Ultrafast laser-induced nanogratings in sodium germanate glasses.

We report ultrafast laser inscription of nanogratings possessing form birefringence in binary sodium germanate glasses in a wide range of Na2O content from 3 to 22 mol.%. A minimal number of laser pulses required to induce noticeable form birefringence is shown to grow exponentially with Na2O content in glass. Atomic force microscopy showed similarity of their periodical structure and period value to those in the nanogratings formed in fused silica. A sharp pulse duration threshold below which laser pulses do not induce nanogratings in the studied glasses has been revealed. Formation of a nanograting in 22Na2O·78GeO2 at the studied conditions is accompanied by crystallization of a surrounding submicron layer and partial crystallization inside the nanograting with precipitation of Na2Ge4O9 crystals.