Collisional deactivation mechanism of luminescence in hydrogen-loaded Ge-doped fibers

Abstract

We report experimental results on the temperature and Ge-content dependencies of the decay times of the 3.1 eV emission of twofold-coordinated germaniums, excited at 3.51 eV, in both virgin and H2-loaded germanosilicate optical fibers. For T<110 K, the lifetimes are temperature invariant in the two kinds of fibers; at higher temperatures, they are progressively shortened in the H2-loaded fibers whereas they remain unaltered in the virgin fibers. On increasing the Ge content in the fiber core the lifetime decreases in the whole temperature range. We also found a direct correlation in H2-loaded samples between the dependencies on temperature of the emission lifetime and the diffusion lengths of H2 molecules in silica glass determined from earlier work. These experimental features are explained by assuming an elastic-collisional deactivation mechanism due to interaction with diffusing H2 molecules, described in terms of an Arrhenius law with activation energy approximately 64 meV that operates in competition with the normal radiative process.

Ultraviolet-induced paramagnetic centers and absorption changes in singlemode Ge-doped optical fibers

“…This can occur either by reaction of GLPC in ground state with H 0 produced by breaking of H 2 on paramagnetic centers [35], or by direct reaction of H 2 with GLPC in excited-triplet state [14,33]. The latter process leads to an observed reduction of lifetime of GLPC in excited triplet state in presence of H 2 [36]; since we have supposed the generation of Ge(1) to occur by two step absorption of GLPC via the long-lived triplet state, we expect a reduction of the Ge(1) formation rate in presence of hydrogen: this is consistent with the lower initial slope of the H 2 -loaded curve in Fig. 3.…”

Role of diffusing molecular hydrogen on relaxation processes in Ge-doped glass

“…As concerns the process k(H 2 ), its activation above 200 K is qualitatively consistent with the diffusion properties of molecular hydrogen [24][25][26]. Within current experimental results, we cannot determine the precise mechanism lying under k(H 2 ), which could represent either a reaction of excited twofold coordinated Ge with H 2 leading to formation of H(II) centers [9], or an elastic collision with vibrational energy transfer between the two species [3]. Finally, we observe that the excited singlet state is poorly affected by the presence of hydrogen because of its much faster lifetime ($10 4 times lower than s T ).…”

“…The latter finding had already been evidenced and interpreted as due to the activation of an additional collisional non-radiative relaxation channel k(H 2 ) from T 1 (see inset of Fig. 4(a)) [3]. To discuss point (a), it is mandatory to consider also the properties of the decay channels originating from S 1 state.…”

“…For instance, some works revealed that under H 2 loading the ultraviolet (UV) transmittance is improved, because optical absorbing defects are suppressed by saturating dangling Si or O bonds with hydrogen. Moreover, the presence of H 2 improves by several orders of magnitude the photosensitivity [1][2][3][4][5][6]. From the microscopic point of view, photosensitivity has been partially associated with photochemical reactions involving the twofold-coordinated Ge (=Ge •• ) [1,[7][8][9][10][11][12], where the symbols '••' and '@' represent an electron lone pair and bonds with two distinct oxygen atoms, respectively.…”

“…However, several of their optical properties are strongly affected by the presence inside the fiber core of interstitial mobile species [1]. In particular, extensive attention has been paid to explore effects of dissolved molecular hydrogen (H 2 ) in glass under laser exposition [2,3]. For instance, some works revealed that under H 2 loading the ultraviolet (UV) transmittance is improved, because optical absorbing defects are suppressed by saturating dangling Si or O bonds with hydrogen.…”