eV Absorption Band Research Articles

Study on the electronic structures of CsI crystal with F-type color center

The electronic structures of the perfect CsI crystal and the crystal containing iodine vacancy and F, F2 color centers are studied within the framework of the fully relativistic self-consistent Dirac-Slater theory, by using a numerically discrete variational (DV-Xα) method. Calculated results reveal that the existence of the iodine vacancy results in the band gap widening and does not introduce the new energy level in the forbidden band. However, the crystal containing F color centers and F2 color centers have their donor energy levels introduced in the forbidden band. The transition energy from energy level of F color center and F2 color center to the bottom of conduction band is 169 eV (736 nm) and 115 eV, respectively, which are consistent with the experimental results. This result indicates that the 750 nm absorption band originates from the F color center and the 11 eV absorption band originates from F2 color center.

Preparation of thick, crack-free germanosilicate glass films by polyvinylpyrrolidone and study of the UV-bleachable absorption band

With addition of polyvinylpyrrolidone (PVP) transparent, stable GeO 2–SiO 2 sols containing up to 60–80 mol% GeO 2 were synthesized using tetraethyl orthogermanate (TEOG) and tetraethylorthosilicate (TEOS) as precursors for germania and silica, respectively. It was shown by TEM analyses that the PVP can be absorbed onto the colloidal particles providing steric hindrance for the combination and aggregation of particles. These sols were observed showing rapid increase in viscosity within both the early period and the end period of sol aging time, but exhibiting a viscosity value of about 28 mPa s within the rest aging time (45–90 h) satisfying well the requirements for the deposition of thick films by cycles of dip-coating operation. It was determined by TG-DTA and SEM analyses that the densification of GeO 2–SiO 2 gel material with PVP was much more retarded than the gel without PVP resulting in crack-free germansilicate films with a thickness of 3 μm. The crystallization behavior of germansilicate films was enhanced with the increase of GeO 2 content but glass films with a composition of 60GeO 2 · 40SiO 2 was obtained by sintered at 700 °C for 1 h and annealed at 550 °C under a flowing H 2/N 2 atmosphere for 2 h. FT-IR analyse showed that the heat treatment at 700 °C for 60 min was effective to remove the organics and hydroxyl groups in the germansilicate film. An intense 5 eV absorption band was distinctly observed in films. The intensity of this absorption band was found to be effectively bleached by UV illumination. Weak photoluminescence emission bands which originated from the neutral oxygen di-vacancy (NODV) were detected near 375 and 276 nm. Therefore, the 5 eV absorption band observed in this work was mainly caused by the neutral oxygen monovacancy (NOMV). A saturated absorptivity change of the UV-bleachable band after prolonged illumination was found to be 256 cm −1 for the 60GeO 2 · 40SiO 2 films implying the NOMV concentration in the films reached about 3.8 × 10 18 cm −3.

The role of Pb2+ ions in the luminescence of LuAG:Ce single crystalline films

AbstractSteady‐state and time‐resolved emission and excitation spectra and luminescence decay kinetics were studied at 4.2‐350 K under excitation in the 2.5‐15 eV energy range for the undoped and Ce3+‐doped Lu3Al5O12 (LuAG) single crystalline films grown by Liquid Phase Epitaxy method. The spectral bands arising from the single Pb2+‐based centers were identified. Competition between Pb2+ and Ce3+ ions in the processes of energy transfer from the LuAG crystal lattice was evidenced. Due to overlap of the 3.61 eV emission band of Pb2+ centers with the 3.6 eV absorption band of Ce3+ centers, an effective energy transfer from Pb2+ ions to Ce3+ ions occurs, resulting in the appearance of slower component in the luminescence decay kinetics of Ce3+ centers and decrease of the Ce3+‐related luminescence intensity. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence characteristics of Pb 2+ centres in undoped and Ce 3+-doped Lu 3Al 5O 12 single-crystalline films and Pb 2+→Ce 3+ energy transfer processes

At 4.2–350 K, the steady-state and time-resolved emission and excitation spectra and luminescence decay kinetics were studied under excitation in the 2.5–15 eV energy range for the undoped and Ce 3+-doped Lu 3Al 5O 12 (LuAG) single-crystalline films grown by liquid phase epitaxy method from the PbO-based flux. The spectral bands arising from the single Pb 2+-based centres were identified. The processes of energy transfer from the host lattice to Pb 2+ and Ce 3+ ions and from Pb 2+ to Ce 3+ ions were investigated. Competition between Pb 2+ and Ce 3+ ions in the processes of energy transfer from the LuAG crystal lattice was evidenced especially in the exciton absorption region. Due to overlap of the 3.61 eV emission band of Pb 2+ centres with the 3.6 eV absorption band of Ce 3+ centres, an effective nonradiative energy transfer from Pb 2+ ions to Ce 3+ ions takes place, resulting in the appearance of slower component in the luminescence decay kinetics of Ce 3+ centres and decrease of the Ce 3+-related luminescence intensity.

