Optical Active Centers Research Articles

Near-infrared laser operation of Cr^3+ centers in chromium-doped LiInGeO_4 and LiScGeO_4 crystals

Laser action has been demonstrated in two new tunable Cr3+-doped crystals. Absorption, emission, and excitation measurements for various crystal orientations have been performed for chromium-doped LiInGeO4 and LiScGeO4 single crystals. From the spectroscopic measurements, two optical active centers were identified: tetrahedrally coordinated Cr4+ and octahedrally coordinated Cr3+. Gain-switched, tunable laser operation of Cr3+ centers has been demonstrated in both crystals. An ultrawide tuning range from 1150 to 1480 nm for LiInGeO4 and 1220-1380 nm for LiScGeO4 has been achieved. To our knowledge this is the widest bandwidth and the most near-infrared shifted wavelength range for lasing ever demonstrated for Cr3+ ions.

Process intensification with biocatalysts: dynamic kinetic resolution and fluorous phase switch with continuous extraction

Kinetic resolution of racemates with the help of enzymes has become a widely adopted method for the synthesis of chiral intermediates and compounds with biological activity. However, the inherent limitation to 50% yield of any one stereoisomer restricts the use of this method in industrial practice, when only one of the stereoisomers is required. The yield can theoretically be improved to 100% through either a chemical reaction, which inverts the stereochemistry at the optical active center, or through racemization of the unwanted isomer, followed by additional enzymatic resolution. The combination of the racemization of the slower reacting stereoisomer with kinetic resolution in a one-pot reaction is termed dynamic resolution. We describe the dynamic resolution of secondary alcohols through enzymatic stereoselective transesterification and heterogeneously catalyzed racemization of the alcohol over several zirconia-containing catalysts. In order to facilitate the separation of the products, we used a fluorous phase-switching technique coupled with fluorous extraction. The continuous extraction in a membrane contactor allows for facile recovery of the fluorous tagged species in a scaleable operation. The unoptimized scheme offers over 90% conversion with ca. 75% enantiomeric excess (e.e.).

Green, red and infrared Er-related emission in implanted GaN:Er and GaN:Er,O samples

Er-related luminescence near 1.54 μm (∼805 meV) is observed under below band gap excitation at 4.2 K in GaN:Er and GaN:Er,O implanted samples. The spectrum of the recovered damage samples is a multiline structure. So far, these lines are the sharpest ones reported for GaN. Well-resolved green and red luminescences are observed in implanted samples. The dependence of luminescence on the excitation energy as well as the influence of different nominal fluence and annealing conditions is discussed. Combining the results obtained from photoluminescence and Rutherford backscattering spectrometry, different lattice sites for the optical active Er-related centers are identified.

Confinement induced enhancement of the emission in Er-implanted Si/SiO 2 quantum wells fabricated on SOI substrates

Single Si/SiO 2 quantum wells were fabricated using silicon on insulator (SOI) substrates, the upper barrier being a thin thermally oxidized Si layer. Samples were Er and O implanted at low energy in order to center their concentration profile in the well, and then annealed. A bulk Si sample implanted under the identical conditions was used as a reference. We present low temperature photoluminescence results using UV excitation in order to create photogenerated carriers mainly in the Si well. We show that the 1.54-μm photoluminescence is much more intense for Er in a well than for Er in implanted bulk silicon. We show that infrared emission occurs through an energy transfer mechanism from Si to Er centers. Confinement prevents the escape of photogenerated carriers towards the substrate and promotes energy transfer towards optical active erbium center.

Cathodoluminescence of crystalline and amorphous SiO 2 and GeO 2

Cathodoluminescence (CL) and its temperature-dose behaviour are presented for different crystalline and amorphous modifications of SiO 2 and GeO 2 as well as for Ge-doped SiO 2 layers. The crystalline samples include four-fold coordinated Si and Ge in hexagonal quartz and quartz-like crystals, respectively, as well six-fold coordinated atoms in tetragonal rutile-like crystals. The detected luminescence bands, in general, are attributed to three optical active luminescence centres: the two-fold coordinated silicon (=Si:) and germanium (=Ge:) centre, respectively, the non-bridging oxygen hole centre (NBOHC) and the self trapped exciton (STE). The first ones, the oxygen deficient centres (ODC), are especially developed in both, in the tetragonal crystal rutile-like modifications as well as in glassy states. The huge violet luminescence in Ge-implanted SiO 2-layers is attributed to the two-fold coordinated Ge in the silica matrix.

Spectroscopic properties of Cr-doped Sc 2O 3

Cr-doped Sc 2O 3 crystals were grown and investigated spectroscopically. Octahedrally coordinated Cr 3+ was observed in the crystals with crystal field parameters of D q=1490 cm −1, B=590 cm −1, C=3227 cm −1, thus D q/ B=2.53, and C/ B=5.47. The Cr 3+ emission covers the spectral region between 700 and 1100 nm exhibiting a double exponential decay with lifetimes of 66 and 23 μs at room temperature. An additional optical active center correlated with chromium is also observed. This center exhibits strong absorption between 750 and 1100 nm and emission between 1050 and 1800 nm.

