43 research outputs found
One photon and two photon process in photo-decomposition of germanium oxygen deficient centres
No full textThere has been much debate on the fundamental mechanism of photosensitivity in germanium-doped silica optical fibres. The importance of the effect, which was discovered by K.O.Hill in 1978, has been well demonstrated by the still rapidly increasing application areas that it has in fibre optics. Photo-induced index changes as high as ~10 in H2 loaded fibres have enabled very strong gratings to be written conveniently into optical fibres to produce filters, wavelength defining reflectors, dispersion compensators, and sensors. However, the connection between the photosensitivity and the germanium-related oxygen deficient centre (GODC), first proposed by Hand and Russell, is well recognised. Despite uncertainty on the exact microscopic structure of the GODC, the energy levels of the centre is well understood through spectroscopic studies of the centre. A diagram of such an energy level system is depicted in figure 1, with singlet states marked by S and triplet states T. Despite the fact there has been much work been done on the photo-decomposition of GODCs, there is still work to be done for a thorough understanding the process. Potentially there are two possible reaction path ways. One is a direct two photon reaction producing a photoelectron in the conduction band. Most of the photoelectrons will recombine with no net effect and some will be trapped at various sites to create a net change. The second one is a single photon process involving the long lived triplet state and a near by trapping site, probably situated at the next co-ordination sphere. The two situations are shown in figure 1. Here we report an experimental study on the reaction path way of the GODCs, demonstrating that there are both single photon and two photon process existing under different conditions
Effects of molecular hydrogen on low-intensity UV photochemistry of germanosilicate glasses
No full textThe one-quantum photo-reactions of oxygen deficient centers (GODC) in H2 loaded germanosilicate preforms and fibers are studied using time-resolved photo-luminescence and ESR spectroscopy
Effects of Ge concentration on one-quantum UV photo-reactions of oxygen deficient centers in germanosilicate glasses
No full textThe effects of Ge concentration, spectroscopic and photochemical inhomogeneity on GODC photochemistry in germanosilicate fibers and preforms are established and their mechanisms are discussed
UV laser-induced current in germanosilicate fibres with built in electrodes
No full textThe nature of photocurrents is studied in germano-silicate single-mode fibres with built in electrodes. It is shown that irradiation by excimer KrF laser pulses at electric field intensities up to 1.6 MV cm-1 there causes a charge formation and transfer in the core of the fibre and the injection of photoelectrons from metal electrodes to the fibre cladding. Both effects facilitate the flow of electric current through the sample. The dependences of the photocurrent on the electric field strength, the energy density in the loser pulse, the fibre temperature, and the impurity hydrogen in the fibre core are obtained. <br/
One photon and two photon process in photo-decomposition of germanium oxygen deficient centres
No full textUV 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
Effect of electric field on one-quantum photodecay of oxygen-deficient centers in germanosilicate fibers
No full textThe 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
Direct observation of ultraviolet laser induced photocurrent in oxygen deficient silica and germanosilicate glasses
No full textUV laser induced ionization 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.<br/
CARS diagnostics of near-critical fluid in small mesopores
No full textDue to the high spacial resolution and theinterference nature, coherent anti-Stokes Ramanscattering (CARS) spectroscopy is well suited forthe diagnostics of composites based on transparentnanoporous hosts. In particular, the adsorption of afluid on the walls of nanopores and the formation ofa condensed phase in their volume leads to obvioustransformation of the CARS spectra. Recently wehave developed a model which describes thebehavior of molecular spectra at isothermalcompression in cylindrical nanopores. Calculationsbased on the model have shown a good agreementwith the experimental results for carbon dioxide innanoporous glass with pores of diameter of severalnanometers. Here we use the developed approach toinvestigate the phase behavior of carbon dioxide inglass nanopores at near-critical temperatures. It hasbeen experimentally shown that condensation innanopores occurs at relatively low pressures atsubcritical and even at supercritical temperatures.The analysis based on the developed model allowsto reveal some qualitative and quantitativecharacterizations of the shift of critical point
