Analysis on the evolution of subwavelength ripples fabricated by ultrafast laser pulses on lithium niobate crystal surface

Currently, periodic structures on lithium niobate crystal surface exhibit extensive application prospects in related fields of on-chip photonic-integrated platforms and nanophotonics. We fabricated periodic ripple structures on the surface of lithium niobate crystal by femtosecond laser pulses irradiation and observed the evolution of these structures irradiated by 50,100,500,1000 laser pulses respectively. The morphology of surface structure became more uniform with the increasing number of laser pulses. Especially fabricated by 1000 pulses, regular ripples were found over the ablation area. The direction of ripples is perpendicular to the laser polarization and the period is around 190 nm, which was calculated by 2D-Fast Fourier Transform. By Finite-Difference Time-Domain method, we simulated the effect of the initial periodic structure on subsequent energy distribution. Numerical simulation results show that energy is deposited in the grooves between the ripples. Therefore, the ablation of grooves is more efficient, and the ripples morphology resulting from subsequent laser pulses irradiation becomes more uniform. The simulation results are consistent with the experimental results. This research is considerably valuable for controlling precisely periodic micro- and nanostructures formation and providing innovative laser manufacturing technology for wide bandgap material.