Ab initio calculations of the oxygen–vacancy dipoles and M centers in CaF 2

We performed ab initio calculations of complex defects in CaF 2 namely oxygen–vacancy (OV) dipoles and F 2 centers (M centers) by means of the hybrid B3PW method, in which Hartree–Fock exchange is mixed with density functional theory (DFT) exchange functionals, using Becke’s three parameter method, combined with the non-local correlation functionals by Perdew and Wang. Our calculated optical absorption bands for the OV center (6.40 eV, 6.52 eV and 6.66 eV) are in good agreement with the complex experimental 6.7 eV absorption bands consisting of at least two elementary peaks with separation of 0.48 eV. The defect levels induced by the oxygen impurity between the conduction and valence bands consist mainly of oxygen p orbitals and are split into three bands. Vacancy s orbitals form the vacancy band near the fermi-energy. Several configurations of the (OV) 2-dimer were calculated. The association energy for the most favorable one is 0.55 eV. The results for the M center, the simplest aggregation of two F centers, show that the β band absorption in CaF 2 is predominantly due to the presence of M centers. The direct optical gap for M centers in CaF 2 is 2.22 eV and is in good agreement with the corresponding experimental value of 2.38 eV.

Sources of optical absorption between 5.7 and 5.9eV in silica implanted with Si or O

To determine if the only source of optical absorption between 5.8 and 5.9eV is the E′ center (absorbing at 5.85eV) two separate suites of type III silica samples were implanted, one with Si and one with O. Several ion energies were used for implantation to produce layers 600 and 570nm thick in the Si case and in the O case, respectively. Concentrations of implanted ions in the layers ranged from >0.02to<2.1at.%, with uncertainties in the concentrations less than ±5%. Optical absorption measurements were made from 2.0to6.5eV and electron spin resonance (ESR) measurements were performed at ∼20.3 and 33GHz at temperatures in the 77–100K range. Several ESR active defects were observed including the E′γ center (O vacancy), the nonbridging oxygen hole center, the peroxy radical, a possibly oxygen related center, labeled OS, and a component around g=2.0026. In the O-implantation case, by comparing the increasing optical absorption at 5.85eV with the observed decrease in the ESR E′ densities with increasing O concentration, we conclude that an oxygen related band is created and its intensity increases with increasing O concentration. In the silicon case, the optical absorption coefficient and the E′γ density as a function of Si implantation dose have a correlation coefficient that is ∼1, within reasonable errors. We conclude that in the O-implantation case the absorption between 5.8 and 5.9eV is in part due to an O related defect state in addition to the E′γ center while in the Si case it is mainly the E′ center that is responsible for the 5.85eV absorption band.

Radiation processes in oxygen-deficient silica glasses: Is ODC(I) a precursor of E′-center?

The accumulation of radiation-induced defects under non-destructive X-ray and destructive cathodoexcitation was studied in pure silica KS-4V glasses possessing an absorption band at 7.6eV. The correspondence between the existence of this band and the creation of the E′-center by radiation was checked. Detection of induced defects was accomplished by measurement of the luminescence during irradiation, post irradiation afterglow or phosphorescence, induced optical absorption, and thermally stimulated luminescence. In all samples, these observed phenomena associated with charge trapping and recombination on the oxygen-deficient luminescence center. Others centers of luminescence were not significant contributors. In some samples, the intensity of the 7.6eV absorption band was deliberately increased by treatment in hydrogen at 1200C for 100h. The intensity of luminescence in hydrogen-treated samples was smaller because of the known quenching effect of hydrogen on the luminescence of oxygen-deficient centers. The optical absorption method does not reveal an induced absorption band for the E′-center in the hydrogen-free samples with different levels of oxygen deficiency. Therefore, we did not detect the transformation of the defect responsible for the 7.6eV absorption band or the ODC(I) defect into the E′-center. In the hydrogen-treated sample, the absorption of the E′-center was detected. The E′-centers creation in the hydrogen-treated sample was associated with precursors created by hydrogen treatment (≡Si–O–H and ≡Si–H) in the glass network. The destructive e-beam irradiation reveals an increase with dose of the ODC luminescence intensity in the sample exhibiting a small 7.6eV band. That means that the corresponding luminescence centers are created. Optical absorption measurements in that case reveal the presence of E′-centers and a broad band at 7.6eV. A compaction of the irradiated volume was detected. Therefore, we conclude that the E′-center is produced by heavy damage to the glass network or by the presence of precursors.

