Growth Of ZnWO₄ Crystal And Research On Stimulated Raman Scattering Properties

The third-order nonlinear optical effect of crystal material—Stimulated Raman scattering(SRS)has the function of laser frequency conversion,can broaden the existing laser band,and obtain lasers with wavelength ranges covering the visible,near-infrared,and even mid-infrared bands.At the same time,the stimulated Raman scattering light source has the same high monochromaticity,high directivity and high coherence as the laser light,and can play an important role in medical,military,energy,and even human daily life.Therefore,the study of new Raman crystals and the use of existing lasers to obtain new wavelength laser output has become a major hotspot in today's materials field.Stimulated Raman scattering is a non-elastic process that generates large amounts of heat and various thermal effects,which affect the crystal damage threshold and laser device design.Therefore,the required stimulated Raman materials require high thermal conductivity to achieve efficient heat transfer.Zinc tungstate(ZnWO4)crystal has larger unit cell parameters.According to the analysis of Einstein-Debye model,it may have high thermal conductivity.At the same time,tungstate radicals can produce effective Raman gains and have large gain coefficients.The crystal has a suitable melting point,which can be grown by Czochralski(Cz)to obtain large-size,high-optical quality crystal,the advantages of being resistant to deliquescence,stable quality at room temperature,and a wide transmission band.So the ZnWO4 crystal may be a class of excellent stimulated Raman crystals.However,the basic physical properties of ZnWO4 crystal have not been studied systematically so far,especially the performance of SRS is still blank.In this dissertation,the requirements of stimulated Raman scattering technique for wide transmission spectrum,large gain coefficient,and high thermal conductivity Raman crystals were studied.A new type of stimulated Raman scattering material,zinc tungstate crystal,was grown and a ZnWO4 crystal was grown.Its comprehensive properties,including structure,thermal,optical,and Raman spectroscopy,demonstrate that this type of crystal has a high thermal conductivity,a large Raman gain coefficient,a broad transmission spectrum,and is an excellent stimulated Raman scattering crystal.The properties of stimulated Raman scattering have been studied,and out-of-chamber yellow Raman scattering and 2.25 ?m stimulated Raman laser output have been achieved.The related research provides a class of stimulated Raman scattering crystals with excellent overall performance,which may have important applications in stimulated Raman scattering,especially mid-infrared.The main work is as follows:1.Growth of ZnWO4 crystal(1)ZnWO4 crystals were grown successfully by the Czochralski method with large size and high optical properties;the raw materials used were ZnO and WO3 with a purity of 4N(99.99%).(2)Optimize the crystal growth process,master the high-quality ZnWO4 crystal growth technology.Optimize the process parameters such as seed crystal,temperature field,growth atmosphere,growth rate,annealing during crystal growth,and successfully achieve the growth of high optical quality ZnWO4 crystal,the largest crystal dimensions are ? 35 mm × 70 mm.2.Basic physical properties of ZnWO4 crystal(1)The structure of the ZnWO4 crystal obtained from the growth was characterized and the X-ray powder diffraction(XRD)data was compared with a standard diffraction card.It can be confirmed that the grown ZnWO4 crystal has a monoclinic structure,a C2/n point group,and a P2/c space group.The calculated unit cell parameters were a=4.6920 A,b=5.7186 A,c=4.9284 A,and ?=90.642°.(2)The density of ZnWO4 crystal at room temperature was measured.Experimentally,the crystal density measured by the buoyancy method was 7.862 g/cm-3;theoretically,the crystal density was calculated to be 7.892 g/cm-3.(3)The thermal properties of ZnWO4 crystal were characterized systematically,including specific heat,thermal expansion,thermal diffusivity,and thermal conductivity,and calculates the relationship of density with temperature.Among them,the thermal conductivity along the c-axis has reached 5.412 Wm-1K-1,this data is twice that of BaWO4 and SrWO4,which is favorable for accelerating heat transfer during stimulated Raman scattering.It is verified that this type of crystal has a high thermal conductivity and a large thermal focusing length,which is favorable for the heat conduction and stimulated Raman scattering device design during stimulated Raman scattering.3.The optical properties of ZnWO4 crystal(1)The absorption spectra and infrared transmission spectra of ZnWO4 crystal at room temperature were studied.The crystal was found to be in the range of 432 nm to 6.17 ?m with a transmission rate of up to 80%,and the infrared cut-off edge is 9.8?m,covering the visible,near-infrared,and mid-infrared wavelengths.It is suitable for wide-band stimulated Raman scattering from visible to mid-infrared.(2)Spontaneous Raman spectra of ZnWO4 crystal at room temperature were studied,including normal state and Raman spectra with different geometric configurations.The Raman spectra under different vibration modes are analyzed to provide a theoretical reference for the study of subsequent stimulated Raman scattering properties.4.ZnWO4 crystal stimulated Raman scattering performance characterization(1)The external cavity yellow stimulated Raman scattering performance of 532 nm fundamental frequency light was studied.In this experiment,the 558.9 nm stimulated Raman scattering laser can be obtained with a fundamental frequency of 532 nm.When the pump light energy reaches 0.28 mJ,the energy of the stimulated Raman scattering light at 558.9 nm is 0.07 mJ,and the conversion efficiency can reach 25%.(2)The stimulated Raman scattering performance of the crystal under LD pumping was studied.The stimulated Raman scattering effect of ZnWO4 crystal was used to obtain 2.25 ?m stimulated Raman scattering.When the pump frequency was 2.5 kHz and the input power is 30 W,the Raman laser output power is 187 mW and the pulse width is 5.7 ns.The research shows the prospect of this kind of crystal in the wide-band stimulated Raman scattering.Through the above research,this paper confirms that ZnWO4 crystal has excellent stimulated Raman scattering effect,is an excellent stimulated Raman scattering crystal,and predicts its potential application in the mid-infrared waveband.