Germanium Oxygen Deficient Centers Research Articles

Luminescence of unfused 95%SiO2–5%GeO2 amorphous films with fluorine additive: No evidence for presence of GeODC(I) defects found

Photoluminescence (PL) of unfused amorphous germanosilicate films with fluorine additive is studied in 2–8.5eV spectral range. Experiments are based on films deposited on silica substrates by means of the surface-plasma chemical vapor deposition (SPCVD). Films of about 100μm in thickness with “high F” (~4.2wt.%) and “low F” (~0.5wt.%) fluorine content have been fabricated for the experiments. KrF (248nm), ArF (193nm) and F2 (157nm) excimer lasers are used to pump PL. It is found that absorption and luminescence associated with germanium oxygen deficient centers (GeODCs) in “high F” and “low F” films differ. In the “high F” unfused film absorption coefficient of the band at 5eV as well as intensity of the blue PL band at 3.1eV are significantly greater. This film proves features of the so called GeODCs(II), which symbolize twofold coordinated germanium defects in silica network. In the “low F” unfused film absorption band at 5eV is feebly marked. Poorly resolved PL intrinsic to GeODCs(II) can be detected in this film under the KrF laser pump. The most significant PL features are revealed under deeper UV pump by ArF and F2 lasers. Spectral positions of PL bands excited by these lasers correspond to GeODCs(II). However PL decay kinetics dramatically differs from that one intrinsic to GeODCs(II). Noticeable growth of PL intensity caused by permanent (half an hour and more) exposure to ArF and/or F2 lasers takes place, indicating GeODC(II) formation. It is found that considerable body of fluorine additive has the same effect as profusion for “low F” and/or fluorine free germanosilicate amorphous material synthesized by SPCVD. In the “high F” film yield of GeODC(II) PL pumped by the F2 laser remains high. This speaks for the suppression of the competitive 7.6eV absorption band associated with SiODCs(I) by fluorine additive indicating a decrease in the content of this type of defects in the material. High yield of GeODC(II) luminescence pumped by deep UV photons as well as hypothetical similarity of SiODC(I) and GeODC(I) permit one to conclude that GeODCs(II) are the only defects dominating in the materials under study.

Effect of processing parameters on control of defect centers associated with second-order harmonic generation and photosensitivity in SiO2:GeO2 glass preforms

In this research, the effect of the H2/O2 ratio and the processing temperature parameters on the inducing and enhancement of the defect centers associated to the second-order optical non-linearity in SiO2:GeO2 glass preforms, prepared by vapor-phase axial deposition method, have been investigated. The formation of germanium oxygen deficient centers and the development of paramagnetic structures induced in the glass preforms after X-ray irradiation were investigated using UV–Vis absorption spectroscopy and electronic spin resonance. The results indicate that the concentration of germanium oxygen deficient centers increases exponentially when the H2/O2 ratio decreases, while the processing temperature increases, simultaneously. The electronic spin resonance spectra profiles, shows that defects of the electron trapped centers type [Ge(1), Ge(2)] are induced by the effect of X-ray irradiation. An efficient generation of defect centers associated to the second-order optical non-linearity in SiO2:GeO2 glass preforms, occurring in samples prepared with low H2/O2 ratios and high processing temperatures, have been observed.

Dependence of H 2/O 2 ratio and GeO 2 content on the enhancement of second-order non-linearity related defects in Ge-doped optical fiber preforms

