The radiative lifetime of the ⁴I_13/2 level of Er ³⁺ was modified up to 15% by changing the microstructure in laser-floating-zone-grown Er ³⁺ -doped Al ₂ O ₃– ZrO ₂ eutectic melt growth composites. Erbium enters only the higher-refractive-index ZrO ₂ phase, which occupies 30% of the volume of the sample. The lifetimes of ⁴S_3/2 (545 nm) and ⁴I_13/2 (1520 nm) Er ³⁺ levels were measured at 10 K and room temperature in different microstructures. The variation in the lifetime of this last level with interphase spacing is due to the change of the available electromagnetic modes at 1520 nm due to the change in the optical environment of the luminescent ion. © 2002 American Institute of Physics.