Rare earth (RE)-doped micro/nano-crystals have become a focus of attention due to their potential use as microscopic probes for monitoring local environment parameters, super-resolution imaging and precision tracking, particularly for biological applications. However, the polarization state of upconversion luminescence, especially that of single particles, has been rarely discussed. Here we report a distinct luminescence anisotropy in NaYbF₄:Er(2%) microrods, which can be considered as a case of maximal Yb concentration in a series of well-known upconversion systems, NaYF₄:Yb(x%),Er(2%). In addition to considerable dominance of the red band in the Er³⁺ emission spectrum, we have shown that upconversion luminescence is highly sensitive to the polarization state of the laser source. It is worth noting that the high-resolved polarization response provided by the Stark effect and consequent transitions between Stark-split levels, makes NaYbF₄:Er micro-rods promising as efficient upconversion probes for orientation tracking. We demonstrate the ability to measure both the rod position with 0.7 μm resolution and its orientation by means of confocal luminescence microscopy. These findings reveal a fundamental insight into the anisotropic character of upconversion luminescence of RE-doped single particles and provide great opportunities for various applications as individual anisotropic probes, especially in biological tissues where dominant red emission is highly desirable.

中文翻译：

---

单上转换 NaYbF4:Er 棒中的偏振发光


稀土（RE）掺杂的微/纳米晶体因其潜在的用途而成为人们关注的焦点，因为它们可以用作监测局部环境参数、超分辨率成像和精确跟踪的显微探针，特别是在生物应用中。然而，上转换发光的偏振态，特别是单粒子的偏振态，却很少被讨论。在这里，我们报告了 NaYbF ₄ :Er(2%) 微棒中明显的发光各向异性，这可以被视为一系列众所周知的上转换系统 NaYF ₄ 发射光谱中红波段占据相当大的主导地位之外，我们还发现上转换发光对激光源的偏振态高度敏感。值得注意的是，斯塔克效应提供的高分辨率偏振响应以及随之而来的斯塔克分裂能级之间的跃迁，使得NaYbF ₄ :Er微棒有希望作为定向跟踪的高效上转换探针。我们展示了通过共焦发光显微镜以 0.7 μm 分辨率测量杆位置及其方向的能力。这些发现揭示了对稀土掺杂单粒子上转换发光的各向异性特征的基本见解，并为作为单独的各向异性探针的各种应用提供了巨大的机会，特别是在非常需要占主导地位的红光发射的生物组织中。