Efficient Visible-to-UV Photon Upconversion Systems Based on CdS Nanocrystals Modified with Triplet Energy Mediators

Developing high-performance visible-to-UV photon upconversion systems based on triplet–triplet annihilation photon upconversion (TTA-UC) is highly desired, as it provides a potential approach for UV light-induced photosynthesis and photocatalysis. However, the quantum yield and spectral range of visible-to-UV TTA-UC based on nanocrystals (NCs) are still far from satisfactory. Here, three different sized CdS NCs are systematically investigated with triplet energy transfer to four mediators and four annihilators, thus substantially expanding the available materials for visible-to-UV TTA-UC. By improving the quality of CdS NCs, introducing the mediator via a direct mixing fashion, and matching the energy levels, a high TTA-UC quantum yield of 10.4% (out of a 50% maximum) is achieved in one case, which represents a record performance in TTA-UC based on NCs without doping. In another case, TTA-UC photons approaching 4 eV are observed, which is on par with the highest energies observed in optimized organic systems. Importantly, the in-depth investigation reveals that the direct mixing approach to introduce the mediator is a key factor that leads to close to unity efficiencies of triplet energy transfer, which ultimately governs the performance of NC-based TTA-UC systems. These findings provide guidelines for the design of high-performance TTA-UC systems toward solar energy harvesting.     

Photon Upconversion Hydrogels for 3D Optogenetics

The ability to optically induce biological responses in 3D has been dwarfed by the physical limitations of visible light penetration to trigger photochemical processes. However, many biological systems are relatively transparent to low-energy light, which does not provide sufficient energy to induce photochemistry in 3D. To overcome this challenge, hydrogels that are capable of converting red or near-IR (NIR) light into blue light within the cell-laden 3D scaffolds are developed. The upconverted light can then excite optically active proteins in cells to trigger a photochemical response. The hydrogels operate by triplet–triplet annihilation upconversion. As proof-of-principle, it is found that the hydrogels trigger an optogenetic response by red/NIR irradiation of HeLa cells that have been engineered to express the blue-light sensitive protein Cry2olig. While it is remarkable to photoinduce the clustering of Cry2olig with blanket NIR irradiation in 3D, it is also demonstrated how the hydrogels trigger clustering within a single cell with great specificity and spatiotemporal control. In principle, these hydrogels may allow for photochemical control of cell function within 3D scaffolds, which can lead to a wealth of fundamental studies and biochemical applications.     

掺Er3+的新型GeS2-In2S3-CsI玻璃双光子吸收绿光上转换发光研究

用熔融急冷法制备了掺Er3+的80GeS2-10In2S3-10CsI(mol%)硫卤玻璃样品,研究了该样品的吸收光谱和上转换光谱,分析了Er3+离子在该玻璃中的上转换发光机理.应用Judd-Ofelt理论计算分析了Er3+离子在该样品中的强度参数Ωt(t=2,4,6)、自发辐射跃迁几率A、荧光分支比β等光谱参数.在980 nm LD泵浦激发下,首次在该种玻璃中观察到强烈的绿光(527 nm、548 nm),分别对应于4H11/2→4I15/2和4S3/2→4I15/2的跃迁,且548 nm绿光强度大于527 nm绿光强度.同时研究了绿光(527 nm、548 nm)上转换发光强度随泵浦激发功率的变化,其发光曲线拟合斜率分别为1.90和2.06,表明绿光是双光子吸收过程.研究结果表明:掺Er3+含铟(In)硫系玻璃有望应用于可见光激光器、高密度光储存、三维色彩显示等领域.     

