P. V. Chernov

Inhomogeneous nature of UV absorption bands of bulk and surface oxygen-deficient centers in silica glasses

Photochemical and photoluminescence studies of oxygen-deficient centers stabilized in the bulk and on the surface of silica glasses clearly demonstrate the inhomogeneous nature of the absorption and luminescence spectra of oxygen-deficient centers. The conclusion is drawn that the inhomogeneity of the absorption spectra is due to the dispersion of the energy of the S0-S1 transition, while the inhomogeneity of the luminescence spectra is due to the dispersion of the energy barrier of intersystem crossing. The inhomogeneous nature of the oxygen-deficient centers in silica glasses is assumed to be caused by a small dispersion in the geometrical parameters of different groups of centers with similar chemical properties.

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 (GODCs) in germanosilicate fibers excited by short UV laser pulses are used to determine the cross section of UV absorption of GODCs 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 GODCs, the saturation of triplet luminescence signals, and the analysis of the rate equations that govern the laser photoexcitation and relaxation processes in GODCs.

Spectroscopic Features of Silica Glasses Doped with Tin

The absorption, photoluminescence, and photoluminescence excitation spectra are investigated for tin-doped silica glasses synthesized by different methods. In all the glasses studied, two new centers belonging to the tin dopant in the SiO2 network are revealed in addition to the well-known oxygen-deficient center with the absorption band at 4.9 eV. One new center is associated with the absorption band at 5.9 eV and the photoluminescence bands at 2.7 and 3.6 eV, whereas the other center is characterized by the absorption band in the range of 4.56 eV and the photoluminescence band at 2.95 eV. Both new centers are identified as oxygen-deficient centers. The latter center is an analog of the centers observed in pure silica glasses synthesized in a nitrogen atmosphere and in germanosilicate glasses prepared by the MCVD and SPCVD methods. It is revealed that the formation of the oxygen-deficient center is one of the main channels of incorporating tin into the SiO2 network in all tin-containing silica glasses. Consideration is given to the reasons for the lower efficiency of the photodecay of oxygen-deficient centers in the SiO2 network in tin-containing glasses as compared to that in germanosilicate glasses and for the relatively high efficiency in the formation of the photoinduced refractive index in optical fibers with a core consisting of tin-containing silica glass.

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.

Color Centers in Sulfur-Doped Silica Glasses: Spectroscopic Manifestations of an SO2 Interstitial Molecule

The absorption spectra of sulfur-doped silica glass are investigated under different external actions (laser irradiation, saturation of samples in a hydrogen atmosphere, and thermal annealing). It is found that the absorption band at 400 nm has a vibrational structure. A correlation between changes in the intensities of the absorption bands at 237 and 400 nm under different actions is revealed. Reasoning from analysis of the results obtained, a model is proposed according to which these bands are assigned to an S+2 interstitial molecular ion. It is demonstrated that none of the optical centers considered in this work is responsible for the observed ESR signal.

PHOTOINDUCED SECOND-HARMONIC GENERATION IN FIBERS DOPED WITH RARE-EARTH IONS

Photoinduced second-harmonic generation in silica fibers doped with Er(3+), Sm(3+), and Tb(3+) has been investigated. Er(3+)-doped fibers have been found to tune a chi((2)) grating easily to the mode structure of the laser radiation, whereas Sm(3+)-doped fibers have been found to possess the greatest resistance to chi((2)) grating erasure by radiation at 532 nm. From the beginning of the preparation processes, the third harmonic at 355 nm is registered in all the fibers.

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.

Correlation of defect centers with photoinduced second-harmonic generation in Er- and Sm-doped aluminosilicate fibers.

The spectral dependence of photoinduced optical losses after chi((2)) writing have been obtained in Sm- and Er-doped fibers. The results show that Sm(2+) and Er(2+) can be a source of photoelectrons during fiber preparation.

Unstable radiation colour centres in pure silica fibres: the nature and properties

Review results on the investigation of the nature and properties of certain radiation color centers (RCC) arising in (gamma) -irradiated pure silica core fibers was presented in our work. These RCC with the absorption bands 670,800 nm and the low-temperature IR- absorption are unstable at the room temperature and bring the large contribution in the induced absorption value of fibers at (lambda) equals 850 nm.