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.

Regulation of sensitivity of Yb concentration to power-dependent upconversion luminescence colors

Controlling the power density of exciting light is a widely applied technological approach to dynamically tuning emission spectra to yield desirable luminescence properties, which is essential for various applications in laser devices, cancer cell imaging, biomarker molecule detections, thermometers and optoelectronic devices. However, most of upconversion systems are insensitive to power regulation. In this study, a series of Yb/Ho doped NaYF₄ microrods with different Yb concentrations was synthesized by using a sodium citrate-assisted hydrothermal method. The dependence of upconversion characteristics of NaYF₄:Yb/Ho microrods on Yb concentration and excitation power density are investigated in detail by a laser confocal microscopy system. The emission spectra exhibit discrete upconversion emission characteristic peaks that can easily be assigned to ⁵F₃→⁵I₈ (at about 488 nm), ⁵F₄, ⁵S₂→⁵I₈ (at about 543 nm), ³K₇, ⁵G₄→⁵I₈ (at about 580 nm) and ⁵F₅→⁵I₈ (at about 648 nm) transitions of Ho, respectively. The upconversion spectra and synchronous luminescence imaging patterns show that the luminescence ratio of red to green is not only dependent on the Yb concentration, but also sensitive to the excitation power. With Yb concentration increasing from 5% to 60%, the sensitivity of the power-controlled red to green luminescence ratio largely increases from 0.1% to 13.0%, corresponding to a clear luminescent color modification from green to red. These results indicate that the power-tuned red-to-green-luminescence ratio can be used as a method of measuring and evaluating Yb doping concentration. We attribute the sensitivity tuned by Yb concentration to the differences in population approach and upconversion mechanism for the red and green luminescence. By recording the slope of luminescence intensity versus exciting power density in a double-logarithmic presentation, we detect a small slope for the green emission relative to that for the red emission, especially at a high Yb concentration. These results indicate that the red upconversion process may be a three-photon process. The exciting power induced color adjusting is therefore explained by preferential three-photon population of the red emission due to the high ⁵S₂→⁵G₄ excitation rate, which is verified by down-conversions of emission spectra. Our present study provides a theoretical basis for the spectral tailoring of rare-earth micro/nano materials and supplies a foundation for the applications in rare-earth materials.