Professor, Department of Display Engineering, Hoseo University, KOREA

Speech Title: Photon energy up-conversion processes in Er³⁺ doped KYF₄ crystal
Abstract: Photoluminescence (PL) study shows that with KY_1-xEr_xF₄ crystals incident photon energy is up-converted to produce PL band around 410 nm upon excitation by 488 nm or 532 nm laser radiation. From temporal dependence of the PL, it is seen that the energy transfer between excited Er³⁺ ions is responsible for the up-conversion of the photon energy. In contrast to conventional PL that shows reduction of PL intensity at elevated temperatures, the intensity of this up-converted PL increases with increasing temperature, which is an indication that the energy transfer process is a phonon-assisted one. Phonons are compensating for the energy mismatch between the two transitions involved in the energy transfer. In addition to this up-conversion by energy transfer, we find a secondary up-converted PL around 520 nm when excited by 532 nm light. By the incident power and temperature dependence, this secondary up-conversion is shown to arise from level crossing mechanism that leads to a thermal equilibrium between the two levels ²H(2)_11/2 and ⁴S_3/2. We also discuss the pressure-induced structural phase transformation of KYF₄ crystal as monitored by the PL of Er³⁺ ions under high pressure.
Keywords: up-conversion, energy transfer, photoluminescence, Er³⁺ ion