Chromatic Dispersion in Ge-Doped SiO<sub>2</sub>-Based Single Mode Fibres due to Temperature Dependence of the Ultraviolet Absorption: Numerical and Experimental Results

The main information transmission limitation in optical communications employing SiO2 optical fibres is imposed by the optical pulses temporal broadening, resulting from the dispersive properties of the transmission medium (chromatic dispersion). Usually, this dispersion is modelled assuming a -0.4 meV/°C dependence of the 11 eV gap energy. We have monitored the 4.3 eV absorption band, with temperature and obtained a thermal dependence of -0.026 meV/°C. With this value we can predict the first and second order chromatic dispersion for a wavelength of 1550 nm.

Fabrication of sol–gel 70GeO2–30SiO2 thick films from TEOG and DEOS and investigation of the 5 eV band

Crack-free germanosilicate films, 3.6 µm-thick and containing up to 70 mol% germanium dioxide, which are inaccessible through the conventional sol–gel process, were fabricated using tetraethyl orthogermanate (TEOG) and diethylorthosilicate (DEOS) as precursors for germania and silica, respectively. The studies using viscosity, TEM and SEM revealed that DEOS contributed largely to stabilizing the GeO2–SiO2 sol and suppressing crack formation in thick films. XRD study showed that the films remained amorphous after being sintered at 600 °C in air for 60 min and annealed at 550 °C under a flowing H2/N2 atmosphere for 120 min. An intense absorption band at around 241 nm was distinctly observed in the films. The intensity of this absorption band was found to be effectively bleached by UV illumination. Weak photoluminescence emission bands which originated from the neutral oxygen di-vacancy were detected near 375 and 275 nm. Therefore, the 5 eV absorption band observed in this work was mainly caused by the neutral oxygen monovacancy. A saturated absorptivity change of the UV-bleachable band after prolonged illumination was found to be 389 cm−1 for 70GeO2–30SiO2 films.

Optical absorption properties of TiO2-doped silica glass in UV–VUV region

Optical absorption properties of TiO2-doped and non-doped silica glass (type I) were studied to investigate the effect of titanium (Ti) ion doping on absorption spectra changes produced by 7.2eV irradiation. The samples were prepared by polishing down to a thickness of about 100μm to resolve the absorption bands in the VUV range and to measure the color center created or eliminated in the irradiated surface region. For both kinds of silica glass, the magnitude of the 7.6eV absorption band of a thin sample (<100μm) decreased at the early stage of irradiation, while that of the 5.8eV absorption band increased. In the thick sample (>100μm), the 7.6eV band magnitude increased even in the early stages of irradiation. These results indicate that the type I silica glass absorbs <7eV light, which is much smaller than band gap energy of SiO2 glass, and produces Si–Si bonds. The magnitude of these changes were smaller in the TiO2-doped silica glass. The molar extinction coefficients of TiO2 in silica glass were determined in the 4.0–8.0eV range by using 47–150wtppm Ti containing samples. To avoid perturbation by absorption due to Si–Si bonds, the samples were heated at 950°C in hydrogen atmosphere. Two absorption maxima were observed at 6.2 and 7.6eV, and the corresponding molar extinction coefficients were estimated to be 3.0×104 and 5.5×104 Lmol−1cm−1, respectively. The 7.6eV absorption in the TiO2 doped samples prevents bond breaking by VUV photon for type I silica glass.

