T. G. Yukina

Interaction of Pr3+ optical centers in the Y2SiO5 crystal

It is shown on the basis of studies of the optical spectra and luminescence decay of nonequivalent Pr3+ optical centers in the Y2SiO5 crystal that the occupation of the nonequivalent cation sites of the crystal lattice by the activator ions is irregular. For excitation in the region of the optical transitions 3H4→1D2in the temperature interval 6–80 K there is no transfer of electronic excitation energy between Pr3+ ions localized in different cation sites. The luminescence decay law of the Pr3+ optical centers is determined by the concentration of activator ions and has a complicated non-single-exponential character; the quenching of the luminescence of the Pr3+ ions is due to the formation of dimers of these ions.

Spiky structure of coherent emission from optically thick media

The spiky structure of the spontaneous emission from praseodymium ions in the LaF3 lattice on the 3P0-3H6 transition under coherent pulsed excitation of the adjacent 3H4-3P0 transition has been studied experimentally and theoretically. The experimental results are in good agreement with the theoretical conclusions based on analysis of the emission dynamics of an excited ensemble of three-level systems in the mean field approximation. The generation of a spiky structure is interpreted as the emission of a cnoidal wave pulse. The recorded random pattern of time intervals between the spikes reflects the sensitivity of the cnoidal wave period to fluctuations in the system. The developed theory has been found to be related to models of deterministic chaotic dynamics.

The nature and mechanism of charging of electron traps in Lu2SiO5:Ce3+ crystals

Investigation of the thermoluminescence (TL) properties depending on the temperature, UV irradiation dose, and activator ion concentration in Lu2SiO5:Ce3+ (LSO:Ce) crystals and measurements of the decay of recombination radiation during photostimulated release of the excitation energy accumulated in these crystals showed that a part of Ce3+ impurity ions exhibit ionization leading to the injection of electrons into the conduction band. The conduction band of a LSO:Ce crystal is involved in two opposite processes: the charging of electron traps and the recombination of electrons with Ce4+ ions. The transport of electrons to the traps has a diffusion character: electrons possess a significant mobility [D(315 K)=10−3 cm2/s] and can diffuse away from the donor ions to distances exceeding the lattice parameter of the crystal. The results of experiments with controlled atomic packing of LSO:Ce nanoclusters unambiguously demonstrated the key role of the structure of oxyorthosilicates in the formation of electron traps. The existing two-center model does not provide adequate description of the properties of electron traps and TL in LSO:Ce crystals.

Microscopic nature of Pr3+ optical centers in Y2SiO5, Lu2SiO5, and Gd2SiO5 crystals

The results of investigations of the energy spectrum, energy relaxation channels, and dynamics of electronic excitations of Pr3+ optical centers in Y2SiO5, Lu2SiO5, and Gd2SiO5 crystals are reviewed. It is shown that the crystal field of the ligands of nonequivalent cationic sites in these crystals preserves the quasisymmetry of distorted octahedra and tetrahedra; the Pr3+ activator ions are nonuniformly distributed over the nonequivalent cationic sites in Y2SiO5. For high concentrations of activator ions (>1at.%) in Y2SiO5 crystals, the luminescence is quenched as a result of the migration and trapping of the electronic excitations energy of optical centers by traps. For Y2SiO5: Pr3+ the low-temperature mechanism of dephasing of the optical transitions of Pr3+, which is not characteristic for crystals and is due to the interaction of impurity ions with two-level systems of a multiwall adiabatic potential, is analyzed.

Manifestation of quasi-symmetry of the cation sites of Gd2SiO5, Y2SiO5, and Lu2SiO5 in the spectra of the impurity ion Pr3+

The quasi-symmetry of the inequivalent cation sites in the crystals Gd2SiO5, Y2SiO5, and Lu2SiO5 is established on the basis of an analysis of the features of the low-temperature optical spectra of the impurity ion Pr3+. One type of cation site of the crystals Y2SiO5 and Lu2SiO5 manifests the quasi-symmetry of a distorted octahedron, and the other type, that of a distorted tetrahedron. The parameters characterizing the energy spectrum of the free Pr3+ ion in the crystalline field of the cation sites are determined.

