Periodic subwavelength ripples on lithium niobate surfaces generated by tightly focused sub-15 femtosecond sub-nanojoule pulsed near-infrared laser light

On exposure to 85 MHz sub-15 fs pulsed 800 nm Ti:sapphire laser light in the focal spot of a high-numerical aperture oil immersion objective, characteristic periodic nanostructures emerged on the surface of z-cut congruent LiNbO3 crystals. Close to the ablation threshold shallow ripples oriented parallel to the laser polarization appeared on the surface at a periodicity Λ∥ ≈ 200 nm as well as ripples running perpendicular to the laser polarization at almost the same period (Λ⊥ ≈ 190 nm). Nanostructure formation involved melting and resolidification of LiNbO3 as evidenced by scanning electron microscopy images of structural peculiarities. Micro-Raman spectroscopy demonstrated that the resolidified material crystallized in the original structural phase. Ripple formation is attributed to plasma wave interference patterns that arise in the electron–hole plasma generated by multiphoton and avalanche collisional excitation. (paper)