Luminescence quantum efficiency and local optical density of states in thin film ruby made by ion implantation

Single crystal (0001) oriented, sapphire samples were implanted with 150 keV Cr ions at fluences between 6.0×1014 and 4.0×1015 Cr/cm2. The peak concentrations ranged from 0.04 to 0.28 at %. Characteristic photoluminescence of the R lines at 694.3 and 692.9 nm was observed. Annealing at 1450 °C for 2 h increased the luminescence intensity by a factor of 45, due to the increasing fraction of substitutional Cr ions as confirmed by Rutherford backscattering spectrometry. The Cr luminescence decay rate in an annealed sample implanted with 3.0×1015 at/cm2 at 300 K is 299 s−1. Decay rates were also measured for samples covered with a range of transparent liquids (refractive index n=1.33–1.57), showing a clear increase with increasing refractive index of the liquid. This effect is explained by the increase of the local optical density of states in the Cr-implanted region. By comparing the measured data with the calculated optical density of states the radiative decay rate is found to be 164±10 s−1 and the internal quantum efficiency ∼50%. The quantum efficiency decreases slightly for increasing Cr concentration.