived and numerical analyzed. The results indicated that the crystallization front velocity was influencedby crystal thickness, crucible size and temperature gradient, furthermore, the effectswere calculated and analyzed in detail.One of the important conditions for high-quality large Nd:YAG laser crystal isthe excellent thermal mass transfer. Therefore, the mechanism of melt flow patternsin Nd:YAG crystal growth process by HDS method was derived and numericalanalyzed, in order to make clear of the main mechanism of the melt flow patterns.Meanwhile, the distribution of Nd³⁺ions in crystal was studied, and the relation withtemperature gradient was discussed as well. The results indicated that thedistribution was superior to that of other growth methods, and the HDS method wasmore suitable for Nd:YAG crystal growth.The growth defects of HDS-grown Nd:YAG crystal were carried out by meansof laser scanning confocal microscope and scanning electron microscope. Theresults manifested that scattering particles （bubbles, small inclusions and largeinclusions） and dislocations were the main growth defects, and the distribution andformal reasons of growth defects were discussed. Meanwhile, the distribution,density and categories of dislocations in grown crystal were also analyzed bychemical etching and XT methods, which indicated that the quality of crystal centerwas better than that of the crystal edge, and the dislocation density of the center wasrelatively lower.Absorption and fluorescence spectra of HDS-grown Nd:YAG crystal werecharacterized and analyzed. The results shown the maximum absorption at808nmwith a wide FWHM and strong emission peak at1064.59nm was coincident withtheoretical value （1064.15nm）, which proved the well optical quality of HDS-grownNd:YAG crystal. The intensity parameters were obtained to be Ω₂=0.5022×10^-20cm²,Ω₄=3.3506×10^-20cm²and Ω₆=5.1653×10^-20cm²based on the Judd-Ofelt theory, andabsorption cross section, radiative life time and emission cross section and otherparameters were estimated as well. Meanwhile, the laser conversion efficiency ofoutput power-input power was37.44%.