Research On Growth And Properties Of Rare-earth Doped BaY₂F₈ Laser Crystals

Rare earth-doped fluoride crystals are widely used in communications,remote sensing,ranging and other fields due to their low phonon energy and excellent luminescence properties.It has been one of the research hotspots of laser crystal materials since it was discovered.The typical band emission obtained has a wide range of applications by these crystal materials,such as 2.0?m,2.9?m.At present,the traditional crystal growth methods mainly include the pulling method(Cz),the Bridgman-Stockbarger method(B-S)and the temperature gradient method(TGT).For crystals with low structural symmetry and excellent performance,it is significant to obtain rare earth-doped fluoride crystals with large size and multi-band emission.In this dissertation,based on the rich energy level structure of Ho³⁺ions and the potential for multi-wavelength emission in the range of infrared band,and other sensitizers or desensitizers are doped to adjust the emission wavelength and optical properties of the crystal.On the one hand,the process parameters,crystal structure and crystal density are studied.On the other hand,the change of phase structure and spectral parameters of different doping schemes are systematically investigated to predict the laser performance of crystals.The specific work can be listed as follows:High-purity Re,Ho:BaY₂F₈ crystal growth material was successfully prepared by co-precipitation method and atmosphere sintering furnace.The suitable sintering temperature is650?and the sintering time is 2.5 h.Combined with interface theory,phase change driving force theory and solid-liquid interface shape theory,the temperature gradient parameters and crystal growth parameters of the drop furnace temperature field are optimized.A single crystal of BaY₂F₈ is obtained when the radon drop speed is 0.2-0.5 mm/h.The self-designed Bridgman furnace was used to successfully cultivate BaY₂F₈ seed crystals and prepare different series of Re,Ho:BaY₂F₈ crystals.Based on the advantages of the 889 nm pump source matching with the ⁵I5 energy level of the Ho³⁺ion,the upper energy level of radiation transition on the 3.9?m(⁵I₅?⁵I₆)was directly pumped,and the effective emission output of 2.0?m and 3.9?m was obtained respectively.This scheme effectively avoids the complexity of the optical parametric laser system and reduces the energy loss of the pump source.And the emission cross section at 3.9?m was calculated to be 7.759×10^-20 cm².And through a self-designed laser test system,an effective laser output of 3.9?m was obtained.The output energy of 3.9?m laser is about 5.6m J with an optical-to-optical conversion efficiency of 5.8%.Although the Ho:BaY₂F₈ crystal obtains emission output near 2.0?m,the pump energy is preferentially used for 3.9?m emission,resulting in the low spectral parameters of the Ho:BaY₂F₈ crystal in the 2.0?m band.In response to this,the sensitizer Yb³⁺ions are doped in the crystal to increase the absorption of the pump source energy,which is used to improve the spectral performance of Ho³⁺ions in the 2.0?m band.According to the results of the absorption spectrum,the 980 nm laser source is used to pump the Ho,Yb:BaY₂F₈ crystal,and the radiation characteristics and fluorescence decay curves of Ho,Yb:BaY₂F₈ crystals at1.2?m and 2.0?m are studied in detail.Through the sensitivity of Yb³⁺ions,the upper energy level(⁵I₇)corresponding to 2.0?m of Ho³⁺ion is significantly enhanced.The spectral parameters of Ho,Yb:BaY₂F₈ crystals are calculated in detail by combining the spectrum and energy level lifetime data,such as absorption and emission cross section,gain cross section,which confirm the excellent optical properties of the crystal in the 2.0?m band.Both Ho:BaY₂F₈ and Ho,Yb:BaY₂F₈ crystals achieve emission output by enhancing the upper energy level of the radiation transition,but it is difficult to find an effective pump source or sensitizer to achieve Ho³⁺ion emission in the 2.9?m(⁵I₆?⁵I₇)band.Based on the characteristics and advantages of the four-level structure in the laser system,the aim is to achieve ⁵I6?⁵I7 radiation transition of the number of particles in reverse by doping the desensitizing Pr³⁺ions to weaken the Ho³⁺:⁵I₇ energy level,so as to obtain a 2.9?m band emission.With the modified Bridgeman method,1%Ho,%Pr:BaY₂F₈(=0,0.2,0.5,0.8,1.2)crystals were successfully prepared.The emission spectra showed that the doping of Pr³⁺ions weakened the intensity of the emission peak at 2.0?m and enhanced the intensity of the emission peak at 2.9?m of the crystals.As the doping concentration of Pr³⁺ions increased,the lifetime of the ⁵I7 level of Ho³⁺ions decreased from 2.03 ms to 0.23 ms,thus promoting the number inversion of Ho³⁺ions at 2.9?m.By calculating the gain cross section,the particle inversion percentage of 2.9?m is 36.8%.As for Ho,Pr:BaY₂F₈ crystal,the energy transfer efficiency from Ho:⁵I7 level to Pr:³F2 level was calculated to be 88.7%.This concept can be extended to more radiation transitions that are difficult to achieve population inversion,in order to study the laser output of different rare earth ions in more abundant wavelength bands.In order to develop the emission potential of Ho³⁺ions in the 2.0?m band,Nd³⁺ions were doped to further optimize the spectral parameters of the 2.0?m band.This scheme avoids pump energy from being used for up-conversion emission,and improves the efficiency of infrared band emission.With the traditional vertical Bridgman method,1%Ho,x%Nd:BaY₂F₈ crystals(=0,1,1.5,2,2.5,3)were grown and investigated.The monoclinic phase of Ho,Nd:BaY₂F₈ crystals was determined as C2/m space group with an X-ray diffractometer.In the fluorescence spectra,near-infrared radiation at 1.3?m and 2.0?m was obtained,which shows that Nd³⁺ion is an effective sensitizer,and the energy transfer efficiency is up to 73.7%.The spectral parameters of the crystal are calculated by combining2.0?m radiation spectrum and 2.0?m fluorescence lifetime of Ho,Nd:BaY₂F₈ crystals.Among them,the strongest emission cross section is 11.52×10^-20 cm²,this is an order of magnitude higher than the emission cross-section value obtained at 2.0?m with the Yb³⁺ion as the sensitizer,which is of great significance for obtaining an efficient 2.0?m laser output.