Amplifying Excitation-Power Sensitivity of Photon Upconversion in a NaYbF₄:Ho Nanostructure for Direct Visualization...

Controlling excitation power is the most convenient approach to dynamically tuning upconversion that is essential for a variety of studies. However, this approach suffers from a significant constraint due to insensitive response of most upconversion systems to excitation power. Here we present a study of amplifying excitation power-sensitivity of upconversion in Ho³⁺ ions through the use of a NaYbF₄ host. Mechanistic investigation reveals that the sensitive response of Ho³⁺ upconversion to excitation power stems from maximal use of the incident energy enabled by concentrated Yb³⁺ sensitizers. This allows us to sensitively tune the red-to-green emission intensity ratio from 0.37 to 5.19 by increasing the excitation power from 1.25 to 46.25 W cm^-2, which represents a 5.6-fold amplification of the tunability (from 0.19 to 0.49) offered by Yb/Ho (19/1 mol %) codoped NaYF₄. Our results highlight that the excitation-power sensitive upconversion emission can be exploited to experimentally visualize electromagnetic hotspots.