Two-photon absorption induced dangling bonds in amorphous silica revealed by molecular dynamics simulations

The amorphous silica (a-silica) has been troubled by dangling bonds induced strong optical absorption of intense ultraviolet (UV) rays, where the two-photon absorption (TPA) plays a critical role in the formation of dangling bonds. Through the combination of Keldysh theory and molecular dynamics, we studied the effects of TPA on the evolution of dangling bonds in a-silica, where the ab initial calculations need a larger size of amorphous structure. The damage precursor of (Si-O)(3) rings experimentally observed was reproduced, a process where the strained Si-O bonds acted as the TPA site. We showed that TPA played a critical role in initiating dangling bonds and the following densification process depressed it. Formation channels for oxygen deficiency center I (ODC(I)) and ODC(II) were shown, as well as the interconversion with Frenkel-type defects. Our research describes the static and dynamic properties of dangling bonds and gives an interpretation of experimental observations.