[Frequency up-conversion properties of Er3+ /Yb3+ co-doped zinc oxide powders]

Er3+ /Yb3+ co-doped ZnO powders were prepared by the high temperature sintering method with starting composition of (mol%) 95ZnF2-4. 8Yb2 O3-0. 2Er2 O3. Microstructure analysis by X-ray diffraction (XRD) showed that the sample consists of two phases, i. e. ZnO and YbF3, which verified that the ZnF2 was oxidized during the high temperatue sintering Composition analysis by scanning electron microscope (SEM) and spectroscopic measurements showed that the Er3+ and Yb3+ ons were successfully used in doping the lattice of ZnO, but most of Yb3+ ions were in the YbF3 phase. These results indicated that the up-conversion luminescence was emitted from ZnO, not from YbF3. Under the excitation of 980 nm diodelaser, four strong up-conversion emissions peaks centered at 658, 538, 522 and 409 nm, corresponding to the transitions 4F9/2 --> 4I15/2, 4S3/2 --> 4I15/2, 2 H11/2 --> 4I15/2 and 2 H9/2 --> I15/2, respectively, were observed. Especially, a strong red up-conversion emission was observed, which is different from that the green up-converted luminescence is dominated in glass and ceramics. Three important cross energy transfer (CRET) processes between Er3+ ions played an important role for this. Under 488 nm Ar+ laser excitation, intense violet (409 nm), weak blue (466, 450 nm) and ultraviolet (379 nm) up-conversion luminescence originating from the transitions 2 H9/2 --> 4I15/2, 2P3/2 --> 4I11/2, 4 F3/2 /4 F5/2 --> 4I15/ 2 and 4G11/2 --> 4 I15/2, respectively, were obtained. The dependence of up-conversion intensities on excitation power indicated that two-photon absorption processes were responsible for the violet luminescence under 488 nm excitation, and the violet up-converted luminescence was achieved through the forward and back energy transfer between Er3+ and Yb3+ ions. Our results show that ZnO as a host material has the potential applications in the up-conversion red phosphors and ultraviolet laser materials.