In this research, we studied the influence of the process conditions through the H 2/O 2 ratio and the GeO 2 content, on the formation of germanium oxygen deficient centers and paramagnetic defects center induced by X-ray irradiation in SiO 2:GeO 2 glass preform prepared by the vapor-phase axial deposition method. The absorption band at around 5.1 eV, characteristic of germanium oxygen defect center was observed in all glass preform sample absorption spectra. The absorption coefficient increased when the GeO 2 concentration increased, however, the absorption was higher in samples prepared with high H 2/O 2 ratio. The electronic spin resonance spectrum profiles suggest that paramagnetic structures, such as, electron trapped centers and positively charged germanium oxygen deficient centers, assigned as Ge(1) and Ge(2), respectively, are induced in all samples after being X-ray irradiated for 1 h. It was found that the concentration of induced paramagnetic centers is linearly proportional to germanium oxygen defect center concentration and that the efficiency in the enhancement of paramagnetic defect centers associated with second-order optical non-linearity increased as H 2/O 2 ratio used in the fabrication process was decreasing. It was concluded that SiO 2:GeO 2 glass preform with low H 2/O 2 ratio and higher GeO 2 content must be prepared for the enhancement of the second-order non-linearity.

Dependence of H2/O2 ratio and GeO2 content on the enhancement of second-order non-linearity related defects in Ge-doped optical fiber preforms

In this research, we studied the influence of the process conditions through the H2/O2 ratio and the GeO2 content, on the formation of germanium oxygen deficient centers and paramagnetic defects center induced by X-ray irradiation in SiO2:GeO2 glass preform prepared by the vapor-phase axial deposition method. The absorption band at around 5.1 eV, characteristic of germanium oxygen defect center was observed in all glass preform sample absorption spectra. The absorption coefficient increased when the GeO2 concentration increased, however, the absorption was higher in samples prepared with high H2/O2 ratio. The electronic spin resonance spectrum profiles suggest that paramagnetic structures, such as, electron trapped centers and positively charged germanium oxygen deficient centers, assigned as Ge(1) and Ge(2), respectively, are induced in all samples after being X-ray irradiated for 1 h. It was found that the concentration of induced paramagnetic centers is linearly proportional to germanium oxygen defect center concentration and that the efficiency in the enhancement of paramagnetic defect centers associated with second-order optical non-linearity increased as H2/O2 ratio used in the fabrication process was decreasing. It was concluded that SiO2:GeO2 glass preform with low H2/O2 ratio and higher GeO2 content must be prepared for the enhancement of the second-order non-linearity.

Photosensitivity of germanosilicate fibres and preforms doped with nitrogen inhomogeneously over the cross section

The effect of the inhomogeneous distribution of nitrogen impurity on the photosensitivity of germanosilicate fibres is studied. For this purpose, single-mode fibres with different alternations of the core layers sintered in nitrogen and oxygen atmospheres were specially prepared by the MCVD technique. Photoinduced variations in the quadratic nonlinear susceptibility and refractive index of these fibres were recorded. It is shown that the recording efficiency depends on the radial distribution of germanium oxygen-deficient centres, which can be controlled at the stage of sintering through a nonuniform doping by nitrogen over the cross-section.

Influence of oxygen deficiencies and hydrogen-loading on defect luminescence in irradiated Ge-doped silica glasses

The influence of ionizing radiation on the optical properties of various Ge-doped silica glasses in the UV–vis range was examined. Hydrogen-loaded and hydrogen-free germanosilicate glass samples containing varying amounts of oxygen deficiencies were X-ray irradiated and changes in their respective absorption and luminescent properties were measured. The results show that germanium oxygen deficient centers (GODCs) are necessary for significant absorption changes in the hydrogen-free samples. In the hydrogen-loaded samples, large absorption changes were observed in the 200–550 nm range regardless of GODC content. Emission bands at 615 and 680 nm were also found to result from irradiation in these samples. Furthermore, it was found that X-ray radiation drastically increased the concentration of germanium lone pair centers (GLPCs) in H 2-loaded samples with low initial GODC concentration.

