Synthesis and upconversion emission studies of CaYF:Ho/Yb phosphor and its applications in optical thermometry, fingerprint detection, and security ink.

In this work, a CaYF:Ho/Yb upconversion phosphor was synthesized and its structural, morphological, and optical properties were studied. The upconversion emission study was performed at an excitation pump power density of 5 W cm and emissions at 544 nm, 650 nm and 747 nm due to the F(S) → I, F → I and F(S) → I transitions of the Ho ion, respectively were observed. From temperature-dependent upconversion spectra temperature sensitivity was calculated and sensitivity is found to be 14 × 10 K for the synthesized upconversion phosphor. The photothermal conversion efficiency of the prepared sample in ethanol medium was tested. Moreover, the sample was used to develop latent fingerprints on various surfaces and good contrast in recorded images is observed. The invisible ink was prepared using the upconversion phosphor and then written words were recorded in upconversion emission mode.