γ-ray-induced bleaching in silica: Conversion from optical to paramagnetic defects

We report experimental results on optical and ESR measurements performed in $\ensuremath{\gamma}$-irradiated natural silica samples having different content of OH groups. A partial bleaching of the optical absorption band ${B}_{2\ensuremath{\beta}}$ at 5.15 eV and the related photoluminescence emissions at 3.1 eV and 4.2 eV is observed together with the growth of an ESR doublet split by 11.8 mT. The kinetics of the two processes as a function of the $\ensuremath{\gamma}$ dose are correlated and depend on the OH content. Our experiments indicate the occurrence of a $\ensuremath{\gamma}$-ray-induced conversion, from optically active centers to paramagnetic ones and vice versa, changing the relative concentration of the two defects until a steady state is reached. A conversion model is proposed in which $\ensuremath{\gamma}$-ray-activated H atoms either are trapped in optically active defects, giving rise to paramagnetic ones, or interact with the last ones, restoring the original optical centers. Our results are consistent with a structural model in which the optical active center is a twofold coordinated Ge and the 11.8 mT doublet originates from a $\mathrm{O}={\mathrm{Ge}}^{\mathit{\ensuremath{\cdot}}}---\mathrm{H}$ center.

Optical Er-doping of Si during sublimational molecular beam epitaxy

We report the first application of sublimation molecular beam epitaxy to grow uniformly and selectively doped Si : Er layers with a high concentration of Er and O (up to ∼10 19 cm −3) and high quality of crystalline structure. All samples exhibit photoluminescence close to 1.54 μm up to 200 K. New optical active Er center with their main lines at 6502, 6443, 6393, 6342, 6337 and 6268 cm −1 was observed in the PL spectra.

Cathodoluminescence of Quartz, Silica and Thin Amorphous SiO2 Layers

ABSTRACTCathodoluminescence and its temperature-dose behaviour of different modifications of SiO2 are presented. The detected luminescence bands are attributed to three optical active luminescence centers: the twofold coordinated silicon center (=Si:), the non-bridging oxygen hole center (NBOHC) and the self-trapped exciton (STE). The experiments are correlated with infrared absorption measurements. Existing structural models are discussed with reference to our results.

A review of recent contributions on biologically active products of arachidonate conversion

Leukotrienes (LTs) C 4, D 4 and E 4, the recognized components of slow reacting substance of anaphylaxis (SRS-A), have previously been shown to have contractile activities for guinea pig pulmonary and ileal smooth muscles; LTB 4 has been shown to possess chemotactic activity for neutrophils in vitro. Based on data obtained by the use of structural analogs of the SRS-A LTs and LTB 4, we have recently determined a number of the structural bases for the biological function of each moiety. With regard to the SRS-A leukotrienes, analogs differed from the native structures in the position of the peptide side chain and/or the hydroxyl group, the number and position of ethylenic bonds, the chirality at optically-active centers, or the structures of the four polar substituents in the C-1 to C-6 region. Analogs of LTB 4, differing in the stereochemistry of their ethylenic bonds, were evaluated for chemotactic activity both in vitro, using human neutrophils, and in vivo intracutaneously in the rhesus monkey. We propose that true receptors exist on the pulmonary parenchyma of the guinea pig for the SRS-A LTs and on the primate neutrophil for LTB 4. Further, LTC 4, LTD 4 and LTE 4 have been shown to elicit a wheal and prolonged flare in human skin, whereas LTB 4 evokes a time-dependent induration. The interaction of these secondary mediators may be critical to a fully developed host inflammatory response to both immunologic and non-immunologic injury.

Investigation of optical active centres of the irradiated lithium niobate

This article is devoted to the investigation of optical active centres (o.a.c.) in mono-domain crystals of LiNbO 3. O.a.c. were formed by γ- and electron radiations and detected by spectrophotometric analysis, thermoluminescence (TSL) and optically in the microsecond region. Several types of stable and short-lived colour centres accumulated in the crystal lattice during γ- and e-radiation and these are distributed over a wide interval of energies. From our results we could pick out the next three types of centres: (I) the short-lived centres ( γ max =650 nm and τ 1 2 ≈ 10×10 -6)s) ; (II) the colour-centres with τ 1 2 =(2.4−18.6)×10 -3s and TSL max -77° C; (III) centres stable at room temperature but which annealed at t=350–400° C. These centres had some component of optical density at 360–600 nm with γ max =390, 470 and 520 nm. The presence of optical centres with such different half-lives (from 10 μs to stable at room temperature) allows us to assume a cascade mechanism for the formation of these centres in single crystals of lithium niobate.