Er3+/Yb3+共掺光纤激光器中能量上转换的抑制

通过对掺Er^3 和Er^3 /Yb^3 共掺光纤激光器的输出功率进行数值模拟发现：高浓度的Er^3 /Yb^3 共掺光纤激光器的最大量子转换效率比同等浓度下的掺Er^3 光纤激光器的最大量子转换效率高出一倍。说明由于Yb^3 的加入，Er^3 /Yb^3 共掺光纤激光器有效地抑制了能量上转换，提高了受激转换效率。     

Removing the Obstacle of Dye‐Sensitized Upconversion Luminescence in Aqueous Phase to Achieve High‐Contrast Deep Imaging In Vivo

Although upconversion nanoparticles (UCNPs) have drawn increasing attention for their unique photophysical characteristics, they suffer from a bottleneck of low luminescence efficiency due to the poor absorption coefficient of Ln³⁺. Dye sensitization has provided thousands‐fold enhancement of upconversion luminescence (UCL) in organic solvents because of the remarkably improved light absorption ability, but the sensitization of UCL in aqueous phase is only less than 20 folds by far, with unknown restrictive factors. Herein, the aggregation‐caused quenching (ACQ) of dyes is revealed as the most important reason limiting dye sensitization in aqueous phase, and the problem is circumvented through delicately modulating the physical properties of dyes and their assembly manner with UCNPs. By further alleviating energy back transfer (EBT) from Ln³⁺ to dyes, more than 600‐fold enhancement of UCL is achieved in aqueous phase. The as‐obtained dyes modified UCNPs show good biocompatibility and high signal contrast when applied for deep in vivo imaging.     

Yb3+/Ho3+共掺碲酸盐玻璃的能量传递和上转换发光

制备了75TeO2-20ZnO-(3-x)La2O3-2Yb2O3-xHo2O3(TZLH x,x=0,0.2,0.4,1,2 mol %),75TeO2-20ZnO-(4.6)La2O3-0.4Ho2O3系列玻璃样品,研究了975nm泵浦下,在Yb3 离子浓度确定的情况下,Ho3 离子浓度的变化对Ho3 上转换发光的影响并分析了上转换发光机制.结果表明,随着Ho3 离子含量的增大,549nm绿光强度先增大而后减小.在TZLH-0.4玻璃中,即含有0.4mol%Ho2O3时,绿光强度达到最大值.而当Ho2O3含量超过0.4mol%时,绿光强度明显降低.基于Yb3 和Ho3 的能级图及上转换机制建立了速率方程,通过数值求解速率方程拟合实验测量寿命曲线,得出了Yb3 与Ho3 之间的能量转移系数CDi(i=2,3,4).     

Nanostructured Polymers Enable Stable and Efficient Low-Power Photon Upconversion

Photon upconversion based on sensitized triplet–triplet annihilation (sTTA-UC) is a wavelength-shifting technique with potential use in actuators, sensing, and solar technologies. In sTTA-UC, the upconverted photons are the result of radiative recombination of high-energy singlets, which are created through the fusion of metastable triplets of two annihilator/emitter molecules. The emitter triplets are populated via energy transfer (ET) from a low-energy absorbing light-harvester/sensitizer. The process is highly efficient at low powers in solution but becomes relatively ineffective in solid matrices since the limited molecular mobility precludes bimolecular interactions. The realization of efficient solid-state upconverters that exhibit long-term stability and are compatible with industrial fabrication processes is an open challenge. Here, nanophase-separated polymer systems synthesized under ambient conditions that contain the upconverting dyes in liquid nanodomains is reported. The nanostructured polymers show an excellent optical quality, an outstanding upconversion efficiency of up to ≈23%, and excellent stability in air, with only negligible performance losses over a period of three months. Moreover, the dyes’ confinement in nanosized domains <50 nm results in an increased effective local density of chromophores that enables hopping-assisted ET and TTA and confers to the upconversion process peculiar kinetics that enhances the material performance at low powers.     