The nature of activation centers in Y2SiO5:Pr3+, Gd2SiO5:Pr3+, and Lu2SiO5:Pr3+ crystals

A complex study of the energy spectra and relaxation channels for the excitation energy of activation centers in Y2SiO5:Pr3+, Lu2SiO5:Pr3+, and Gd2SiO5:Pr3+ was performed. An analysis of the low-temperature optical spectra showed that the energy parameters and the character of field splitting of the 1D2 and 3H4 activator ion terms were substantially different in crystals of different crystallographic types. The pseudosymmetry effect was observed in splitting of the 1D2 and 3H4 terms of Pr3+ ions situated in nonequivalent crystal lattice cation sites of Y2SiO5 and Lu2SiO5. Activator ions nonuniformly populated nonequivalent cation sites of the Y2SiO5 crystal lattice. At high activator ion concentrations (>1 at %), luminescence decay in Y2SiO5 could not be described by a simple exponential time dependence. The complex luminescence decay law was caused by activator ion excitation energy migration and capture by acceptors. The role of energy acceptors was played by activator ion dimers.

Superradiant emission of LaF3:Pr3+ in resonator

Experimental data concerning superradiant emission from the LaF3 medium doped with impurity praseodymium ions are presented in the cases of a free medium and when the medium is placed into an optical resonator. The spike structure of superradiance is registered and studied. When the medium is placed inside a cavity, a new channel of energy removal by superradiance related to the cavity mode appears; the old noncavity channels are preserved. The duration of superradiance in the cavity channel is decreased, and regular modulation arises. These peculiarities of superradiance induced by the presence of an optical resonator are explained.

Selective spectroscopy of Pr3+ impurity ions in Y2SiO5, Gd2SiO5, and Lu2SiO5 crystals

Based on a study of the low-temperature optical spectra of Pr3+ activator ions in Y2SiO5, Gd2SiO5, and Lu2SiO5 crystals, it is shown that the parameters and character of the crystal-field splitting of the 1D2 and 3H4 terms of the impurity ions are substantially different in crystals belonging to different crystallographic types. In Y2SiO5 and Lu2SiO5 crystals a pseudosymmetry effect is observed in the splitting of the 1D2 term for ions localized in inequivalent cation sites. The activator ions nonuniformly occupy the inequivalent cation sites as their concentration is increased. At high concentrations of activator ions (∼1 at. %) the optical absorption spectra exhibit spectral lines belonging to dimers of activator ions.

Anomalies in the concentration quenching of luminescence in doped Y2SiO5:Pr3+ nanocrystals

In Y2SiO5:Pr3+ nanocrystals, an ordered phase is observed in the 1D2 luminescence decay curves of Pr3+ ions at their anomalously low concentration (0.5 at %). This effect is caused by the predominant accumulation of activator ions near the nanocrystal surface, which provides relaxation of the elastic strains arising as a result of the misfit between the ionic radii of Pr3+ and Y3+. The concentration quenching of Pr3+ luminescence is due to cooperative cross relaxation.

Concentration quenching anomalies of activated Y2SiO5:Pr3+ nanocrystal luminescence

For the fist time in Y2SiO5:Pr3+ nanocrystals, the ordered stage in the 1D2 luminescence decay curves for Pr3+ ions has been observed at anomalously low doped ion concentration (0.5 at %). This effect is caused by preferred location of the activator ions in the near-surface layer of the nanocrystal that provides the relaxation of elastic tension arising due to the difference of ionic radii of Pr3+ and Y3+ ions. Concentration quenching of Pr3+ luminescence is caused by the cooperative cross-relaxation.