Optical Properties And First Principles Calculations Of The(Cr,Ca):YAG Crystals

(Cr,Ca):YAG crystals has been widely used in tunable and mode-locked laser and Q-switch laser,which is primarily attributed to its outstanding mechanical stability,high thermal conductivity and excellent optical properties.The crystals of YAG,Cr:YAG and?Cr,Ca?:YAG were grown by an optical floating zone furnace.The XRD and SEM were used to analysis the crystals' micro-structure,and we obtained abundant micro-structures information of those crystals;Shimadzu UV-2700 spectrometer were used to test the absorption spectra and optical transmittance while photo-luminescence meter were used to test the emission spectra of them,which help us investigate the optical properties of those crystals.In addition the DOS and band structure of YAG,Cr:YAG?Cr:0.6 at.%?and?Cr,Ca?:YAG?Cr,Ca:0.6 at.%?were studied based on the density functional theory of first principles,and meaningful results have been achieved:?1?The XRD and SEM results of different doped concentrations of?Cr,Ca?:YAG crystals powder shown that only the crystal particles of the seed rods with uniform size,few reunion phenomenons and few holes can be used to grow good quality single crystals.?2?Through The Rietveld refinement of the XRD patterns of Cr:YAG?Cr:0.6 at.%?and?Cr,Ca?:YAG?Cr,Ca:0.6 at.%?crystals' powder and the analyzed of the XRD spectra,we obtained the simulated XRD spectra and the Rwp are 12.75%and 12.55%which shown that the simulation were very good.And the fitting function were Tomandl Pseudo-Voigt.And we also obtained some lattice constants of those crystals' powder and found that the doped ions changed the lattice structures.?3?The emission spectra of the different concentrations doped?Cr,Ca?:YAG crystals indicated that the emission spectra of?Cr,Ca?:YAG crystal with four peaks at 679.5nm,688nm,706.5nm,725nm.The peak near 688nm is due to 4E2—4A2 transitions of Cr³⁺ ions.And the peaks near 679.5nm,706.5nm and 725nm are due to the 2E—4A2 transitions of Cr³⁺ ions.?4?The absorption spectra of the different concentrations doped?Cr,Ca?:YAG crystals shown that when the concentration of Cr is 0.04at.%and Ca ions is 0 that the peaks are near 430nm and 600nm which concentrated by the 3B1?3A2??3A2 and 3B1?3A2??3E?3T1?transmissions of Cr³⁺ ions.And when the concentration of Ca ions is equal to that of Cr ions the absorption peaks are near 465nm and 612nm.And with the concentration of Ca ions increased from 0 to the concentration of Cr ions that the absorption peaks changed from 430nm and 600nm to 465nm and 612nm.?5?Through the measurement of the absorption edges of YAG,Cr:YAG?Cr:0.6 at.%?and?Cr,Ca?:YAG?Cr,Ca:0.6 at.%?crystals,It was found that the optical band gaps of these crystals are about 6.52eV,5.38eV and 6.31eV,respectively.And the calculated band gaps of these crystals are 3.72eV,1.98eV and 2.76eV respectively.The calculation results are in agreement with the experimental results.?6?The DOS and absorption edges of YAG,Cr:YAG?Cr:0.6 at.%?and?Cr,Ca?:YAG?Cr,Ca:0.6 at.%?crystals shown that in a Cr:YAG crystal the Cr³⁺impurity introduces a band of 3d occupied state near the Fermi level,which significantly narrowed the band gap of Cr:YAG.In a?Cr,Ca?:YAG crystal,as the Ca²⁺ ions substitute for the Y³⁺ ions,some of the Cr³⁺ ions will change into Cr4+ ions for the balance of the charge,which increases the band gap of the?Cr,Ca?:YAG crystal in comparison with that of the Cr:YAG crystal.