Plasmon‐Enhanced Blue Upconversion Luminescence by Indium Nanocrystals

Numerous endeavors have been undertaken to gain enhanced upconversion luminescence via surface plasmon resonance (SPR) generated by specially designed nanostructures of noble metals (e.g., Au, Ag). However, the SPR response of these metals is usually weak in the ultraviolet (UV) region because of their intrinsic electronic configurations; thus, only green and red upconversion emissions can undergo significant plasmonic enhancement yet without selectivity, while an efficient approach to selectively enhancing the blue upconversion luminescence has been lacking. Herein, by integrating the pronounced UV SPR of silica‐coated indium nanocrystals (InNCs) with blue‐emission upconversion nanoparticles (UCNPs) of NaYbF₄:Tm, an up to tenfold selective luminescence enhancement at 450 nm is obtained upon 980 nm laser excitation. Precise manipulation of the silica shell thickness suggests an optimal working distance of 3 nm between InNCs and UCNPs. This study has, for the first time, realized selective blue upconversion luminescence enhancement by using an inexpensive, non‐noble metal material, which will not only enrich the fundamental investigations of SPR‐enhanced upconversion emission, but also widen the applications of blue light‐emitting nanomaterials, for example, in therapeutics.     

An Advanced Tunable Multimodal Luminescent La4GeO8: Eu2+, Er3+ Phosphor for Multicolor Anticounterfeiting

The development of advanced luminescent materials is of great importance to the anticounterfeiting application and still confronts with lots of challenges. At present, most anticounterfeiting luminescent materials are based on a monotonous photoluminescence model, which is easily faked by substitutes. Therefore, in this work, a multimodal La₄GeO₈: Eu²⁺, Er³⁺ material is reported, which can emit red, purple, baby blue, and green light under the increased excitation wavelength from 250 to 380 nm. Meanwhile, the phosphor also shows green upconversion luminescence under the NIR (980 and 808 nm) laser irradiation. Moreover, the phosphor features excellent stability and humidity resistance against harsh conditions. Based on the integrated feature, a functional anticounterfeiting application is designed. Results demonstrate that the multimodal luminescent feature can be easily detected by using a portable ultraviolet lamp or NIR (808 or 980 nm) laser. The unique characteristic will be complicated to counterfeit and show high-level security in the field of advanced anticounterfeiting.     

Highly Secure Plasmonic Encryption Keys Combined with Upconversion Luminescence Nanocrystals

This study proposes a novel and highly secure encryption technology based on plasmonic‐enhanced upconversion luminescence (UCL). The technology can be realized by a disordered plasmonic nanostructure composed of a transferred metal nanoparticle–UC nanocrystals (UCNC)–metal (tMUM) film using the graphene transfer process, in which the metal nanoparticles that formed on the graphene layer are transferred using Scotch tape. The plasmonic tMUM film strongly enhances the UCL by a factor of 200 mainly because of the excitation of the gap plasmon polaritons. Meanwhile, the UCNCs in direct contact with the metal film result in luminescence quenching caused by a nonradiative process. Herein, a highly secure anti‐counterfeit film is developed, which is very hard to duplicate and cannot be reused, using two conflicting features (i.e., emission enhancement and quenching phenomena). The UCL is strongly amplified only when the first (i.e., a random metal nanoparticle array) and second (i.e., UCNCs on a Ag film) codes are very precisely overlapped as designed, thereby generating the originally designed final code. Therefore, our novel high‐level security device is expected to be easily applied to protect and identify genuine products.     

Tm3+,Yb3+共掺的氟铝基玻璃的能量传递与上转换发光

在 970 nm L D激发下 ,研究 Tm3 ,Yb3 共掺的 Al F3 基 (AYF,AZF)玻璃能量传递和上转换发光性质。发现对于 Tm3 的浓度猝灭 ,蓝光跃迁比近红外荧光跃迁表现更明显 ;对 4 76 nm蓝光跃迁 ,AYF和 AZF玻璃 Tm3 最佳掺杂浓度都为 0 .1mol- % ;而对 793nm的近红外跃迁 ,Tm3 最佳掺杂浓度为 0 .3～ 0 .5mol- %。AYF玻璃的 Yb3 离子最佳掺杂浓度为 5mol- % ,AZF玻璃则为8～ 10 mol- %。 Tm3 (3 F4 )→ Yb(2 F5/ 2 )反向能量传递是引起 Yb3 离子对上转换荧光的浓度猝灭的重要原因。     

