Orthogonal excitation polarization dictated by site symmetry in upconversion luminescence of ????−NaYF4:Er3+ microrods

Here, the authors build a symmetry-based framework for rare-earth upconversion luminescence via deterministic polarization correlations among sequential absorption steps, previously assumed to be uncorrelated. Using polarization-resolved spectroscopy and crystal field modeling, they assign full Stark levels and irreducible representations for Er${}^{3+}$ in \ensuremath{\beta}-NaYF${}_{4}$ microrods and identify an approximate ${C}_{3}$ site symmetry. Since excited-state absorption inherits ground-state polarization constraints, both downshifting and upconversion luminescence exhibit excitation-wavelength-tunable, region-selective orthogonal excitation polarization responses.