低强度飞秒激光激发下CdTe和CdTe/CdS核壳量子点的荧光上转换研究

利用400 nm和800 nm不同波长的低强度飞秒激光,对CdTe和CdTe/CdS核壳量子点溶胶进行激发,研究其稳态和时间分辨荧光性质.800 nm飞秒激光激发下,CdTe和CdTe/CdS核壳量子点产生上转换发光现象,上转换荧光峰与400 nm激发下的荧光峰相比蓝移最多达15 nm,而且蓝移值与荧光量子产率有关.变功率激发确认激发光功率与上转换荧光强度间满足二次方关系,时间分辨荧光的研究表明荧光动力学曲线服从双e指数衰减.提出表面态辅助的双光子吸收模型是低激发强度上转换发光的主要机理.CdTe和CdT 关键词： CdTe量子点 CdTe/CdS核壳量子点 时间分辨荧光 上转换荧光     

980nm激光激发下Er3+和Yb3+共掺杂氟氧玻璃的上转换发光

制备了一种新型的上转换发光材料,它不仅具有较高的上转换发光效率,而且还避免了氟化物基质的缺点。其组分为58.52%PbF₂-34.43% GeO₂-3% Al₂O₃-0.05% Er₂O₃-4%Yb₂O₃,其中GeO₂为玻璃形成体氧化物,PbF₂和Al₂O₃为调整剂,以共掺杂Er³⁺和Yb³⁺离子为上转换研究的对象。测量了该玻璃系统在980nm半导体激光器激发下的上转换发光光谱,观察到很强的658nm的红光和548,526nm的绿光,而且红光的发射强度远远强于绿光。通过对上转换发光强度与激发强度关系曲线的拟合,表明此材料的绿光发射和红光发射都为双光子过程。研究了激发光的工作电流与上转换荧光强度的关系,讨论了其上转换发光特性。     

Yb^3＋/Er^3＋掺杂氟氧化物微晶玻璃的制备与发光性能

采用高温熔融法制备了Yb3＋/Er3＋掺杂的氟氧化物发光微晶玻璃,确定了最佳熔化温度（1 100℃）和退火温度（440℃,480℃）。测定得到基质玻璃的透过率为85%,掺入稀土后,透过率有所下降,并出现了稀土离子的特征吸收峰。980 nm半导体激光器（LD）激发下样品的上转换发射光谱存在4个明显的发射峰,分别为410,532,546和656 nm,对应于2H9/2→4I15/2,2H11/2→4I15/2,4S3/2→4I15/2和4F9/2→4I15/2跃迁。研究了不同Yb3＋/Er3＋（摩尔分数）和Er3＋浓度对上转换发光强度的影响,当Yb3＋∶Er3＋=4∶1、Er3＋摩尔分数为1.5%时,上转换发光强度达到最高。根据发光强度与泵浦功率之间的关系,确定了上转换发射均为双光子过程。讨论了Yb3＋,Er3＋离子间的能量传递,建立了上转换发光机制。     

Up-conversion luminescence polarization control in Er~(3+)-doped NaYF_4 nanocrystals

We propose a femtosecond laser polarization modulation scheme to control the up-conversion(UC) luminescence in Er~(3+)-doped NaYF_4 nanocrystals dispersed in the silicate glass. We show that the UC luminescence can be suppressed when the laser polarization is changed from linear through elliptical to circular, and the higher repetition rate will yield the lower control efficiency. We theoretically analyze the physical control mechanism of the UC luminescence polarization modulation by considering on- and near-resonant two-photon absorption, energy transfer up-conversion, and excited state absorption, and show that the polarization control mainly comes from the contribution of near-resonant two-photon absorption. Furthermore, we propose a method to improve the polarization control efficiency of UC luminescence in rare-earth ions by applying a two-color femtosecond laser field.     