稀土离子上转换发光的研究

综述了上转换发光材料、发光机制中的一些物理问题及影响发光效率的因素，并介质了当前的研究现状。     

钨酸锌中Yb^3＋—Tm^3＋间接敏化上转换和共振能量传输

在807nmLD激发下，Tm^3 ，Yb^3 ：ZnWO4中，Yb^3 可以提高Tm^3 486nm(^1 G4→^3 H6）的荧光强度2个数量级以上。其机理Tm^3 -Yb^3 -Tm^3 间接敏化下的共振能量传输。实验表明486nm和404nm处的上转换荧光强度分别随泵浦激光功率的1.8和2.4次幂变化。     

Band Gap Engineering Improves the Efficiency of Double Quantum Dot Upconversion Nanocrystals

Solution‐processed core/multishell semiconductor quantum dots (QDs) could be tailored to facilitate the carrier separation, promotion, and recombination mechanisms necessary to implement photon upconversion. In contrast to other upconversion schemes, upconverting QDs combine the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. Nevertheless, their upconversion quantum yield (UCQY) is fairly low. Here, design rules are uncovered that enable to significantly enhance the performance of double QD upconversion systems, and these findings are leveraged to fabricate upconverting QDs with increased photon upconversion efficiency and reduced saturation intensities under pulsed excitation. The role of the intra‐QD band alignment is exemplified by comparing the upconversion process in PbS/CdS/ZnSe QDs with that of PbS/CdS/CdSe ones with variable CdSe shell thicknesses. It is shown that electron delocalization into the shell leads to a longer‐lived intermediate state in the QDs, facilitating further absorption of photons, and enhancing the upconversion process. The performance of these upconversion QDs under pulsed excitation versus continuous pumping is also compared; the reasons for the significant differences between these two regimes are discussed. The results show how one can overcome some of the limitations of previous upconverting QDs, with potential applications in biophotonics and infrared detection.     

Yb3+对掺铒碲酸盐玻璃红外和上转换发光的影响

研制了Er3+/Yb3+共掺TeO2-ZnO-La2O3玻璃,测试了Er3+离子的吸收谱、荧光谱和上转换发光谱,系统研究了975 nm抽运下Yb3+离子对于Er3+离子1.5 μm波段荧光性能及其上转换发光性能的影响.结果表明,随着碲酸盐玻璃中Yb2Os含量的增加,Yb3+离子对Er3+离子的能量传递增强,Er3+离子1.5 μm波段的荧光强度和上转换发光强度相应增大,但后者相对于前者增加更为迅速.通过对粒子数速率方程的理论模拟,计算结果与实验测量结果较为一致,表明Er3+/Yb3+共掺碲酸盐玻璃是一种优良的潜在上转换激光器增益介质.对上转换发光分析表明,强烈的绿光和红光激发是基于双光子的吸收过程.     

980 nm抽运时掺铒光纤放大器中的上转换发光效应研究

采用980nm抽运的掺铒光纤放大器（EDFA）中存在上转换发光效应。能级分析和光谱扫描结果表明上转换辐射光为绿色荧光。波长为538nm和514nm，其产生机理为铒离子的激发态吸收效应（ESA）。从理论和实验两方面分析了抽运功率和信号功率这两个放大参量对上转换绿色荧光的影响，结果表明，存在一个特殊抽运功率值，当抽运功率小于该值时，上转换绿色荧光的抽运效率随抽运功率的增加而快速增大；抽运功率大于该值时，上转换绿色荧光的抽运效率变化缓慢，基本保持稳定。掺铒光纤放大器工作在线性放大状态下，输入信号的有无和功率大小对绿色荧光影响很小；掺铒光纤放大器工作在饱和状态下，绿色荧光功率随输入信号功率增加而增加。     

