Y2O3:5%Er nanocrystalline powder was prepared by low temperature combustion method. The crystal structure and
morphology were analyzed by means of XRD and HRTEM. The resultant powders were sintered into translucent
ceramics at 1570°C in vacuum for 6 hours. The micrograph of unpolished surface and fracture surfaces showed that the
sintered Y2O3:Er ceramics with average grain size at about 10μm had homogeneous micro-structure and low pore
volume. Under the excitation of 980 nm, 808 nm and 785 nm diode lasers, respectively, very strong green and red
upconversion emissions from Er3+ ions were observed, the power dependence of upconversion emission intensity was
measured to deduce the upconversion mechanism. A trend of upconversion intensity increase first and then decrease with
the excitation time was also found for the first time.
Powder samples Cs2NaGdCl6:10%Er3+ and Cs2NaGdCl6:10%Er3+,20%Ho3+ investigated were prepared. Upconversion and down-conversion spectra from both Er3+ and Ho3+ ions was measured and investigated at room temperature under excitation into the 2H11/2 levels of Er3+ ions by using a 514.5 nm Ar+ laser. The observed emissions were all clearly assigned and analyzed according to the energy level diagrams of Ho3+ and Er3+ ions. It was concluded that many emissions with peaks at 423 nm (5G5 -> 5I8), 492 nm (5F3 -> 5I8), 587 nm (5G4 -> 5I6), 657 nm (5F5 -> 5I8) and 760 nm (5I4 -> 5I8) were all from Ho3+ ions, although Ho3+ ions cannot be excited by a ground-state-absorption (GSA). Therefore, all the emissions from Ho3+ ions were caused by multistep energy transfer from Er3+ to Ho3+ ions. And the 388 nm and 410 nm upconversion emissions were assigned to be 4G11/2->4I15/2 and 2H9/2->4I15/2 transitions of Er3+ ions, respectively. The possible upconversion mechanism for them was deduced to be excitation state absorption (ESA) and energy transfer (ET). In addition, the result that the slopes of 388nm and 410nm upconversion emissions of Er3+ ions were smaller in Er3+-doped than Er3+, Ho3+-codoped Cs2NaGdCl6 crystals was explained successfully.
The upconversion fluorescence was recorded at room temperature and investigated in LiKGdF5: 2%Er3+, 0.4%Tb3+ single crystal grown by the hydrothermal synthesis technique under 514.5 nm and 785 nm laser excitation, respectively. Under 514.5 nm laser excitation, four strong upconverted emission bands with peaks at 410 nm (violet), 470 nm, 486 nm and 492 nm (blue) were obtained. The former two emission bands were assigned to be corresponding to 2H9/2 -> 4I15/2 and 2P3/2 -> 4I11/2 transitions of Er3+ ions, and the latter two are possibly corresponding to 5D4 -> 7F6 of Tb3+ and 4F7/2 -> 4I15/2 of Er3+. The power dependence for 410 nm and 470 nm indicates that they arose from two-photon upconversion processes. While for 486 nm and 492 nm emission, the logarithmic slope is 0.98, which was explained by mutiphonon assisted upconversion process and energy transfer from 4F7/2 (Er3+) to 5D4 (Tb3+). Under 785 nm laser excitation, besides four weak upconverted emissions mentioned above, three strong emissions with peaks at 523 nm (2H11/2 -> 4I15/2), 550 nm ( 4S3/2 -> 4I15/2) and 660 nm (4F9/2 -> 4I15/2) were also observed. The possible upconversion mechanism for these seven emissions was all given with the help of the power dependence of upconversion emission intensity and the energy level diagram of Er3+ ions.