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Zero-phonon electronic transitions of Cr3+ impurity ions in solid solutions and nanocrystals are studied. In the series of Lu3xY3-3xAl5O12 (0

The impact of the surrounding gaseous media on optically excited fluorescence of YAG:Ce nanoparticles is examined. The results have shown that the observed effects have resulted from gas-molecule-assisted electron transfer between the nanoparticles and from modification of the surface of the particles by adsorption of gas molecules.

Abstract: Excitation of YAG:Ce3+ crystals and nanocrystals was performed at λ exc =473–584nm over a wide temperature range. It was observed that the luminescence of both nano- and single-crystal YAG:Ce samples is efficiently excited with photon energies well below the Ce3+ absorption band and at least 1650cm−1 below the ZPL of the 4f1(2F5/2)−4f05d1 transition, located at 489nm. The studies of Ce3+ fluorescence spectra as a function of temperature and excitation wavelength and of their temporal evolution point to the role of phonon-assisted nonradiative energy transfer between different groups of Ce3+ centers in the excitation mechanism. [Copyright &y& Elsevier]

Abstract: Sol–gel technology-produced MgO:Cr3+ and MgO:Mn4+,Li+ nanocrystals (particle size ~5nm) were studied with laser spectroscopy techniques. The low-temperature fluorescence line narrowing (FLN) experiments were performed for the inhomogeneously broadened R-line of cubic Cr3+ centers. The observed spectra and their dependence on excitation wavelength are significantly different from that reported in literature for the bulk single crystals, and thus demonstrate specific inhomogeneous broadening mechanisms in nanocrystals. The nature of inhomogeneous broadening of spectral lines of impurity ions in nanocrystals is discussed. The site-selective spectroscopy of MgO:0.02%Mn,0.4%Li nanocrystals (Li added for charge compensation) showed that no lines in the emission spectra can be ascribed to Mn4+ centers with cubic symmetry. [Copyright &y& Elsevier]

Abstract: The fluorescence spectra of nanocrystalline powder samples of Sc2O3:Ce3+ prepared by the sol–gel process were studied. The strong shift of the fluorescence spectra with the excitation wavelength was observed. The effect of the surrounding gaseous medium on 4f05d1→4f1 fluorescence of Ce3+ ions in nanoparticles, discovered earlier in YAG:Ce3+, was also observed and studied in Sc2O3:Ce3+. The fluorescence intensity and the excitation power dependence of fluorescence dramatically depend on the pressure of the gaseous media. [Copyright &y& Elsevier]

The temperature dependencies of the widths of R1(E¯→4A2) and R2(2A¯→4A2) lines of Cr3+ and Mn4+ in ferroelectric Li2Ge7O15 (LGO) crystals were measured. It was observed that at low temperatures R2(2A¯→4A2), lines are significantly broader than R1(E¯→4A2) lines. The effect may be explained by a fast (∼ps) one-phonon 2A¯→E¯ relaxation. The unusually large direct processes rates within 2E states may be explained by the specific vibrational spectrum of LGO, in which low-frequency optical vibrations are present, including the soft modes responsible for phase transition. The contribution of these modes to the vibrational density of states at frequencies close to 2A¯−E¯ splitting Δ is observed in the vibronic sidebands of Cr3+ R1-lines. [Copyright &y& Elsevier]

We report optical studies of the new material – Cr3+-doped lithium–germanate glass, containing Li2Ge7O15 (LGO) nanocrystalline particles. While only broadband 4T2–4A2 fluorescence from the low-field octahedral Cr3+ sites was observed from Cr3+ ions in the glass, in LGO nanocrystals high-field Cr3+ centers emit 2E–4A2 (R–lines) fluorescence. The process of crystallization in the course of isothermal annealing of glasses was monitored spectroscopically and the nucleation of LGO crystallites was observed starting from the smallest clusters. Using the 2E–4A2 fluorescence spectra it is possible to detect the ferroelectric phase transition in LGO:Cr3+ nanocrystals, whose critical temperature was found to be similar to that of the bulk crystals. Long-lived spectral holes were burned in the inhomogeneously broadened R-lines of Cr3+ in LGO nanocrystals at low temperatures. The linear temperature dependence of hole widths shows that the homogeneous broadening of 4A2–2E transitions of Cr3+ in nanocrystals is due to interaction of Cr3+ electronic levels with the two-level systems (TLS) of the surrounding glass. The range of the Cr3+-TLS elastic dipole–dipole interaction is estimated. [Copyright &y& Elsevier]

Abstract: The results of optical studies of Cr3+ and Eu3+ ions doped nanocrystalline ferroelectric BaTiO3 produced by the sol–gel process (particle size 20–40nm) are reported. The Cr3+ impurity ions 2E–4A2 (R-lines) fluorescence spectra in BaTiO3 revealed significant differences from that reported in literature for the bulk material. At least three types of Cr3+ centers were found in the spectra. The temperature dependence of optical second harmonic generation in nanocrystalline BaTiO3: Eu3+ shows a strong hysteresis in C4v–Oh ferroelectric phase transition region, which was explained by the ordering effects in the system of electric dipole moments of dipole nanocrystals. The temperature dependencies of radiative lifetime of Eu3+ 5D0 excited level reveal some hysteresis too. The possibility of influence of the ordering in the system of BaTiO3:Eu3+ nanocrystals on the effective index of refraction of the medium and thus on the Eu3+ 5D0 radiative lifetime, due to the local field effects is discussed. [Copyright &y& Elsevier]