Luminescence properties of Y$_3$Al$_5$O$_{12}$ crystal doped with Tm$^{3+}$ and Ho$^{3+}$ ions have been investigated. Upconverted luminescence in the UV to green region was observed by the red laser-light excitation of Ho$^{3+}$ and Tm$^{3+}$ ions into the $^5$F$_5$ and $^3$F$_2$ states, respectively. The emission processes of upconverted and down-converted luminescences have been proposed by analyses of the emission and excitation spectra and decay times. The sequential two-photon excitation process (STEP) is only responsible for the upconverted luminescence following Ho$^{3+}$($^5$F$_5$) excitation, while the upconverted luminescence on the excitation of Tm$^{3+}$($^3$F$_2$) is attributed to STEP and energy transfer upconversion (ETU) process.

Keywords: Y$_3$Al$_5$O$_{12}$, Tm$^{3+}$, Ho$^{3+}$, Upconversion, Luminescence, Energy transfer