Systematic Control of Structural Changes in GeO₂ Glass Induced by Femtosecond Laser Direct Writing

Abstract

We report the structural changes inside germania glass induced by femtosecond laser pulses. Inspection by polarization microscopy and secondary electron microscopy indicate that the periodic nanostructures consist of oxygen defects such as ODCs (oxygen deficient centers) and NBOHCs (nonbridging oxygen hole centers) for laser pulse energy less than 0.2 lJ. However, the glass network was dissociated and O 2 molecules were generated for laser pulse energy greater than 0.4 lJ. Two different structural changes, form-birefringence and dissociation, were induced in GeO 2 glass, depending on the laser pulse energy. The form-birefringence exhibited by the nanogratings in GeO 2 glass is larger than that in SiO 2 glass for pulse energy less than 0.2 lJ, as the density of nanovoids enclosed by ODCs in GeO 2 glass is higher than that in SiO 2 glass. Arrhenius plots of the phase retardation caused by the nanogratings in GeO 2 and SiO 2 indicate that the oxygen defects are relaxed at a temperature 100°C above the glass-transition temperature.

Nanoscale Phase Separation in Lithium Niobium Silicate Glass by Femtosecond Laser Irradiation

“…This stood for a long time as the only example of a subwavelength structure in silica glass. Recently, fs laser‐induced nanostructures in other glasses have been reported: firstly, close to SiO 2 (e.g., doped silica), GeO 2 (a glass with an atomic arrangement close to SiO 2 ), TiO 2 ‐containing silicate glasses, and two multicomponent borosilicate glasses (with a few mol% of Na 2 O and Al 2 O 3 , BK7 (Schott), and Borofloat 33 (Schott) or alkali‐free aluminoborosilicate glass). For all of these, a similar nanostructure to the one observed in silica glass was described.…”

Direct writing of birefringent elements by ultrafast laser nanostructuring in multicomponent glass

“…15 Furthermore, doping of pure silica glass with Ge, F, and P was demonstrated to affect the threshold for nanograting formation. 16 Recently, evidence for femtosecond laser-induced nanogratings has been observed in glasses other than SiO 2 , including several studies reported nanogratings in GeO 2 glass, [17][18][19] binary titanium silicate glass (ULE, Corning), 20,21 and two multicomponent borosilicate glasses (BK7 and Borofloat 33, Schott). 20,22 Germanium dioxide is formed by a three-dimensional network of [GeO 4 ] tetrahedra with all bridging oxygen atoms, whereas ULE glass also possesses structure similar to pure SiO 2 24 were not revealed using scanning electron microscopy despite the induced retardance was of the same level as in Borofloat 33.…”

Tunability of form birefringence induced by femtosecond laser irradiation in anion‐doped silica glass

“…Using a focus of the femtosecond pulses as a local heat source in a multicomponent glass, we confirmed that the structural changes are isotropic according to the temperature distribution . While, as opposed to the formation of LIPSS on the various materials, the polarization‐dependent bulk nanogratings were observed only in a relatively simple oxides . Meanwhile, the doping effects in amorphous oxides including radiation‐induced structural changes have been widely investigated.…”

“…It is known that the nanogratings consisting of oxygen deficiency defect nanoplanes including the nanopores with the diameter below several tens nm were self‐aligned perpendicular to the laser polarization . Such structures are the origin of the strong refractive index contrast leading to the optical anisotropy . The third one being the change in the oxygen deficiency defect concentration.…”

“…To probe the matter within the nanoplanes, we observed the sample surface cleaved along the nanostructure planes using a field‐emission gun scanning electron microscope . Such observations were carried out using a low accelerating voltage and very low current can reveal the original characteristics of the sample surface without a conductive coating . Figure shows the secondary electron images (SEIs) on the cleaved surface of a laser traces written in SiO 2 glass samples.…”