Effect of Pulse Repetition Rate on Ultrafast Laser-Induced Modification of Sodium Germanate Glass.

We report an unexpected pulse repetition rate effect on ultrafast-laser modification of sodium germanate glass with the composition 22NaO 78GeO. While at a lower pulse repetition rate (~≤250 kHz), the inscription of nanogratings possessing form birefringence is observed under series of 10-10 pulses, a higher pulse repetition rate launches peripheral microcrystallization with precipitation of the NaGeO phase around the laser-exposed area due to the thermal effect of femtosecond pulses via cumulative heating. Depending on the pulse energy, the repetition rate ranges corresponding to nanograting formation and microcrystallization can overlap or be separated from each other.