Microscopic reaction mechanisms in hydrogen-loaded germanosilicate fibers: Formation of divalent Ge defects

Abstract

We have investigated the microscopic mechanisms for the thermal reactions of H2 with germanosilicate optical fibers by means of first-principles quantum chemical techniques. As a first reaction step, it has recently been shown that H2 reacts with a Ge -0 bond to form Ge -H and SiO -H. In this work, we have carefully considered the activation energies for two additional reaction channels at the Ge -H site which lead to divalent Ge defects. A channel involving hydrogen migration from Ge to a neighboring 0 is found to be energetically favored significantly and is likely to be the dominant pathway for the formation of divalent Ge.The discovery of refractive index changes in germaniurndoped silicate optical fibers upon UV irradiation is an exciting recent development that has led to considerable research efforts in this area. Diffraction phase gratings have been imprinted directly into the core of optical fibers, and this technology is likely to play an important role for future fiber devices such as lasers, mode couplers, and sensors. ' More recently, it has been observed that the refractive index changes can be greatly enhanced by loading the germanosilicate fibers with molecular hydrogen. ' In addition, recent experiments have shown that similar large refractive index changes can also be achieved by rapid thermal heating. ' Mechanistic understanding of the underlying chemical reactions is just beginning to emerge based on contributions from both experiments" and theory. Experimental studies by Lemaire et al. with several different fiber samples found that the Ge content correlates directly with the amount of OH formed. Later Raman spectral data by Greene et aI. showed clear signatures of SiO -H and Ge -H bond-stretching modes. The experimental data suggest that a chemical reaction occurs preferentially at a Ge site and that H2 breaks the Ge -0 bond of a =Ge-0 -Si= unit to form Ge -H and Si0 -H. Recently, we have performed first-principles quantum chemical calculations to investigate the thermal reactions of H2with Ge -0 and Si -0 bonds and found that the energy barrier for breaking a Si -0 bond is about 0.5 eV higher than that of breaking a Ge -0 bond. Assuming similar preexponential factors, the reaction probability at a Ge site should be the dominant mechanism even at temperatures as high as 1000 C.Our previous theoretical study considered only the initial step in the chemical reactions of Hz. There are several other features seen experimentally which require a detailed consideration of additional steps. For example, Lemaire et al. determined that a strong 242-nm band grew in proportion to the OH increases in Game-heated fibers. The 242-nm band has been proposed to correspond to divalent Ge defects. Also, under photoinitiated conditions, the H2 reaction leads to the formation of the dihydride =GeH2 as evidenced by Raman-scattering measurements where a peak at 2135 crn has been observed. Greene et al. assigned this peak to be the GeHz stretch mode based on the frequency shift of about 50 cm ' from the position (2...

Reactions of divalent Ge in hydrogen loaded germanosilicate optical fibers: Identification of a new monohydride defect

“…7,8 The Raman spectral data by Greene et al 6 identified a few of the important products of chemical reactions between H 2 and germanosilicate fibers such as wSiO-H, wGe-H ͑monohydride͒, and vGeH 2 ͑dihy-dride͒. 7,8 We obtained transition states and the associated activation barriers of some of the reaction sequences to identify those with relatively smaller energy barriers. 7,8 We obtained transition states and the associated activation barriers of some of the reaction sequences to identify those with relatively smaller energy barriers.…”

“…The possible reaction paths leading to the above products were investigated by first-principle quantum chemical calculations in our previous papers. 8 It is generally accepted that the 240 nm absorption band in germanosilicate fibers is due to germanium oxygen deficiency centers ͑GODC͒ and a likely candidate for GODC is the divalent Ge defect. As a first step, the reaction occurs at a Ge site where a Ge-O bond is broken by H 2 to form wGe-H and wSiO-H. 7 A subsequent hydrogen migration from Ge to a neighboring O can lead to the formation of divalent Ge defects.…”

“…12 The minima are characterized by all positive frequencies while the transition states have one and only one imaginary frequency. In order to obtain accurate energies, these MP2 calculations have been performed with a large basis set containing 9s, 6p, 4d, 1f contracted functions on Ge, 13 6s, 5p, a͒ 2d, 1f on Si, 12,18 4s, 3p, 2d, 1f functions on O, 12,19 and 3s, 1p functions on H. 12,19 As mentioned earlier, the first key reaction step considered in our previous studies 7, 8 This leads to the formation of -Ge-H, also a monohydride, but occurring at a divalent Ge site rather than the tetravalent site. 14 Electron correlation effects have been included by means of Mo "ller-Plesset ͑MP͒ perturbation theory.…”

“…However, divalent Ge tends to form a complex with the oxygen lone pair of a nearby OH group 8 and hence the reverse reaction barrier may become somewhat larger. However, divalent Ge tends to form a complex with the oxygen lone pair of a nearby OH group 8 and hence the reverse reaction barrier may become somewhat larger.…”

Role of hydrogen loading and glass composition on the defects generated by the femtosecond laser writing process of fiber Bragg gratings

“…It is clear that in both FBG writing processes the writing laser creates defects in the glass network. In the case of the UV modification of silica-based glass fibers, much work has been done on identifying the kinds of defects generated [8][9][10].…”