Multicolor upconversion emission of Ho3+ in single NaYF4 microrod

Lanthanide-doped upconversion nano/micro-materials exhibit superior and fascinating optical properties due to their unique 4f–4f transitions. They have been extensively investigated in various fields. Thus, studying the upconversion optical characteristic, especially for Er3+, Tm3+, and Ho3+ ions, is extremely important and useful. In this work, a strong upconversion emission with a candy-like fluorescent pattern has been discovered in a single NaYF4: Yb3+/Ho3+ microrod via confocal microscopy under NIR 980 nm excitation. When the Yb3+ ion-doped concentrations and the excitation power are increased, the Ho3+ ions show an increased blue emission intensity and high R/G ratio in single NaYF4 microrod, which is due to the cross-relaxation between Ho3+ ions. The corresponding upconversion emission behaviors and physical mechanisms have been studied on the basis of the spectral characteristics of Ho3+ ions and rate equations. Meanwhile, the candy-like emission pattern of single NaYF4 microrod is explained by establishing optical models, which because of the occurrence of fluorescence total reflection. The present study is of great importance in NaYF4 micro-materials doped with rare-earth ions, thereby providing new applications in optical anti-counterfeiting, 3D-displays, and wave-guides.