On-off blinking and multiple bright states of single europium ions in Eu³⁺:Y₂O₃ nanocrystals

We show that continuously illuminated single europium ions incorporated in yttrium oxide (Eu³⁺:Y₂O₃) nanocrystals (5-15 nm diameter) undergo on-off blinking on a variable time scale ranging from hundreds of milliseconds to several seconds. We observe both a pump-intensity dependent "duty factor" (on-time as a percentage of total measurement time), and quantum jumps between at least three well-defined luminescence intensity levels (bright states) from individual nanoparticles. Interesting switching or oscillation between different bright levels was also observed with a modulation rate that is dependent on pump-laser-intensity. These features of single-ion luminescence are not observed for larger particles with multiple chromophores. We propose that these effects derive from pump laser-induced fluctuations between different quasi-stable Eu³⁺ symmetry sites that effectively modulate the electric dipole transition moment.