Concentration enhanced red upconversion in nanocrystalline ZrO2 : Er under IR excitation

The effects of Er3+ concentration in ZrO2 on the red (680 nm) and green (545 nm) upconversion emission from Er3+ under near infrared radiation (NIR) excitation (at both 1438 and 962 nm) are presented. For the highest Er3+ concentration used, the red band is almost quenched under VIS (489 nm) excitation, whereas it is enhanced under NIR excitation. Such a difference between photoluminescence and upconversion is explained taking into account the concentration dependent cross-relaxation process (2H11/2 + 4I15/2) → (4I9/2 + 4I13/2). Experimental results suggest that two- and three-photon upconversion processes are taking place under 962 nm and 1438 nm excitation, respectively.