Photorefractive effect and photoinduced quadratic nonlinear susceptibility in germanosilicate fibres fabricated in nitrogen and helium atmospheres by the MCVD technique

Single-mode optical fibres were fabricated from a germanosilicate glass by the method of modified chemical vapour deposition (MCVD), which used sintering of a porous glass in a reducing (helium or nitrogen-containing) atmosphere. The optical fibres exhibit a high photoinduced change in the refractive index and a high efficiency of recording quadratic nonlinear susceptibility compared to a standard germanosilicate fibre. Sintering, both in nitrogen and in helium atmospheres, was shown to increase the concentration of germanium oxygen-deficient centres in glass. It is likely that nitrogen enters into a germanosilicate glass in the concentration that is sufficient to modify the glass structure and to additionally increase its photosensitivity. The replacement of oxygen or silicon in the close vicinity of an oxygen vacancy by nitrogen may play a key role in the photosensitivity enhancement owing to the formation of additional valence bonds and blocking of recombination processes.

Effect of electric field on one-quantum photodecay of oxygen-deficient centers in germanosilicate fibers

The effect of a strong electric field (up to 106 V/cm) on one-quantum UV photoexcitation and photodecomposition of germanium oxygen deficient centers (GODC) in twin-hole germanosilicate fibers with internal wire electrodes is reported. The fiber has been irradiated with the UV light of a deuterium lamp and triplet photoluminescence of GODC has been used to monitor the kinetics of its photodecay. Applying such an electric field did not affect the spectral characteristics of GODC but increased the rate of their one-quantum photodecomposition, while direct photoionization and charge separation did not take place. We have also shown that this effect is caused by the suppression of secondary photoinduced recombination processes of intermediates, rather than by acceleration of primary photodecomposition of GODC.

Enhanced photosensitivity in germanosilicate fibers exposed to CO_2 laser radiation

We report a novel method to increase the UV photosensitivity of GeO2:SiO2 optical fibers based on exposure to CO2 laser irradiation before grating writing. Fibers treated with a CO2 laser can produce gratings with refractive-index modulation two times greater and a Bragg wavelength that can be 2 nm longer than those of untreated fibers. Experiments on GeO2:SiO2 preform samples treated with a CO2 laser in a way similar to the fibers showed a marked increase of the 242-nm absorption band, which is associated with an increase of germanium oxygen-deficient centers and is believed to be responsible for the higher photorefractive response.

Fiber Bragg Grating Growth Simulation Using an Inverse Scattering Matrix Approach

ABSTRACTFiber Bragg gratings are playing an important role in the field of optical telecommunications and sensors. We have simulated the growth of gratings, longitudinally written in the core of germanosilicate optical fibers by an Ar+ laser. Following a Kramer-Kronig model, absorption in the Germanium Oxygen deficient centers band (∼240 nm) is considered to be responsible for the fiber photosensitivity and a two photon absorption process models the dynamics of the refractive index variation.The spatial propagation of light along the grating is described by the coupled wave theory and is calculated by using an inverse scattering matrix technique. The predicted physical properties of the gratings, reflectivity and temporal dispersion of propagating light as a function of wavelength and input power, show that a sustained growth is possible. A good agreement with previous simulations and experiments is found.

One Photon and Two Photon Process in Photo-Decomposition of Germanium Oxygen Deficient Centres

UV photon-induced transformation of germanium oxygen deficient centres (GODC) in germanium-doped silica glass have been studied using photocurrent measurements, absorption and fluorescence bleaching. It has been identified that the photocurrent are generated via a two photon effect. Evidence have been found suggesting that the UV photon-induced destruction of GODCs is achieved via two reaction pathways, a single photon pathway and a two photon pathway. The process is discussed.

