Regulated polarization degree of upconversion luminescence and multiple anti-counterfeit applications

Polarization upconversion luminescence (PUCL) of lanthanide ions (Ln³⁺) has been widely used in single particle tracking, microfluidics detection, three-dimensional displays, and so on. However, no effective strategy has been developed for modulating PUCL. Here, we report a strategy to regulate PUCL in Ho³⁺-doped NaYF₄ single nanorods based on the number of upconversion photons. By constructing a multiphoton upconversion system for Ho³⁺, we regulate the degree of polarization (DOP) of PUCL from 0.590 for two-photon luminescence to 0.929 for three-photon upconversion luminescence (UCL). Furthermore, our strategy is verified from cross-relaxation between Ho³⁺ and Yb³⁺, excitation wavelength, excitation power density, and local site symmetry. And this regulation strategy of PUCL has also been achieved in Tm³⁺, with the DOP ranging from 0.233 for two-photon luminescence to 0.925 for four-photon UCL. Besides, multi-dimensional anti-counterfeiting display has been explored with PUCL. This work provides an effective strategy for regulating PUCL and also provides more opportunities for the development of polarization display, optical encoding, anti-counterfeiting, and integrated optical devices. Graphical abstract: (Figure presented.)