Optical and Structural Investigations on Mn-Ion States in MOCVD-grown Ga1−xMnxN

ABSTRACTThe incorporation of Mn into GaMnN epilayers by MOCVD growth was investigated. Samples with high Mn concentrations lead to room temperature ferromagnetism. In addition an absorption band around 1.5 eV was observed. Intensity and linewidth of this band scaled with the Mn concentration and with the room temperature (RT) saturation magnetization. This band is assigned to the internal Mn3+ transition between the 5E and the partially filled 5T2 levels of the 5D state. The broadening of the absorption band is introduced by the high Mn concentration. Recharging of the Mn3+ to Mn2+ was found to effectively suppress these transitions resulting also in a significant reduction of the RT magnetization. The pronounced sensitivity of the relative position of the Fermi level and 1.5 eV absorption band can be used to predict the magnetization behavior of the Ga1−xMnxN epilayers. The absence of doping-induced strain was observed by Raman spectroscopy. The structural quality, the presence of Mn2+ ions were confirmed by EPR spectroscopy, meanwhile no Mn-Mn interactions were observed.

Luminescence and absorption in type III silica implanted with multi-energy Si, O and Ar ions

Some effects on the photoluminescence band at 4.4 eV and optical bands in the 4.8–5.2 eV range produced by multi-energy (highest energy 320 keV) Si and O implantations in (type III) silica have been measured. Samples were implanted with multi-energy Si ions, with O ions, and with Ar ions to form a region of constant concentration. The depth of this region was ∼600 nm with the transition region, from zero to constant concentration being ∼100 nm thick. Concentrations ranged from approximately 0.02–2.1 at.%. We propose that in these samples photoluminescence and absorption bands due to the various combinations of oxygen vacancies will increase with increasing Si concentration with related O related defects changing at much smaller rates. In the absorption spectra of samples implanted with O, bands associated with oxygen related defects increase with increasing concentration, while those due to various forms of O vacancies will change at much smaller rates. To test the hypothesis that the observed absorption bands in the 4.8–5.2 eV range are the source of the 4.4 eV luminescence band, we have compared the changes in the three absorption bands in the 4.8–5.2 eV range with changes in the luminescent intensity of the 4.4 eV band. Based on the intensity of the luminescence in the Si implanted samples compared to the intensity in the Ar and O implanted samples and the expectation that the Si implanted samples will have a preponderance of oxygen deficient sites, we conclude that the 4.4 eV emission is due to oxygen deficient defects. The changes in the amplitude of the 5.01 eV absorption band is more similar to the changes in the 4.4 eV emission than to any other band in the O samples. However, none of the bands have a convincing correlation with the 4.4 eV emission.

Growth of H(II) centers in natural silica after UV laser exposure

The post-irradiation increase in the amplitude of the electron spin resonance doublet split by 11.8 mT, associated with the H(II) centers (Ge H), was measured in dry and wet natural silica irradiated at room temperature with UV photons at 266 nm from a Nd:YAG laser. The concentration of these paramagnetic defects increases on increasing the delay time after the UV exposure, in a time scale of few hours and its final magnitude depends on the number of laser shots and on the OH content. The generation of H(II) centers is correlated with the bleaching of the 5.1 eV absorption band ascribed to the twofold coordinated Ge (Ge ) so supporting the occurrence of a conversion process between these defects whose kinetics is governed by the diffusion of molecular hydrogen in the silica matrix.

Luminescence and relaxed excited state origin in CsI:Pb crystals

Emission and excitation spectra, luminescence polarization and decay kinetics have been studied for CsI:Pb crystals in the 0.36–300 K temperature range. The origin of the excited states responsible for the optical characteristics has been discussed. It has been concluded that the doublet ≈3.70 eV absorption (excitation) band is caused by the electronic transitions into the Pb 2+ triplet state split due to the presence of a cation vacancy near a Pb 2+ ion, while the higher-energy bands are of the charge-transfer origin. Like in CsI:Tl, four emission bands of CsI:Pb have been found to belong to the main luminescence centres. Two emission bands, peaking at 3.1 and 2.6 eV, are suggested to arise from the triplet relaxed excited state of a Pb 2+ ion. Two visible emission bands, peaking at 2.58 and 2.23 eV, are interpreted as the luminescence of an exciton localized near the Pb 2+ ion.

