Nanocrystalline BaTiO3, doped with Eu and Cr: Fluorescence spectroscopy and spontaneous ordering effects

The results of optical studies of Cr³⁺ and Eu³⁺ ions doped nanocrystalline ferroelectric BaTiO₃ produced by the sol-gel process (particle size 20-40 nm) 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 C_4v-O_h 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.     

Optical characterization of synthetic opals

The results of a structural-optical characterization of synthetic opals are presented. Information on the growth-induced features of the opal structure was derived from an analysis of the position and width of the one-dimensional photonic band gap. The structure of the samples was found to vary substantially along the growth axis coinciding with the [111] direction of the fcc lattice. It was shown that the regions corresponding to early stages in the opal structure growth are typically strongly disordered, which manifests itself, in particular, in the crystallites being misoriented relative to the sample growth axis. It was concluded that the regions of synthetic opals most suitable for application as photonic crystals are those corresponding to later growth stages.     

Phonon pulse detection in germanium and silicon with a fluorescence contact-type phonon spectrometer

Laser pulse produced nonequilibrium acoustic phonons in Ge and Si single crystals have been detected at several phonon frequencies in the THz range by the fluorescence from an insulting YF₃:Tn³⁺ film on the sample surface.     

Luminescence of excitons in slightly asymmetric double quantum wells

We report the first studies of exciton luminescence spectra from asymmetric double quantum wells (DQWs) of very similar width. The DQWs were of GaAs/AlGaAs and the differences in widths of the coupled wells were one or two monolayers. The coupled direct and indirect exciton states anticross with a resonance splitting of 1.33 meV. An additional luminescence line appearing at low temperatures is identified as a localized indirect exciton. Fiz. Tverd. Tela (St. Petersburg) 39, 735–739 (April 1997)     

Complex interaction of polarized light with three-dimensional opal-based photonic crystals: Diffraction and transmission studies

Polarization characteristics of light interaction with the photonic crystal of a-SiO₂ synthetic opals were studied under the conditions of low dielectric contrast. We analyzed 3D diffraction patterns of monochromatic light and calculated optical transmission spectra of oriented samples. The diffraction patterns are found to change with the polarization of incident light, indicating a strong polarization dependence of photonic stop bands in synthetic opals. It is shown theoretically there exists a critical angle, θ_c, of the p-polarized light incident on the (h k l) crystal plane, at which the resonance contribution to Bragg diffraction vanishes.     

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

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.     

Optical characterization of Cr3+ centers in LiNbO3

Low-temperature luminescence spectra of stoichiometric Cr:LiNbO₃, congruent Cr:LiNbO₃ and congruent Cr,Mg:LiNbO₃ were studied. Dominant low-field and minor high-crystal-field optical centers are the Cr³⁺ impurity ions that preferentially occupy Li⁺ sites (Cr_Li) in the Cr:LiNbO₃ crystals. Low-field centers related to Cr³⁺ substitution of Nb⁵⁺ (Cr_Nb) occur in addition to Cr_Li in co-doped Cr,Mg:LiNbO₃ samples. Application of high hydrostatic pressure leads to the transformation of dominant Cr³⁺ centers from low- to high-field type due to strong pressure-induced blue shift of the ⁴ T ₂ state, resulting in its crossing with the ² E state of Cr³⁺. This level-crossing effect was observed for the dominant Cr³⁺ _Li and Cr³⁺ _Nb centers at pressures that correlate well with estimations based on the ⁴ T ₂-² Eenergy gap (230 cm^-1 and 1160 cm^-1) and on the rate of their pressure-induced change (14.35 and 11.4 cm^-1/kbar, respectively). We also studied inhomogeneous broadeningof the ² E?⁴ A ₂transitions at ambient pressure for the minor high-field “defect” Cr³⁺ _Li centers in congruent LiNbO₃. A fine structure in the spectral response of these centers was observed. The obtained results are discussed on the basis of a microscopic hierarchic model for perturbed Cr³⁺ ions in the LiNbO₃ lattice. Received: 25 June 2001 / Published online: 2 November 2001     

Ferroelectric-specific stark effect in stoichiometric LiNbO3:Fe at room temperature

By means of EPR spectroscopy of LiNbO(3):Fe at room temperature (RT) it is shown that the Stark effect in ferroelectric crystals can be different from that observed in other materials. Novel properties appear when an external E field reverses the direction of the spontaneous polarization, the direction of the linear Stark shift stays the same with a reversal of the E field. The corresponding spectral line shifts can occur over a long time scale (hours). These properties seem to be a general feature of the Stark effect in ferroelectric crystals when the external E field exceeds the coercive field.     

Radiative transitions and spectral-hole burning in MgO: Cr<Superscript>3+</Superscript> 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.     

Optical spectroscopy of opal matrices with CdS embedded in its pores: Quantum confinement and photonic band gap effects

Il Nuovo Cimento D - The spectra of transmission and reflection of synthetic opal which has 3-dimensional periodic structure were measured at different orientations of incident beam relative to the...