S. P. Feofilov

Radiative transitions and spectral-hole burning in MgO: Cr3+ nanocrystals

Luminescence spectra of Cr3+ ions in nanocrystalline MgO obtained by sol-gel technology were studied. The radiative lifetime of excited 2E states of cubic and tetragonal Cr3+ centers was found to be considerably longer than that of bulk single crystals. This effect is caused by a modified effective refractive index of the inhomogeneous medium. Burning of long-lived spectral holes in the 4A2-2E transition profile of cubic centers was observed, which sets nanocrystalline samples strongly apart from bulk crystals, where no hole burning was found.

Selective laser spectroscopy of RE3+ and Mn4+ in sol-gel technique produced Al2O3

Sol-gel produced nanocrystalline highly porous transparent y-Al 2 O 3 doped with RE and TM ions and α-Al 2 O 3 ceramics, produced by its annealing, were studied with selective laser spectroscopy techniques. The site-selective spectroscopy of RE 3+ ions in α-Al 2 O 3 reveals that these ions form centers in corundum with one dominating type. The FLN studies of homogeneous line widths of 5 D 0 - 7 F 0.1 transition in γ- and α-Al 2 O 3 extend the low-temperature hole-burning data (Feofilov et al., Phys. Rev. B 54 (1996) 3690). It is shown that Mn 4+ -doped Al 2 O 3 is potentially effective photochemical hole-burning material and self-gated persistent hole-burning in α-Al 2 O 3 : Mn was demonstrated.

Fluorescence spectra and homogeneous line widths of Cr3+ in glass nucleating ferroelectric Li2Ge7O15

Abstract We report optical studies of the new material – Cr 3+ -doped lithium–germanate glass, containing Li 2 Ge 7 O 15 (LGO) nanocrystalline particles. While only broadband 4 T 2 – 4 A 2 fluorescence from the low-field octahedral Cr 3+ sites was observed from Cr 3+ ions in the glass, in LGO nanocrystals high-field Cr 3+ centers emit 2 E– 4 A 2 (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 2 E– 4 A 2 fluorescence spectra it is possible to detect the ferroelectric phase transition in LGO:Cr 3+ 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 Cr 3+ in LGO nanocrystals at low temperatures. The linear temperature dependence of hole widths shows that the homogeneous broadening of 4 A 2 – 2 E transitions of Cr 3+ in nanocrystals is due to interaction of Cr 3+ electronic levels with the two-level systems (TLS) of the surrounding glass. The range of the Cr 3+ -TLS elastic dipole–dipole interaction is estimated.

Luminescence and laser spectroscopy of highly porous γ-Al2O3 doped with 3d- and 4f-ions: Effects of spatial phonon confinement

Abstract Inhomogeneously broadened luminescence spectra of Cr3+, Eu3+ and Pr3+ ions in sol-gel-grown monolithic highly porous γ-Al2O3 were observed. In FLN and HB experiments, the interaction of ions with the size-resonant terahertz acoustic vibrations of crystalline nanoparticles which form the porous network was studied. The incorporation of Eu3+ and Pr3+ into α-Al2O3 (corundum) lattice is demonstrated in annealed (T > 1200 °C) sol-gel-prepared samples.

Optical generation of nonequilibrium terahertz resonant vibrational excitations in highly porous aluminium oxide

Abstract The dynamics of terahertz phonons generated at liquid helium temperature by laser excitation in sol-gel technology grown transparent highly porous crystalline γ-Al2O3 samples, was studied through the phonon-induced anti-Stokes FLN spectra of Cr3+ impurity ions (2E-4A2 lines). The generated phonon spectrum contains essentially a nonequilibrium narrow peak around ω ∼ 0.6 THz (20 cm−1) with high occupation numbers n ω and decay time ~ 1 ms besides and a slowly cooling (∼tens of ms) thermalized Planckian distribution. The frequency (20 cm−1) of these long-living nonthermalized vibrations is consistent with the lowest acoustic, size-resonant surface mode of γ-Al2O3 spherical nanoparticles with 80 A diameter, which form the network of the sample. Discussed are the nature of the long lifetime and the possible role of these surface modes in the spectral dynamics of optically generated phonons.

Luminescence of Cr3+ impurity ions in Li2Ge7O15 nanocrystals and clusters embedded in lithium germanate glasses

The properties of Cr3+-doped Li2Ge7O15 (LGO) lithium germanate nanocrystals produced in lithium germanate glasses under isothermal heating were studied. The samples were characterized by x-ray diffraction and small-angle scattering, as well as by transmission electron microscopy. The luminescence spectra of the impurity chromium measured in lithium germanate glasses containing LGO crystals revealed transitions in Cr3+ ions residing in the glass phase and in LGO crystallites starting from extremely small clusters. This provided the possibility of following the process of crystallization of the lithium germanate glass from Cr3+ luminescence spectra. The effects observed in the Cr3+ luminescence spectrum revealed a ferroelectric phase transition in LGO nanocrystals embedded in the glass.

Luminescence of lead thiogallate single crystals doped with cerium ions

The luminescence of single crystals of lead thiogallate PbGa2S4: Ce3+ has been studied. The observed nonmonotonic temperature dependence of the integrated luminescence intensity of PbGa2S4: Ce3+ has been explained by the energy transfer from the electronic excitations of the matrix to impurity ions and by the temperature quenching of the Ce3+ ion luminescence. The observed structure of the 4f05d1-4f1 (2F7/2) band of the Ce3+ ion luminescence indicates the existence, at least, two types of Ce3+ ion centers in PbGa2S4.

Optical second harmonic generation study of the ferroelectric phase transition in crystals and nanocrystals of lithium heptagermanate

The D2h-C2v ferroelectric phase transition in crystals of lithium heptagermanate Li2Ge7O15 was revealed by the optical second harmonic generation technique. The phase transition temperature Tc was observed to shift from 10 to 6°C when the samples were doped with chromium (0.05%). No second harmonic signal was detected in the ferroelectric phase of nanocrystalline Li2Ge7O15 samples.

Luminescence of yttrium–aluminum garnet crystals with Eu2+ impurity ions

Luminescence of yttrium-aluminum garnet Y3Al5O12: Eu,Si samples is studied. It is found that luminescence is associated with Eu2+ ions, but does not correspond to intracenter 4f65d1–4f7 transitions. Measurements of the excited state lifetime point to the luminescence mechanism involving charge-transfer states.