Upconversion mechanisms and thermal effects of Er3+ in Er3+ doped yttria nanocrystals

► Excited state absorption cross section of Er3+4I13/2 was determined to be 6.8×10−27 m2. ► Cross relaxation rate of Er3+4I11/2+4I11/2→4I15/2+4F7/2 was determined to be 6200 s−1. ► Effect of temperature on the phonon-assistant absorption is discussed.