Fabrication,Upconversion And Mid-Infrared Luminescence Properties Of Er³⁺ Doped Fluoride Transparent Ceramics

Rare-earth doped transparent ceramics are a kind of optical materials with excellent physical and chemical properties which have attracted extensive attention of researchers in recent years.Among the transparent ceramic materials,alkaline earth fluoride transparent ceramics provide a broad prospect for the development of a new generation of photo-functional transparent ceramic materials because of its low phonon energy,wide light transmission range and low refractive index.The rare-earth ion Er³⁺,which has abundant energy levels and spectral diversity,can emit up-conversion luminescence and 2.7μm mid-infrared luminescence under 980 nm excitation.The potential applications of rare earth ion Er³⁺doped fluoride transparent ceramics in three-dimensional display,bioluminescent labeling,temperature sensor,solid state laser,optical communication,laser guidance attract great interest of researchers.However,the low luminous efficiency and self-termination of the up-conversion and mid-infrared luminescence of Er³⁺makes it difficult for practical application.In this paper,a series of Er³⁺:Ca F₂and Er³⁺:Sr F₂transparent ceramics with different doping concentrations were prepared by co-precipitation method and hot-pressing sintering technology.The microstructure,up-conversion and mid-infrared luminescence properties were studied.Moreover,based on the low efficiency of Er³⁺up-conversion luminescence,two-wavelength excitation strategy and co-doping Dy³⁺as sensitizer were adopted to improve the up-conversion luminescence performance.Based on the self-termination effect of 2.7μm mid-infrared luminescence of Er³⁺,the lifetime difference between the upper and lower energy levels of the ⁴I_11/2→⁴I_13/2transition of Er³⁺was shortened by co-doping the deactivated ion Ho³⁺to suppress the self-termination effect.The research contents and conclusions of this paper are as follows:（1）The Er³⁺:Ca F₂nanoparticles with the Er³⁺doping concentration of 1-5 at.%was synthesized by co-precipitation method,and the corresponding Er³⁺:Ca F₂transparent ceramics were prepared by hot pressing sintering method.All samples showed good transmittance,and the 2 at.%Er³⁺:Ca F₂transparent ceramic exhibited highest transmittance,where the transmittance at 400 nm is 75%and the maximum transmittance is 93%.Under the excitation of 980 nm,the upconversion emission intensity of green and red light of Er³⁺increased with the increase of doping concentration,and the emission intensity ratio of red light to green light also increased.Under the excitation of 808 nm,the green and red upconversion emission intensities of Er³⁺increased with the increase of doping concentration,but the ratio of red to green emission intensities increased first and then decreased with the increase of doping concentration.Under 980+808 nm two-wavelength excitation,the upconversion emission intensity of 5 at.%Er³⁺:Ca F₂transparent ceramic was more than two times of that of single wavelength excitation.The upconversion emission intensity was obviously improved.（2）Using the same method,Er³⁺,Dy³⁺:Ca F₂transparent ceramics with different Er³⁺doping concentration were fabricated.The obtained ceramic samples all presented pure Ca F₂phase.With increasing of Er³⁺doping concentration from 1 at.%to 5 at.%,the cell volume of the sample increased gradually from 162.956（?）³to164.653（?）³.Under the excitation of 900 nm,the green and red up-conversion emission bands of Er³⁺appeared at 540 and 660 nm,and the emission peaks of 505nm,which were not observed under the excitation of 980 nm,also appeared.With the increase of Er³⁺doping concentration,the upconversion emission intensity at 480 nm and 576 nm of Dy³⁺decreased gradually,while the upconversion emission intensity at505 nm and 540 nm of Er³⁺increases gradually.In addition,the fluorescence lifetime at 576 nm of Dy³⁺decreased,while the fluorescence lifetime at 505 nm of Er³⁺increased slightly.All of these were due to the energy transfer process from Dy³⁺to Er³⁺.（3）Er³⁺:Sr F₂nanoparticles with doping concentration of 1-10 at.%was synthesized by co-precipitation method.The effect of sintering temperature on the transmittance and microstructure of Er³⁺:Sr F₂transparent ceramics prepared by hot pressing sintering was studied.900 ^oC is the appropriate sintering temperature for Er³⁺:Sr F₂transparent ceramics.The up-conversion and mid-infrared luminescence properties of Er³⁺:Sr F₂and Er³⁺:Ca F₂transparent ceramics were studied.Under 980nm excitation,with the increase of Er³⁺doping concentration,the green and red up-conversion luminescence intensity of Er³⁺:Sr F₂and Er³⁺:Ca F₂transparent ceramics both gradually increased,the ratio of red and green emission intensity of Er³⁺:Ca F₂transparent ceramics gradually increased,but the ratio of red and green emission intensity of Er³⁺:Sr F₂transparent ceramics decreased gradually.The fluorescence lifetime of green and red up-conversion luminescence in Er³⁺doped Sr F₂transparent ceramics was longer than that of Ca F₂transparent ceramics.In addition,the mid-infrared luminescence intensity of Er³⁺:Sr F₂ceramics increased,while the mid-infrared luminescence intensity of Er³⁺:Ca F₂transparent ceramics decreased slightly when the doping concentration of Er³⁺exceeded 8 at.%,indicating that the quenching concentration of 2.7μm of Er³⁺in Sr F₂transparent ceramics was higher than that of Ca F₂transparent ceramics.When the doping concentration increased from 2 at.%to 10 at.%,the lifetime difference between the upper and lower levels of mid-infrared luminescence of Sr F₂ceramics decreased from 9.91 ms to 3.36 ms,which was much smaller than that of Ca F₂transparent ceramics.（4）Based on Er³⁺:Sr F₂transparent ceramics,Er³⁺,Ho³⁺:Sr F₂transparent ceramics with different Ho³⁺doping concentration were fabricated,and the effect of Ho³⁺on the 2.7μm self-termination effect of Er³⁺was studied.With the increase of the doping concentration of Ho³⁺,the mid-infrared emission intensity of the samples at 2.7μm decreased gradually,and the upper and lower energy levels of the 2.7μm luminescence transition also decreased gradually.This was because there are two energy transfer processes between Er³⁺and Ho³⁺,i.e.,⁴I_11/2(Er³⁺)→⁵I₆(Ho³⁺)and ET2:⁴I_13/2(Er³⁺)→⁵I₇(Ho³⁺).The energy transfer efficiency of ET1 process was always bigger than that of ET2,indicating that the deactivated effect of Ho³⁺on the2.7μm lower energy level of Er³⁺was stronger than that on the upper energy level.After introducing Ho³⁺,the lifetime difference of the upper and lower energy levels corresponding to the 2.7μm emission of Er³⁺decreased from 8.50 ms to 1.12 ms,indicating that the introduction of Ho³⁺can suppress the self-termination effect of 2.7μm emission,but also reduced its emission intensity.Therefore,appropriate Ho³⁺doping concentration should be selected to reduce the difference between upper and lower energy level lifetime and maintain a high upper energy level lifetime.