Polarized luminescence in single upconversion NaYbF4:Er rods

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.