Phonon-assisted energy up-conversion process of Er3+ ions doped in KY F4 crystal

Er3+ ions doped in KY F4 crystal emit blue photoluminescence around 406 nm by energy up-conversion process when excited by 532 or 488 nm laser. By measuring the temperature dependence of up-converted luminescence, we found that the up-conversion process is enhanced with increasing temperature, which is an indication that phonons are assisting the process. This observation is supported by the fact that the energy transfer responsible for the up-conversion involves two transitions that have an energy mismatch.