Thermal Properties Of RECa₄O?BO₃?₃ And Sr₃?BO₃?₂ Crystals

Laser technology has been applied in many fields such as medicine,communication,industry,biology,military,scientific research,etc.In order to adapt to different needs,the laser wavelength should be diversified.However,commercial lasers generally could only emit a single wavelength.The nonlinear effect produced by the interaction of the laser and the crystal can change the frequency(wavelength)of the output light.This nonlinear frequency conversion makes it possible to obtain lasers of different wavelengths,among which the second-harmonic-generation(SHG)and stimulated Raman scattering(SRS)are two commonly used methods of frequency conversion.In this paper,optical grade RECa4O(BO3)3(RECOB,RE = Y,Gd)and Nd:RECa4O(BO3)3(Nd:RECOB,RE = Y,Gd)crystals were grown using conventional pulling method,and millimeter-sized Sr3(BO3)2 crystal grain were obtained as well.The crystal quality and thermal properties of RECOB crystal were characterized and the structure of Sr3(BO3)2 crystal was analyzed.The cracking phenomenon occurred during the growth process of Sr3(BO3)2 crystal pulling method was analyzed by thermal test method,and the Raman performance was compared with Ca3(BO3)2 crystal.Under the condition of small signal approximation,the calculation method of temperature bandwidth in SHG process of RECOB crystal was deduced by considering thermal rotation effect and thermo-optic effect.The influence of different refractive index and thermo-optic coefficient on the calculation results was analyzed.The experiment was designed to test the temperature bandwidth of RECOB crystals in different directions during SHG process.The experimental and theoretical calculation results were compared and the reasons for the differences were analyzed.The distribution of the temperature bandwidth of the RECOB crystal during SHG process in the whole space was obtained.The maximum temperature bandwidth direction of RECOB crystal during type-I SHG was found.Combined with theoretical calculation and experimental test,the optimized direction of GdCOB crystal with both high SHG efficiency and large temperature bandwidth was found.The temperature bandwidth of the YCOB crystal was tested at different fundamental frequency energies,and the practical application of the YCOB crystal was suggested.The spectral properties of Nd:RECOB crystals were studied.The self-frequency doubling(SFD)temperature bandwidth of the direction was determined experimentally.The source of the temperature bandwidth difference between SFD and SHG was analyzed.The main work and results are as follows:Following the stoichiometry,large-scale optical grade RECOB and Nd:RECOB crystals were grown by Cz method.The crystals obtained by the growth of the crystals were confirmed by X-ray powder diffraction,and the results were all pure phases.The quality of RECOB crystals was evaluated by high resolution XRD.The full width at half maximum(FWHM)of the rocking curves of all samples was less than 30",indicating that the crystal lattice integrity is good and the crystal quality is high,which lays a foundation for optical experiments.The thermal expansion and thermal conductivity of RECOB crystals were studied.Their tensor matrices were calculated according to the test results,and their main axis parameters were obtained.The relationship between main axis of thermal expansion,thermal diffusion and crystallographic axis was determined.Considering the thermal rotation and thermo-optic effect,the calculation method of the temperature bandwidth under the small signal approximation was given.For the first time,the temperature bandwidth of the full-space phase matching direction in type-I SHG of GdCOB crystal was calculated theoretically.The influence on the calculation results causing by the different refractive indexes and thermo-optic coefficients were compared.Some samples were selected and the extra cavity single-pass temperature-tuned SHG experiment was designed.The experimental results were basically consistent with the theoretical calculations.The variation of the temperature bandwidth of GdCOB crystal with angle was determined.The direction with largest temperature bandwidth was found to be(135°,47.3°),and the measured temperature bandwidth was>430 ? cm.Through further optimization,it was experimentally determined that GdCOB crystal in(130°,47.8°)direction possessed both a large temperature bandwidth and a high SHG efficiency.This crystal sample obtained SHG efficiency