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 mu m 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.