Excitation power-dependent multicolor upconversion in NaLnF 4 :Er 3+ under 1532 nm irradiation for anti-counterfeiting application.

Upconversion (UC) materials are renowned for their ability to convert low-energy photons into high-energy ones. The manipulation of parameters allows for the observation of multicolored UC luminescence (UCL) within a single material system. While modulation of multicolored UCL commonly relies on excitation at approximately 980 nm, investigation into multicolored UC materials activated by a 1532 nm excitation source remains comparatively scarce. In this work, we introduce NaLnF ₄ :Er ³⁺ as a novel class of smart luminescent materials. When the power density of a 1532 nm laser increases from 0.5 to 20.0 W/cm ² , the emission peak positions remain unchanged, but the red-to-green (R/G) ratio decreases significantly from 18.82 to 1.48, inducing a color shift from red to yellow and ultimately to green. In contrast, no color variation is observed when NaLnF ₄ :Er ³⁺ is excited with a 980 nm laser at different power densities. This power-dependent multicolored UCL of NaLnF ₄ :Er ³⁺ excited at 1532 nm can be attributed to the competitive processes of upward pumping and downward relaxation of electrons on the ⁴ I _9/2 level of Er ³⁺ . By utilizing the unique UC characteristics of NaLnF ₄ :Er ³⁺ , its potential utility in anti-counterfeiting applications is demonstrated. Our research highlights the distinctive optical properties of NaLnF ₄ :Er ³⁺ and provides novel insights into the use of luminescent materials in optical anti-counterfeiting technologies.