倍半氧化物激光陶瓷的研究进展

Research Progress on Sesquioxide Laser Ceramics

倍半氧化物透明陶瓷具有声子能量低、热膨胀系数低、损伤阈值高、热导率高等优势,在固体激光领域有着极大的应用前景。提出了研制高质量倍半氧化物透明陶瓷亟需解决的瓶颈问题,阐述了在倍半氧化物陶瓷制备过程中提高光学质量、减少晶格缺陷的关键措施,总结了稀土离子掺杂倍半氧化物陶瓷在1~3μm波段固体激光器应用中的代表性成果,并对倍半氧化物激光陶瓷的未来发展进行了展望。

Solid-state lasers have the advantages of realizing laser outputs with a high energy and a high peak power,having potential applications in civil and military fields(i.e.,remote sensing,environmental monitoring,medical treatment and optoelectronic countermeasure).Rare-earth ions doped sesquioxide materials as one of the most promising gain media have attracted recent attention due to their low phonon energy,low thermal expansion coefficient and high thermal conductivity.However,large-size sesquioxide single crystals with a good optical quality are difficult to grow because of the high melting points(i.e.,>2400℃)and phase transition point at 2280℃.Fortunately,the sintering of sesquioxide ceramics as an alternative way to prepare laser host materials.It is possible to obtain the materials with a large volume and a high doping concentration,showing a superiority in large-scale production,a feasibility of shape control and better mechanical properties.To achieve high-efficiency and high-power laser oscillation from sesquioxide ceramics,it is crucial to eliminate the main scattering centers inside microstructures,i.e.,residual pores and secondary phases.Raw powders with less agglomeration and high sinterability,combined with appropriate molding methods are fundamental for producing compacts with small pore size and uniform microstructures.This is favorable to avoid differentiate densification,degrading the optical homogeneity and transparency.During the sintering process,it is essential for pores to remain at grain boundaries until the final full densification.This prevents the formation of intragranular pores that are hard to remove.The effective control of grain boundary migration can be achieved to prevent pore-boundary separation through the addition of suitable sintering additives,regulation of sintering atmospheres(i.e.,vacuum,hydrogen and oxygen),and the use of advanced sintering techniques(i.e.,microwave sintering,spark plasma sintering or two-step sintering,etc.).In addition,pressure assisted sintering,including hot pressing and hot isostatic pressing can also improve the densification rate and facilitate the effective removal of residual pores.This even enables the full densification of sesquioxide ceramics without using sintering additives.Zirconia is once widely used as a sintering aid for sesquioxide transparent ceramics.However,it can severely degrade the laser performance of sesquioxide ceramics due to the occurrence of photodarkening phenomenon under high-power pump beam excitation.This may be attributed to the charge imbalance between Zr^(4+)and host cation ions.Consequently,the introduced point defects act as acceptors for the electrons of excited laser ions,forming Zr^(3+)color centers that cause a broad absorption in a wavelength range of 400–700 nm.This seriously affects the laser oscillation efficiency.Therefore,in addition to achieving laser-grade quality compared to single crystals,minimizing the use of sintering additives or regulating the lattice defects are also considered in the fabrication process of sesquioxide ceramics.In general,chemical co-precipitation process for synthesizing well-dispersed powders,combined with vacuum sintering and hot isostatic pressing sintering,is considered as an effective way for fabricating high-quality sesquioxide transparent ceramics.This approach provides a great driving force for densification through well controlling grain boundary diffusion and migration.However,it is necessary to determine suitable sintering curves and microstructure morphology before hot isostatic pressing.Summary and prospects:At present,solid-state lasers based on sesquioxide ceramics primarily emphasize the near-infrared wavelength range.These lasers can be categorized into applications near 1μm(doped with Yb^(3+)and Nd^(3+)),2μm(doped with Tm^(3+)and Ho^(3+)),as well as 1.6μm and 3μm(doped with Er^(3+)),with significant achievements in both high power and ultra-short pulse laser outputs.It is noteworthy that Konoshima Chemical Co.and World-Lab Co.currently dominate the market with their high-quality sesquioxide laser ceramics.it is imperative to elucidate the effect of lattice defects on the laser performance in the future development of sesquioxide laser ceramics.Breakthroughs in fabrication technologies for large-size ceramics with a high optical homogeneity are crucial for laser engineering applications.Complex structure design is also essential to optimize thermal management in high-power laser systems.In addition,there should be also a focus on synthesizing single crystals through sintering methods,indicating a potential for obtaining new laser materials with heavily active ions doping and composite structures.A broad range of laser applications from sesquioxide transparent ceramics can be anticipated through the continuous improvement of powder synthesis,molding methods and sintering techniques.