Green and red up-conversion emissions and thermometric application of Er3+-doped silicate glass

The green and red up-conversion emissions centred at about 534, 549 and 663\,nm of wavelength, corresponding respectively to the ${^{2}}H_{11 / 2} \to {^{4}}I_{15 / 2}$, ${^{4}}S_{3 / 2} \to {^{4}}I_{15 / 2}$ and ${^{4}}F_{9 / 2} \to {^{4}}I_{15 / 2}$ transitions of Er$^{3 + }$ ions, have been observed for the Er$^{3 + }$-doped silicate glass excited by a 978\,nm semiconductor laser beam. Excitation power dependent behaviour of the up-conversion emission intensity indicates that a two-photon absorption up-conversion process is responsible for the green and red up-conversion emissions. The temperature dependence of the green up-conversion emissions is also studied in a temperature range of 296--673\,K, which shows that Er$^{3 + }$-doped silicate glass can be used as a sensor in high-temperature measurement.     

Simulation of Intermediate State Absorption Enhancement in Rare-Earth Ions by Polarization Modulated Femtosecond Laser Field

We extend the third perturbation theory to study the polarization control behavior of the intermediate state absorption in Nd~(3+)ions. The results show that coherent interference can occur between the single-photon and three-photon excitation pathways, and depends on the central frequency of the femtosecond laser field. Moreover,single-photon and three-photon absorptions have different polarization control efficiencies, and the relative weight of three-photon absorption in the whole excitation processes can increase with increasing the laser intensity.Therefore, the enhancement or suppression of the intermediate state absorption can be realized and manipulated by properly designing the intensity and central frequency of the polarization modulated femtosecond laser field.This research can not only enrich theoretical research methods for the up-conversion luminescence manipulation of rare-earth ions, but also can provide a clear physical picture for understanding and controlling multi-photon absorption in a multiple energy level system.     

Green and red up-conversion emissions and thermometric application of Er^3＋-doped silicate glass

The green and red up-conversion emissions centred at about 534, 549 and 663 nm of wavelength, corresponding respectively to the ^2H11/2 → ^4I15/2, ^4S3/2 → ^4I15/2 and ^4F9/2 → ^4I15/2 transitions of Er^3＋ ions, have been observed for the Er^3＋-doped silicate glass excited by a 978 nm semiconductor laser beam. Excitation power dependent behaviour of the up-conversion emission intensity indicates that a two-photon absorption up-conversion process is responsible for the green and red up-conversion emissions. The temperature dependence of the green up-conversion emissions is also studied in a temperature range of 296-673 K, which shows that Er^3＋-doped silicate glass can be used as a sensor in high-temperature measurement.     

Er3+-Yb3+-Tm3+共掺CdF2:PbF2基玻璃中的荧光特性及其上转换机制

在 98 0nm半导体激光激发下 ,在Er3 Yb3 Tm3 共掺玻璃样品中得到了如下的 5条较强的上转换荧光带 ,分别是近红外 (80 0nm) ,红 (6 4 5nm) ,绿 (5 4 5和 5 2 5nm) ,蓝 (4 80nm)及紫 (4 0 7nm)。与Er3 Yb3 共掺样品相比 ,Tm3 的加入使得 4 80nm的蓝光显著增强 ,这应与Tm3 特殊的能级结构有关 ;荧光强度随激发功率变化的双对数曲线表明 4 80nm蓝光发射是双光子激发过程 ,为两个Yb3 的合作上转换敏化发光 ,随着激发功率的增加 ,4 80nm荧光的logI logP曲线的斜率将变小 ,逐渐向下“弯曲”。作者详细的分析了各条荧光带的上转换机制 ;并用速率方程讨论了稳态情况下 4 80nm蓝色上转换荧光强度随激发功率变化的关系 ,其结果与实验一致。     

Up-conversion luminescence research of Er（0.5）：ZBLAN material for volumetric display application when excited by 1520nm laser

