Formation of femtosecond laser-induced nanogratings at high repetition rates

We investigate the formation of self-organized nanostructures inside the bulk of fused silica induced by ultrafast laser pulses at high repetition rates. Three distinct stages of the evolution from random nanostructures to periodic nanogratings are identified. In addition, for the first time nanograting formation in fused silica is observed at repetition rates as high as 9.4 MHz, thereby significantly extending the parameter window for nanograting inscription. Throughout the whole parameter range, a strong dependence of the grating period on the number of incident pulses was revealed.