Diffraction grating fabrication in lithium niobate and KDP crystals with femtosecond laser pulses

We report studies of the modifications induced inside lithium niobate and KDP crystals by Yb:KGW ultrafast laser pulses, having 300-fs pulse duration and operating at 100-kHz repetition rate. By focusing the laser beam with a 0.42 numerical-aperture objective, we have recorded homogeneous volume Bragg gratings in the bulk of the niobate crystal that showed excellent diffraction efficiencies, reaching up to 87%, and remained permanent after thermal annealing for one hour at 150°C. The refractive index modification level was found to be 0.002 in lithium niobate. The results show that lithium niobate is a very promising crystal candidate for microphotonics applications. On the contrary, in the KDP crystal no smoothly modified refractive index zones were created.