Optical microscopy of single ions and morphological inhomogeneities in Sm-doped ${\mathrm{CaF}}_2$ thin films

We have investigated the luminescence of ${\mathrm{CaF}}_2$ thin films doped with very low concentrations of ${\mathrm{Sm}}^{2+}$ ions using scanning confocal optical microscopy at low temperatures. The film morphology was studied independently by atomic force microscopy. The ${\mathrm{Sm}}^{2+}$ ions are homogeneously distributed in the films and show photobleaching. Unexpectedly, on the film surface strongly luminescent small topographic features are observed that are found to contain ${\mathrm{Sm}}^{3+}$ by spectral analysis. The formation of ${\mathrm{Sm}}^{3+}$ is probably due to the presence of oxygen during film growth. In the lowest doped films on-off blinking behavior of isolated luminescent spots provides strong evidence for the first observation of single ions in a crystal.