Ultrashort pulse induced modifications in ULE - from nanograting formation to laser darkening

We report on ultrashort pulse laser induced modifications in ultra-low expansion (ULE) glass. This silicate glass has a significant fraction of TiO sub(2) (7.5 wt%) to ensure a low thermal expansion. Ultrafast laser irradiation generates different kinds of modifications in this glass: so-called nanogratings are formed when low irradiation power is used; while high laser powers result in heat accumulation and induce local melting. In addition, for almost all laser parameters applied, the processed material tends toward darkening. With help of Small Angle X-ray Scattering (SAXS), Focused Ion Beam (FIB) milling and optical retardance measurements we analyzed the structure of laser induced nanopores which are the basic components of nanogratings. Investigation of the mechanisms responsible for laser induced darkening were conducted by multiple techniques such as FIB milling, Electron Spin Resonance (ESR) and Raman spectroscopy. We could identify the formation of hollow cavities filled with molecular oxygen surrounded by a compressed shell of glass which contains trivalent titanium. While light scattering on the cavities causes opacity, the reduction of colorless Ti super(4+) (d super(0)) to blue Ti super(3+) (d super(1)) is responsible for the darkening. By combining the inscription of nanogratings with laser induced darkening it is easily possible to locally tune the type of the modification by three independent degrees of freedom (retardance, orientation of optical axis, amount of darkening) rendering ULE an ideal material for future data storage applications.