Nanostructure formation on lithium niobate surfaces by high-repetition rate sub-15-fs near-infrared laser pulses

Abstract

Due to its electro-optical, acousto-optical, ferroelectric, piezoelectric and nonlinear-optical properties lithium niobate is a material of high technological relevance. Thus, patterning of LiNbO 3 surfaces by laser light may significantly influence the performance of micro-optical devices made of this material. Here, we report on the generation of self-organized nanostructures on surfaces of unpoled LiNbO 3 crystals using tightly focused sub-15 fs pulsed Ti:Sapphire laser light (centre wavelength 800 nm, bandwidth 120 nm, repetition rate 85 MHz) at sub-nanojoule pulse energies. With the LiNbO 3 surface immersed in oil intensities close to the ablation threshold resulted in the formation of shallow ripples of 5 -25 nm in height appearing at a periodicity of approximately 220 nm. The ripples were generated by local melting and resolidification of LiNbO 3 involving minor admixture of hydrocarbons. At intensities well beyond the ablation threshold the LiNbO 3 surfaces were patterned densely with tiny cones of 100 -500 nm in height featuring diameters of a few hundred nanometers. Moreover, lines scanned inside the LiNbO 3 crystals resulted in refractive index changes along the laser traces. In contrast, with the LiNbO 3 surface in air or water, ablation was not observed even at prolonged exposure due to aberrations of the focal spot.