The up-conversion luminescence of the ZBLAN fluoride glass Er(0.5):ZBLAN, when excited by a 1520 nm semiconductor laser, is studied in this paper. The absorption and common-fluorescence spectra are also measured in order to understand the up-conversion clearly. It is found that there are seven strong up-conversion luminescence lines (406.97^m, 410.42 nm), (521.97^m, 527.56 nm), (542.38^m, 549.27 nm), (654.27^m, 665.70 nm), 801.57^m nm, 819.46 nm, and 840.00 nm, which can be recognized as the fluorescence transitions of ({}^2G^4F^2H)_{9/2}→{}^4I_{15/2},{}^2H_{11/2}→ {}^4I_{15/2},{}^4S_{3/2}→{}^4I_{15/2},{}^4F_{9/2}→{}^4I_{15/2}, {}^4I_{9/2}→ {}^4I_{15/2}, ({}^2G^4F^2H)_{9/2}→{}^4I_{9/2}, and {}^4S_{3/2}→{}^4I_{13/2} respectively. Meanwhile, the small up-conversion fluorescence lines 379.20 nm, 453.10 nm and 490.60 nm are the transitions of {}^4G_{11/2}→{}^4I_{15/2},{}^4F_{5/2}→{}^4I_{15/2} and {}^4F_{7/2}→ {}^4I_{15/2} respectively. It is interesting that the slopes of log F－logP curves, the double-logarithmic variation of up-conversion luminescence intensity F with laser power P, are different from each other for these observed up-conversion luminescence, this being valuable for the volumetric display. Comprehensive discussions find that the {}^4G_{11/2}→{}^4I_{15/2}, (^2G^4F^2H)_{9/2}→{}^4I_{15/2}, (^2H_{11/2}→{}^4I_{15/2},{}^4S_{3/2}→{}^4I_{15/2},{}^4F_{9/2}→ {}^4I_{15/2}), and {}^4I_{9/2}→{}^4I_{15/2} up-conversion luminescences are five-photon, four-photon, three-photon, and two-photon up-conversion luminescences respectively. It is found also that the absorption from ground-state {}^4I_{15/2} level to {}^4I_{13/2} level is very large, which is beneficial to the sequential energy transfer up-conversion to occur.     

Simulating resonance-mediated two-photon absorption enhancement in rare-earth ions by a rectangle phase modulation

Improving the up-conversion luminescence efficiency of rare-earth ions via the multi-photon absorption process is crucial in several related application areas. In this work, we theoretically propose a feasible scheme to enhance the resonance-mediated two-photon absorption in Er~(3+) ions by shaping the femtosecond laser field with a rectangle phase modulation. Our theoretical results show that the resonance-mediated two-photon absorption can be decomposed into the on-resonant and near-resonant parts, and the on-resonant part mainly comes from the contribution of laser central frequency components, while the near-resonant part mainly results from the excitation of low and high laser frequency components.So, the rectangle phase modulation can induce a constructive interference between the two parts by properly designing the modulation depth and width, and finally realizes the resonance-mediated two-photon absorption enhancement. Moreover, our results also show that the enhancement efficiency of resonance-mediated two-photon absorption depends on the laser pulse width(or laser spectral bandwidth), final state transition frequency, and intermediate and final state absorption bandwidths. The enhancement efficiency modulation can be attributed to the relative weight manipulation of on-resonant and near-resonant two-photon absorption in the whole excitation process. This study presents a clear physical insight for the quantum control of resonance-mediated two-photon absorption in the rare-earth ions, and there will be an important significance for improving the up-conversion luminescence efficiency of rare-earthions.     

Up-conversion emission in triply-doped Ho/Yb/Tm KGd(WO4)2 single crystals

Detailed spectroscopic studies of the triply doped KGd(WO₄)₂:Ho³⁺/Yb³⁺/Tm³⁺ single crystals (which exhibit multicolor up-conversion fluorescence) are reported for the first time. The absorption spectra of crystals were measured at 10 and 300 K; the room temperature luminescence spectra were excited at 980 nm wavelength. The dependence of the intensity of luminescence on the excitation power for three different concentration of Ho³⁺, Yb³⁺ and Tm³⁺ ions was investigated. Efficient green and red up-converted luminescence of Ho³⁺ ions and weak blue up-conversion luminescence of Tm³⁺ ions were observed in spectra. The red emission of Ho³⁺ ions is more intensive than their green emission. Dependence of the up-conversion luminescence intensity on the excitation power and impurities concentration was also studied; the number of phonon needed for efficient up-conversion was determined for each case. All possible energy transfer processes between different pairs of the impurity ions' energy levels are also discussed.     

