A. Watterich

Ion beam irradiated channel waveguides in Er3+-doped tellurite glass

Erbium-doped tellurite glasses are of great interest for the fabrication of active integrated circuits because of their unique properties in terms of bandwidth and rare earth solubility. The fabrication of multimode channel waveguides in a glass of this family, namely, a sodium-tungsten-tellurite glass, is demonstrated using a high-energy ion beam irradiation technique. Nitrogen ions with dose of 1.0×1016ions∕cm2 and 1.5MeV energy were used for this aim. The waveguiding effect was investigated using the end-fire coupling technique.

Investigation of Cu-doped Li2B4O7 single crystals by electron paramagnetic resonance and time-resolved optical spectroscopy

A low-temperature study of the thermoluminescent dosimeter material, lithium tetraborate (Li2B4O7) doped by Cu, has been carried out by the methods of electron paramagnetic resonance (EPR) and time-resolved polarization spectroscopy using 4–20 eV synchrotron radiation and 1 µs Xe flash lamp pulses in the region 3–6 eV. The observed EPR spectra of an unpaired hole with strong d-character and characteristic hyperfine splittings can be ascribed to Cu2+ substituted at a Li lattice site and displaced due to relaxation. The results on the Cu+-related luminescence strongly support the conclusion about a low-symmetry position of copper impurity ions in the lithium tetraborate lattice. The temperature dependence of the decay kinetics of the Cu+-related 3.35 eV emission indicates a triplet nature for the relaxed excited state of the Cu+ centres. An off-centre position of the Cu+ ion in the relaxed excited state is suggested.

Spectroscopic properties of ZnWO4:Fe single crystals

Abstract The incorporation and charge of iron in ZnWO4:Fecrystals were investigated by absorption spectrometry. The UV bands at 350 nm and the visible band at 460 nm have been found to be related to Fe3+ and Fe2+ ions respectively. Both ions are mainly in substitutional Zn positions. The infrared absorption at 3445 cm−1 is related to Fe3+ OH complexes. The OH− ions compensating the excess charge of Fe3+ are probably situated in interstitial positions.

Electron spin-resonance (ESR) and electron-nuclear double-resonance (ENDOR) study of the self-trapped hole in ZnWO4 single crystals

After x-ray irradiation at 20 K, an intrinsic O- centre was identified by ESR and ENDOR spectroscopy as the self-trapped hole centre in ZnWO4. Observation of one Zn and two strong W superhyperfine interactions allows us to distinguish between two possible trapping sites: the hole resides at the B-type oxygen position which has one Zn and two W nearest neighbours. Broadening of the ESR lines and averaging of the g-value is observed and explained as due to thermally activated hopping of the hole between two energetically equivalent oxygen positions. The activation energy of this reorientation is found to be 0.016±0.003 eV. The thermal decay of the intrinsic O- centre, and its connection to thermoluminescence, has been studied; it shows that this centre cannot be the luminescence centre for the typical TL emission at ~480 nm in ZnWO4. This emission may be due to an intrinsic electron-type defect.

Different incorporation of Cu+ and Cu2+ in lithium tetraborate single crystals

The incorporation site of Cu in the scintillator and thermoluminescent dosimeter material lithium tetraborate Li2B4O7:Cu has been shown to depend on the charge state of the dopant. As confirmed by a refined analysis, the EPR spectra of an unpaired hole can be ascribed to Cu2+ situated near the Li lattice site with C1 symmetry, in contrast to the higher (C2) overall symmetry reported earlier for Cu+ in its relaxed excited state, the latter suggesting interstitial incorporation for the monovalent dopant. Consequences for charge transfer processes involving copper ions are discussed.

Colouration, impurities and non-local charge-compensation in ZnWO4

Abstract The characteristic pink colour of undoped ZnWO 4 single crystals having minimal Fe 2+ concentration is attributed primarily to Cr 3+ impurity ions. Other suggested sources of this colour, namely OH - ions, W 5+ type colour centres or oxygen vacancies are ruled out. The Me 5+ type dopants which enhance the crystals transparency in the visible region are shown to be charge-compensated non-locally by Fe 3+ ions.

Ti3+ centres in reduced ZnWO4: Ti single crystals

Abstract Reduced ZnWO 4 :Ti single crystals were studied by ESR and ENDOR methods, which permitted characterization of a new Ti 3+ centre in C 1 symmetry (labeled as Ti 3+ H). The Ti 3+ ion substituted for W and showed superhyperfine interactions with two neighbouring W ions and an H impurity ion. A more detailed study of the Ti 3+ ( C 2 ) centre, whose g values were previously published, was also achieved. The hyperfine tensor for interactions of the 3 d 1 electron with the nuclear magnetic moments of 47 Ti and 49 Ti were determined.

ESR of W5+-H centers in γ- or UV-irradiated ZnWO4 single crystals doped by Li

Abstract Gamma of UV (254 nm) irradiation of an “as-received” ZnWO 4 :Li crystal, or, UV (366 nm) bleaching of a reduced crystal, produces a W 5+ paramagnetic defect for irradiation at 77 K. The ESR spectra indicate C 1 symmetry for this center. It is characterized by a strong 183 W hyperfine interaction as well as weaker 183 W, 1 H and 67 Zn superhyperfine interactions, permitting the development of a defect model. The defect is designated as the W 5+ H center. It is related to a Mo impurity center reported in reduced ZnWO 4 crystals, although both its production and stability are different.

Fe centers and charge compensation in ZnWO4 single crystals characterized by ESR and i.r. spectroscopy

Abstract In ZnWO 4 :Fe one of the numerous Fe 3+ centers with reduced symmetry ( C 1 ) was identified as an Fe 3+ ion compensated probably by a Zn 2+ vacancy not very close to the Fe 3+ ion. The lack of a proton superhyperfine interaction of the different Fe 3+ complexes with C 1 symmetry and the missing correlation between the absorption coefficient of the i.r. band at 3445 cm −1 and the ESR intensities of the Fe 3+ complexes leads to a modified model for the i.r. active center, namely an OH − ion substituting for O 2− associated with a Zn vacancy and designated as v − OH .

Slab optical waveguides in Er3-doped tellurite glass by N + ion implantation at 1.5 MeV

Slab optical waveguides were fabricated in tung-sten-tellurite glass doped with Er3 + ions by means of nitrogen ion implantation at 1.5 MeV. A wide range of ion doses (from 5·1012 to 8·1016 ions/cm2) was used. Optical characterization, performed by dark-line spectroscopy, revealed that the waveguides were of optical barrier type: the implanted layer exhibited a decrease of the refractive index with respect to the virgin bulk glass, while the region comprised between the sample surface and the end of the ion track acted as an optical guiding structure. It was also demonstrated that a post-implantation annealing process, performed at various temperatures on the samples implanted at higher doses, contributes to the reduction of the barrier region.