Abstract
Microstructural modifications were induced by femtosecond laser irradiation on lithium niobate (LN) crystals and investigated by means of optical microscopy and micro-Raman spectroscopy. Commercial z-cut congruent LN substrates were irradiated with focused ultra-short laser pulses at 810 nm, generated by a Ti:Sapphire oscillator or an amplified Ti:Sapphire laser system. At the focus region, either refractive index changes or material removal were observed by varying the irradiation conditions. The Raman spectrum shows in the illuminated regions the insurgence of Raman modes, which are forbidden in the used-scattering geometry. This phenomenon increases gradually by approaching the ablation region, where niobium oxides and Li-deficient surfaces are formed for the low energy and high repetition rate pulses from the Ti:Sapphire oscillator. Amorphous surfaces are obtained for high energy and low repetition rate pulses from the amplified Ti:Sapphire laser system.
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