Green and red Anti-Stokes emission of U3+: LaCl3produced by infrared laser

Anti-Stokes green and red emission from U3+: LaCl3 can be produced by infrared laser excitation at 975.3 nm, 977.7 nm and 979.4 nm at 8K. The upconversion luminescence intensity dependence upon the excitation laser power was measured and analyzed. The results show that depending on the excitation wavelength, the mechanisms responsible for the upconversion process are two-photon absorption and excited-state absorption.     

Two-photon-excited luminescence in a Tb3+ -doped lithium niobate crystal pumped by a near-infrared femtosecond laser

Blue (487.6 nm), green (544.1 nm), yellow (582.1 nm), and red (623.6 nm) upconversion (UC) luminescences are achieved in a Tb(3+)-doped lithium niobate crystal when an 800 nm femtosecond laser is loosely focused onto the sample at room temperature. The relationship between UC luminescence intensity and the pump energy indicates that a two-photon excitation process is dominant in this UC luminescence phenomenon. The Tb(3+) sensitive temperature dependence of the luminescence intensity is demonstrated via an obvious reduction of luminescence intensity with durative laser irradiation.     

Measurement of infrared level lifetime by upconversion luminescence

Based on repetition frequency-dependent excited state absorption (ESA) upconversion (UC) luminescence, a method to measure the lifetime of an IR intermediate level is proposed so long as ESA UC luminescence can occur in the rare earth ions. The feasibility of this idea is demonstrated via a theoretical simulation. A Er(3+):LiNbO? crystal ESA UC luminescence under femtosecond laser excitation is used to illustrate this measurement method, and the obtained 1.5 μm lifetime of 2.31 ms is shorter than previous reported values. This method can obviate the influence of radiation trapping effect on lifetime measurement, which is crucial in the traditional pulse sampling technique.     

Simulating the resonance-mediated (1+2)-three-photon absorption enhancement in Pr3+ ions by a rectangle phase modulation

Enhancing the upconversion luminescence of rare earth ions is crucial for their applications in the laser sources, fiber optic communications, color displays, biolabeling, and biomedical sensors. In this paper, we theoretically study the resonance-mediated (1+2)-three-photon absorption in Pr³⁺ ions by a rectangle phase modulation. The results show that the resonance-mediated (1+2)-three-photon absorption can be effectively enhanced by properly designing the depth and width of the rectangle phase modulation, which can be attributed to the constructive interference between on-resonant and near-resonant three-photon excitation pathways. Further, the enhancement efficiency of resonance-mediated (1+2)-three-photon absorption can be affected by the pulse width (or spectral bandwidth) of femtosecond laser field, final state transition frequency, and absorption bandwidths. This research can provide a clear physical picture for understanding and controlling the multi-photon absorption in rare-earth ions, and also can provide theoretical guidance for improving the up-conversion luminescence.     

Cascade-avalanche up-conversion in Tm3+:YLF crystals

New efficient mechanisms of long-wavelength excitation of short-wavelength luminescence in a system of rare-earth impurity ions are proposed. The model studied involves a multilevel system of electronic states of a rare-earth ion interacting with the long-wavelength emission that is in resonance with one or several transitions between the excited levels. The concentration of impurity ions is assumed to be sufficiently high, so that the interionic cross-relaxation and the up-conversion play a significant role. Moreover, the model includes processes similar to photon avalanche, in which the photoinduced excitation of a single ion is converted into lower-level excitations of several ions, each of them reentering the process after photon absorption. Numerical solutions of the system of balance equations for a multilevel system are obtained with calculated and semiempirical parameters of self-quenching and up-conversion. It is shown that, in a system of Tm³⁺ impurity ions, short-wavelength luminescence at λ≥0.29 μm can be efficiently excited by radiation at λ?1.11 or 0.649 μm with a moderate intensity.     

