Polarization‐Modulated Upconversion and Downconversion Luminescence in a Lanthanide‐Doped Microparticle from Visible to Near‐Infrared

Abstract Polarization‐sensitive lanthanide luminescent materials offer new opportunities for directional photonics, high‐resolution imaging, and information encryption. However, achieving tunable and reproducible polarization responses, especially in the near‐infrared regions remains challsenging. Here, a comprehensive strategy is reported for modulating polarization response in lanthanide‐doped β‐NaYF 4 :Er 3+ microcrystals by adjusting activator concentration, excitation power, and lanthanide cooping. Through single‐particle optical studies, a broad modulation of polarization degree from 0 to 0.997, with higher‐order multiphoton processes showing enhanced anisotropy is demonstrated. Structural analysis reveals that local symmetry distortion and crystal field effects govern the observed polarization. Co‐doping with Yb 3+ and Ce 3+ enables excited‐state population control via inter‐ionic energy transfer, allowing fine‐tuning of excitation anisotropy. This strategy is further validated in Yb 3+ /Tm 3+ codoped systems, where downconversion emissions at 1219 and 1469 nm exhibited photon‐order‐ and power‐dependent sinusoidal polarization responses. These findings highlight lanthanide‐doped β‐NaYF 4 microcrystals as promising materials for polarization‐resolved photonic devices, with particular potential in infrared detection, optical data storage, and advanced near‐infrared imaging technologies.