Luminescence of Eu$^{3+}$ rare-earth ions in Lu$_2$O$_3$ nanospheres

The kinetics of luminescence of Eu$^{3+}$ ions in Lu$_2$O$_3$:Eu nanospheres with diameters of 100–270 nm and a small standard deviation of the size distribution <15% has been studied. A sharp decrease in the decay time of luminescence of Eu$^{3+}$ ions in the red range with an increase in the diameter of nanospheres has been attributed to the appearance of a photon mode accelerating spontaneous luminescence, which is confirmed by the calculation of ranges of existence of whispering-gallery modes in studied nanospheres.