镍离子激活超宽带近红外发光微晶玻璃研究进展

Research Progress on Ni^(2+)-Activated Ultra-Broadband Near-Infrared Emission Glass-Ceramics

处在六配位八面体晶体场的过渡金属镍离子(Ni^(2+))具有超宽带近红外发光特性,其荧光半高宽是传统稀土离子(如Pr^(3+)、Er^(3+))的6~8倍。Ni^(2+)激活的近红外增益材料有望应用于宽带光放大器和可调谐激光器,引发了中国内外广泛关注。Ni^(2+)掺杂晶体具有极高的发光效率,但晶体存在制备工艺复杂、机械加工困难和难以成纤等问题,限制了此类材料在光纤放大器和激光器领域的应用。玻璃具有易加工和成纤的优点,但Ni^(2+)在玻璃中缺少合适的晶体场(配位场)环境,无法产生高效近红外发光。通过对玻璃进行热处理,可以在玻璃内部原位生成出不同类型的纳米晶体,即获得纳米晶体复合的微晶玻璃,可以为Ni^(2+)提供必需的晶体场。同时,控制玻璃内部生长的晶粒尺寸,使其远小于可见光波长(如小于30nm),可有效减弱瑞利散射,使微晶玻璃具有较低的光学损耗,满足光子器件实际应用要求。讨论了Ni^(2+)激活微晶玻璃的发光机理以及Ni^(2+)激活晶体、单相微晶玻璃、双相微晶玻璃和微晶玻璃光纤在超宽带近红外发光方面的研究进展,同时展望了Ni^(2+)激活超宽带近红外发光微晶玻璃未来研究的发展方向。

Institute of High Energy Physics,Chinese Academy of Sciences;Transition metal nickel ions(Ni^(2+))situated in a six-coordinate octahedral crystal field exhibit a ultra-broad near-infrared(NIR)luminescence,with a fluorescence half width at half maximum(HWHM)6-8 times that of rare-earth(RE)element ions like Pr^(3+)and Er^(3+).Ni^(2+)-activated NIR gain materials can be used in broadband optical amplifiers and tunable lasers,which have attracted attention.Despite their impressive luminescent efficiency,Ni^(2+)-doped crystals have some limitations in optical fiber amplifier and laser applications due to their intricate fabrication processes,machining,and fiber formation.Glass offers some advantages in processing and fiber formation,but lacks a conducive crystal field(coordination field)environment for Ni^(2+)to achieve an efficient NIR luminescence.Various types of nanocrystals can be generated in-situ within the glass via subjecting glass to heat treatment,resulting in the formation of nanocrystalline composite glass-ceramics(GCs).Also,a precise control of grain size within the glass to dimensions smaller than the visible light wavelength(e.g.,less than 30 nm)effectively mitigates the Rayleigh scattering,endowing GCs with reduced optical losses that meet the practical demands of photonic devices.Johnson,et al.investigated the optical properties of Ni^(2+).They prepared MgF2 crystals doped with Ni^(2+)and observed fluorescence emission characteristics and optical laser oscillation phenomena under pulse xenon lamp or tungsten lamp excitation at low temperatures(i.e.,20,77 K,and 85 K).Ohishi et al.reported Ni^(2+)activated LiGa5O8 nanocrystalline GCs with a broadband fluorescence emission centered at 1.3μm with a half-width greater than 300 nm under 976 nm excitation.The lifetimes at 5 k and 300 k were greater than 900μs and 500μs,respectively,with internal and external quantum efficiencies of 100%and 9%,respectively.This emission was attributed to the transition of Ni^(2+)from^(3)_T(2g)(^(3)F)→^(3)A_(2g)(^(3)F)level in the LiGa_(5)O_(8) crystal octahedral coordination.Zhou et al.reported the phenomenon of NIR light amplification in Ni^(2+)dopedβ-Ga_(2)O_(3)nanocrystalline transparent GCs.Zhou et al.designed a special glass system with the composition of SiO2/Na2O/Ga_(2)O_(3)/LaF3=51%/15%/20%/14%(in mole).This glass system can orderly precipitate LaF3 and Ga_(2)O_(3)nanocrystals.For co-doping with Er^(3+)and Ni^(2+),the two active ions can enter the two different nanocrystals,respectively,the physical distance between the two active ions and the local crystal field changes effectively inhibit the energy transfer between the two different active ions,achieving the near-infrared ultra-wideband luminescence of an integrated multi-color visible and Er^(3+)/Ni^(2+)composite.Summary and prospects The existing fluorescence regulation of Ni^(2+)-doped GCs is studied,having a great potential in broadband amplifiers,tunable lasers,non-invasive sensing,infrared night vision sources,and infrared medical diagnosis.Numerous studies indicate that 1)the NIR emission band and bandwidth of Ni^(2+)can be regulated via controlling the types(oxides,fluorides)and modes(single phase or dual phase)of crystals in GCs;2)the luminescence intensity of Ni^(2+)can be further enhanced by energy transfer through sensitizers such as Nd^(3+),Yb^(3+),Cr^(3+),etc..;3)the luminescence intensity of Ni^(2+)can be also enhanced by optical field regulation,such as using noble metal nanocrystals to improve the collection efficiency of pump light.However,Ni^(2+)-activated transparent GCs still have some challenges.The doping concentration of Ni^(2+)in microcrystalline glass and microcrystalline glass fibers is relatively low,usually less than 0.2%(in mole fraction),resulting in low NIR absorption coefficients;The matrix glasses that can carry Ni^(2+)activation are still limited.Exploring multi-component glass matrices that can precipitate new types of crystal phases is one of the main tasks;Residual Ni^(2+)at the glass phase or glass-crystal phase interface still accounts for a large proportion.Some strategies are needed to ensure that most Ni^(2+)enters the target crystals,further improving its infrared optical performance;The gain coefficient of Ni^(2+)-doped microcrystalline glass/fiber is relatively low(<0.3 cm^(-1))due to the limited volume ratio of nanocrystals in the glass(i.e.,crystallization rate)and the influence of surface/interface defects of nanocrystals.Meanwhile,efficient positive feedback cannot be formed due to the lack of high-quality resonant cavities,resulting in only room-temperature luminescence of Ni^(2+),and no laser emission is achieved yet.The prerequisite for achieving laser emission is to obtain laser materials with sufficient gain.The Purcell effect can accelerate the radiation relaxation process of luminescent materials,resulting in an increased radiation probability and a correspondingly increased quantum efficiency of photoluminescence.Based on whispering gallery mode(WGM)glass microspheres,light can be confined in the micron-scale cavity for a long time based on the principle of total reflection.Therefore,it has an extremely high quality factor(i.e.,≥10^(5))and minimal mode volume(i.e.,≤10^(3)μm^(3)),which can fully utilize the Purcell effect to enhance the interaction between light and matter.With the unique advantages of the WGM glass microsphere cavity,the preparation of a new Ni^(2+)-doped microcrystalline glass microsphere laser provides an effective way to break through the physical bottleneck of room-temperature Ni^(2+)laser emission and develop low-threshold,ultra-broadband near-infrared multi-wavelength micro-lasers.