as high as 47.9%under 1064 nm,35 ps,3 mJ and 10 Hz fundamental light.The temperature bandwidth of YCOB crystal was studied.The full spatial distribution of YCOB crystal temperature bandwidth was given for the first time.The temperature-tuned SHG experiment was designed.Combined with the experimental results,the proper refractive index and thermo-optic coefficient were determined.Both experiments and theory showed that the maximum temperature bandwidth of the YCOB crystal was in the XZ plane(149.2°,0°),and the measured value was>490 ? cm.Because it is located in the main plane,the processing in this direction is simpler than the spatial direction,and the direction also has a large effective nonliner coefficient,all of these are valuable for practical applications.The relationship between temperature bandwidth and fundamental frequency energy in this direction was studied.It was found that the larger the fundamental frequency energy was,the smaller the temperature bandwidth would be.In practical applications,the influence of fundamental frequency energy on temperature bandwidth needs to be considered.The absorption and emission spectra of Nd:YCOB and Nd:GdCOB crystals were tested at room temperature.Their absorption cross sections,spontaneous transition probability,fluorescence branching ratio,emission cross section and other parameters were calculated.The temperature bandwidth in maximum defr directions and XZ plane were determined experimentally.The results showed that the temperature bandwidths of the same crystal in different directions were not much different.The temperature bandwidths in two directions of Nd:GdCOB crystal were all 26 ? cm.The temperature bandwidth in(112.7°,37.4°)and(149.2°,0°)of Nd:YCOB crystals were 64 ? cm and 72 ? cm,respectively,The temperature bandwidth of the Nd:YCOB crystal is larger than that of the Nd:GdCOB crystal.The temperature bandwidth of the SFD crystal obtained by the test is far less than the SHG results due to the characteristics of SFD process energy level transition exotherm,the absorption of heat in the cavity at the high power density of the resonant cavity,the absorption emission cross section change,and the high processing precision.Due to the lack of absorption of the emission cross section as a function of temperature,it is not possible to quantify the temperature bandwidth of SFD process as SHG process.Since the frequency doubling crystal has both laser energy level transition and frequency doubling process,its thermal effect is more obvious than the simple SHG process.Experiments showed that after adjusting to the optimum temperature at room temperature,the SFD efficiency could still be improved by lowering the temperature as well.The SFD light did not reach saturation within the experimental temperature range.Therefore,effective cooling means will significantly improve the SFD light output.For the first time,millimeter-scale Sr3(BO3)2 crystal particles were obtained using c-cut Ca3(BO3)2 seed crystal.The Sr3(BO3)2 crystal exhibited high transparency during the growth process,but cracks severely after the temperature was removed.The thermal properties of Sr3(BO3)2 crystals were studied for this phenomenon,and the reasons for the severe cracking phenomenon using the pulling method were analyzed.Thermogravimetry-differential thermal analysis showed that the crystal mass did not change with temperature,and there was a significant endothermic peak around 1250 ?.The temperature point was determined not to be the crystal melting point(higher than 1470 ?),and it was preliminarily judged that there was a phase transition.In-situ temperature-variant X-ray diffraction showed that there was an abnormality in the unit cell parameters with temperature changes.Combined with the thermogravimetry-differential thermal analyses results,Sr3(BO3)2 crystals had a first-order phase transition at 1250 ?.The Raman properties of Sr3(BO3)2 crystal and Ca3(BO3)2 crystal were compared by theoretical calculation and powder Raman test.The strongest Raman shift peak of Sr3(BO3)2 crystal was similar to that of Ca3(BO3)2 crystal.The FWHM value of the frequency shift peak was smaller than that of Ca3(BO3)2 crystal,indicating that Srn(BO3)2 possessed better Raman performance.The red shift of the strongest Raman shift peak in the range of 25?300 ? is only 2.3 cm-1,indicating that the Raman property of this crystal is less affected by temperature,and it is expected to achieve stimulated Raman scattering in a wide temperature range.