Low-energy excitations in the magneto-optical response ofα′−NaV2O5

We employ the sensitivity of high-energy sidebands on the 1 eV absorption band to temperature and applied magnetic field to investigate the phase transition in ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}{\mathrm{NaV}}_{2}{\mathrm{O}}_{5}.$ Data were collected in steady (45 T) and pulsed (60 T) fields between 4 and 45 K. We use the spectral changes to map the $H\ensuremath{-}T$ phase diagram and find a temperature-independent phase boundary at $\ensuremath{\sim}27 \mathrm{T}$ ${(H}_{c})$ which persists above 34 K $[{T}_{c}(0)],$ a kink in ${T}_{c}(H)$ near 27 T, and ${H}^{2}$ behavior of ${T}_{c}(H)$ in the high-field phase with prefactor $\ensuremath{\alpha}=0.09.$ It is notable that the energy scale of ${H}_{c}$ is identical to that of ${T}_{c}(0).$ To understand the high-energy sidebands on the 1 eV band in the optical conductivity, we invoke the idea of low-energy excitations coupled with the optically induced charge transfer excitation. The 34 K phase transition is assigned as a crossover between thermal excitation over the on-rung potential barrier above ${T}_{c}$ and tunneling through the barrier below ${T}_{c},$ and ${H}_{c}$ is associated with a change in the on-rung potential, which modifies the low-energy excitation dynamics and may lead to the formation of field-induced ferromagnetic domains above 27 T.

Nickel related defects in diamond: the 2.51 eV band

Optical and magnetic properties of nickel related defects in as-grown diamond are reviewed. Published and new data on the problem of negatively charged substitutional nickel are presented and discussed. We show that the 2.51 eV absorption band occurs at the Ni s −, the spin–orbit and the Jahn–Teller interactions playing important roles in the optical properties of the defect.

Optical absorption spectra of 10-MeV electron-irradiated paratellurite single crystals

Optical absorption spectra of TeO2 crystals, irradiated at room temperature by up to 2·1018 cm−2 10-MeV electrons and subsequently annealed to 575K, are studied. The dependence of Urbach absorption edge parameters in TeO2 on the electron beam fluence is discussed. The irradiation-induced near-edge broad (2.5–3.5eV) absorption band is shown to be related to oxygen vacancies, annealing at 475–525K.

Radial distribution of some defect-related optical absorption and PL bands in silica glasses

In order to investigate the origins of defect-related optical absorption and photoluminescence (PL) bands in silica glasses, we measured the radial distribution of paramagnetic defect centers, optical absorption band, and PL band in various silica glass rods. We studied the radial distribution of the following: the E ′ center and the 5.8 eV absorption band, non-bridging oxygen hole center and the 1.9 eV PL band and 4.8 eV absorption band, and 5.0 eV absorption band and the 2.7 and 4.3 eV PL bands. As a result of this study, we confirmed the correlation of both the radial distributions. These results suggest the following: the E ′ center is the origin of the 5.8 eV absorption band, the non-bridging oxygen hole center is the origin of the 1.9 eV PL band and 4.8 eV absorption band, and the oxygen vacancy is the origin of the 5.0 eV absorption band, the 2.7 and 4.3 eV PL bands.

Correlation between the 1.5 eV photoluminescence band and peroxy linkage in silica glass

In order to understand the origin of the 1.5 eV PL band in silica glass, we studied the correlation between the 1.5 eV PL band and the peroxy linkage (POL: O 3Si–O–O–SiO 3) or the molecular chlorine models (possible origins of the 3.8 eV absorption band) by laser irradiation at low temperatures (∼20 K). On the basis of our results, we conclude that the origin of the 1.5 eV PL band is not molecular chlorine but POL.

Photochromic effect in irradiated and annealed nearly IIa type syntheticdiamond

We examined the effect of radiation damage and annealing on the opticalproperties of nitrogen-gettered nearly IIa type synthetic diamonds. It was found thatthe 2.156 eV centre, whose absorption is usually very weak, appears in thesediamonds as one of the dominant absorption features. A new vacancy-related vibronicabsorption system with zero-phonon line at 3.420 eV was observed. A pronouncedphotochromic effect was established for the 1.945, 2.085, 2.156, 3.420 and 4.325 eV absorption bands. Of all these bands only the 2.156 eV band remains in theabsorption spectra after a proper photoexcitation. In contrast, for similarly treated typeIb diamonds we did not reveal any photoinduced changes in the 1.945 eV bandabsorption. Based on the results of optical bleaching and thermal recoveryexperiments, we conclude that the assignment of the 2.156 eV centre to the neutralcharge state of the nitrogen-vacancy defect needs further verification.