Ho3+/Yb3+共掺碲酸盐玻璃的上转换发光及能量传递机理

用高温熔融法制备了Ho3+单掺和Ho3+/Yb3+共掺、组分为TeO2-ZnO-Na2O的碲酸盐玻璃,应用Judd-Ofelt(JO)理论计算分析了玻璃的强度参数Ωt(t=2,4,6)、自发辐射跃迁几率A、荧光分支比β和荧光辐射寿命τrad等各项光谱参数。通过测量上转换光谱,着重研究了Yb3+掺杂对于Ho3+上转换发光性能的影响,分析了Yb3+/Yb3+以及Yb3+/Ho3+间的能量传递过程。结果显示,975nm泵浦下Ho3+的上转换发光主要来自于Yb3+/Yb3+间的共振能量传递以及基于单声子和双声子辅助的Yb3+/Ho3+间的能量传递过程,并计算得到了声子贡献比和能量传递微观参数。同时,计算分析了Ho3+:5 I7→5 I8能级跃迁的吸收截面、受激发射截面和增益系数。研究表明,Yb3+/Ho3+共掺TeO2-ZnO-Na2O玻璃可以作为上转换激光器和2.0μm波段固体激光器的潜在增益基质。     

Er3+/Yb3+共掺碲酸盐玻璃的上转换低温特性研究

研究了低温下Er3+/Vb3+共掺碲酸盐玻璃在975 nm激光激发下的上转换光谱与温度的关系.通过上转换光谱测量观察到3处上转换荧光,它们依次为529 nm(2H11/2→4I15/2),546 nm(4S3/2→4I15/2)和669 nm(4F9/2→4I15/2),上转换绿光(546 nm)和红光(669 nm)荧光强度在80 K左右达到了最大值,与300 K温度下相比分别提高了2.198和1.556倍.而529 nm上转换绿光强度随着温度的降低逐渐减弱,直至消失.研究了4S3/2能级和2H11/2能级之间的粒子数分布与温度的关系,并通过理论模型拟合了实验测量的4S3/2能级和2H11/2能级之间的粒子数分布,测量了8～300 K温度范围下,4Sa/2能级和4F9/2的寿命和温度的变化关系以及通过高斯拟合研究了4S3/2能级和2H11/2能级Stark分裂与温度的关系.     

Er3+/Yb3+共掺Bi2O3-GeO2-Na2O玻璃的上转换发光

研究了不同Er3 /Yb3 掺杂比46Bi2O3-44GeO2-10Na2O(B46G44N10)(摩尔分数)玻璃的吸收光谱、红外吸收谱和上转换光谱性质,分析了玻璃中Yb3 敏化Er3 的上转换发光机制。测试了Er3 离子在不同Yb3 浓度下玻璃中的上转换荧光强度,得到最佳的掺杂质量比Er3 ∶Yb3 =1∶6。计算了在980 nm激发下Er2O3质量分数为0.5%和Yb2O3质量分数为3.0%的Er3 /Yb3 共掺B46G44N10玻璃的绿光上转换效率为2.27×10-4。比较了不同基质材料玻璃的上转换效率,结果表明B46G44N10玻璃是一种良好的上转换基质材料。     

Er3+掺杂重金属氧氟锗酸盐玻璃的光谱性质和上转换机理

研究了Er^3 掺杂重金属氧氟锗酸盐玻璃的吸收光谱和上转换光谱性质，分析了玻璃中Er^3 的上转换发光机理，应用Judd—Ofelt理论计算了Er^3 在玻璃中的强度参数Ωt(t=2，4，6)，自发辐射跃迁几率、荧光分支比和辐射寿命。结果表明，在975nm抽运光激发下，观察到强烈的绿光和微弱的红光；绿光和红光发射是由于双光子吸收过程，其上转换机理是能量转换(ET)和激发态吸收(ESA)。拉曼光谱分析表明，对于上转换发光，玻璃结构中的F^-离子起到重要作用。