显示用上转换绿色发光材料NaYF4∶Er,Yb及其特性

采用喷射微波燃烧合成法制备了上转换发光显示器中发绿光的上转换发光材料NaYF4∶Er,Yb.测试了该材料的XRD衍射图谱和发光效率.给出了该材料在1 064 nm三种激光功率激发下的发光光谱.分析了该材料的上转换发光机理,得到545 nm和662 nm峰值发光分别是Er3+的4S3/2→4I15/2和4F9/2→4I15/2跃迁产生的.NaYF4∶Er,Yb具有较强的上转换绿光,同时存在的较弱的红光易于用滤色膜滤除,满足显示对三基色中绿色的要求;并且喷射微波燃烧合成法制备的该材料达到了高分辨率显示应用超细粉体的要求.     

Yb浓度对功率依赖的上转换荧光色彩的敏感度调控

控制激发光功率密度是一种调控红绿荧光比率的简单方法.然而,大多数上转换系统对功率的调控并不敏感.本文通过柠檬酸钠辅助的水热法,合成了一系列具有不同Yb浓度掺杂的NaYF₄：Yb/Ho微米棒.通过激光共聚焦显微镜系统,研究了Yb浓度和激发功率密度依赖的NaYF₄：Yb/Ho微米棒的上转换荧光特性.发射谱和同步荧光成像图案表明：荧光红绿比率不仅敏感于激发功率,而且敏感度依赖于Yb浓度.随着Yb浓度的增加,功率调控的红绿比率的敏感度增加,这暗示了功率调控的红绿比率的敏感度可以作为一种度量和评估Yb掺杂浓度的有效途径和方法.通过上/下转换发射谱、激发谱和功率依赖关系,揭示了功率调控红绿比率的机理,并提出了荧光色彩敏感于功率调控的上转换系统具有的特征和判据.本研究为设计和合成高敏感度的功率调控的上转换材料提供了理论基础和实验数据.     

Er掺杂和Er/Yb双掺杂光电陶瓷PLZT的上转换发光

研究了Er掺杂和Er/Yb双掺杂的锆钛酸铅(PLZT)光电陶瓷的上转换发光特性,观测到Er掺杂的PLZT样品在540,566nm附近的绿色发光峰,且随着掺杂浓度的增大而增强;Er/Yb双掺杂的PLZT样品除540,566nm附近处的绿色发光峰外,还有一个较弱的668nm的红色发光峰。另外,上转换发光强度与激发强度的对数关系曲线表明样品的绿光发射和红光发射皆为双光子过程,并且利用喇曼光谱进一步分析讨论了其上转换发光的机制。实验和理论分析表明该材料有望制成电光调Q的双功能上转换激光器件。     

水溶性CdTe量子点的稳态和纳秒时间分辨光致发光光谱

报道了以飞秒脉冲激光为激发光源的水溶性CdTe量子点(QDs)的稳态荧光光谱和纳秒时间分辨荧光光谱.实验发现CdTe量子点的荧光光谱峰值位置随激发波长变化发生明显移动，激发脉冲波长越长，荧光峰位红移越大.荧光动力学实验数据显示，在400nm和800nm脉冲激光激发下，水溶性CdTe量子点的荧光光谱中均含有激子态和诱捕态两个衰减成分，两者的发射峰相距很近，诱捕态的发射峰波长较长.在800nm脉冲激光激发下的诱捕态成分占总荧光强度的比重比400nm激发下的约高3倍，其相对强度的这种变化导致了稳态荧光发射峰位的红移. 关键词： CdTe 量子点 时间分辨 荧光光谱 上转换荧光