Optical properties of amorphous films assembled from the nanoclusters of Ge - Al mixtures

Two samples of nanocluster-assembled Ge - Al thin films on quartz substrates have been fabricated by co-evaporation and inert-gas condensation. The thin films are amorphous and single phase, and the mean diameters of the nanoclusters are and , respectively. The composition of the nanoclusters in the two films is about 98.7 at.% Ge and 1.3 at.% Al, and the ratio of Ge and oxygen atoms is 1:1.8 (O). From the absorption spectra of the two samples, we estimate the optical gaps to be about 2.8 eV and 1.6 eV from Tauc plots, respectively; these are dependent on the nanocluster sizes and are much larger than those of vacuum-evaporated amorphous Ge and amorphous Ge - Al thin films. Under 3.32 eV (374 nm) excitation, photoluminescence (PL) peaks at 2.80 and 3.00 eV appear for the thin film with the mean diameter of the nanoclusters ; these can be interpreted as manifesting electron transitions from the optical gap (2.8 eV) and mobility gap (3.0 eV). In the absorption spectra of the two samples, a shoulder peak appears at 5.06 eV (245 nm), corresponding to the absorption band of one of the germanium oxygen-deficient centres (GODCs), namely GODC-1. The PL peak at 3.10 eV (400 nm), which arises from GODC-1, is also observed for the two samples.

Dynamics of the refractive index induced in germanosilicate optical fibres by different types of UV irradiation

A Mach—Zehnder fibre interferometer, based on two photoinduced long-period gratings, was used in an investigation of the dynamics of the refractive index Δnind induced in the core of a germanosilicate optical fibre by different types of UV irradiation. It was found that the value of Δnind could be approximated satisfactorily by a power-law function of the radiation dose at all the investigated wavelengths. A comparative analysis of the dynamics of the refractive index in the case of singlet and triplet photoexcitation of germanium oxygen-deficient centres showed that the dominant mechanism of the change in the refractive index by low-intensity continuous UV irradiation was transformation of these centres from an excited triplet state.

Optical absorption and luminescence of germanium oxygen-deficient centers in densified germanosilicate glass

Spectroscopic properties of germanium oxygen-deficient centers were studied in vapor axial deposition and modified chemical-vapor deposition germanosilicate glass after hydrostatic densification by as much as 19%. Gaussian decomposition showed a broadening and a significant shift of the initial absorption bands at 5.11 and 5.42 eV. A threefold decrease of the 3.15-eV luminescence band was revealed, whereas the 4.25-eV luminescence band was only slightly sensitive to densification. This shows that these two bands belong to different luminescence centers. A drastic growth of a new metastable absorption band near 7.3 eV was observed. The pressure-induced changes disappeared as a result of thermal annealing. These changes are discussed with respect to in-fiber grating phenomena.

Direct observation of ultraviolet laser induced photocurrent in oxygen deficient silica and germanosilicate glasses

UV laser induced inonization of silicon oxygen deficient centers (SODC) in silica glasses and germanium oxygen deficient centers (GODC) in germanosilicate glasses has been studied by direct displacement photocurrent measurements. For both SODC and GODC the two-photon (two-step) nature of photoionization has been established. The cross sections of transitions from first excited to upper (ionizable) states were estimated as σ12=6×10−18 cm2 for SODC and (0.5–1.0)×10−19 cm2 for GODC. The effect of pulse-to-pulse degradation of the photocurrent signal caused by the screening effect is applied to estimate the lifetimes and ranges of the free carriers. The conclusion was drawn, that ionized SODC and GODC play the key role for free charge trapping.

Oscillator strengths of UV absorption and luminescence for oxygen-deficient centers in germanosilicate fibers

The time-resolved measurements of triplet and singlet luminescence from germanium oxygen-deficient centers (GODC's) in germanosilicate fibers excited by short UV laser pulses are used to determine the cross section of UV absorption of GODC's at lambda = 248 nm (sigma = 5 x 10(-18) cm(2)), the oscillator strength of UV absorption (f(abs) = 0.027), and UV luminescence (f(lum) = 0.031). The basis for these measurements is the direct luminescence detection of lifetimes of electronically excited states of GODC's, the saturation of triplet luminescence signals, and the analysis of the rate equations that govern the laser photoexcitation and relaxation processes in GODC's.