Effect of sodium oxide content on the formation of nanogratings in germanate glass by a femtosecond laser

We report on the formation and structural evolution of embedded self-organized, polarization-dependent nanogratings in sodium germanate glasses induced by an 800 nm, 1 kHz femtosecond laser. Optical birefringence dependent on the femtosecond laser polarization as well as the sodium oxide content is observed when the sample surface is perpendicular to the laser propagation direction. Scanning electron microscopy images of the written lines reveal the formation of periodic platelet or nanovoid arrays, which are aligned perpendicularly to the laser polarization direction after mechanical polishing. The influences of sodium oxide content on the morphology and period of the nanogratings are discussed.