The surface diffraction gratings on lithium niobate single crystal ablated by a femtosecond laser pulse at a wavelength of
800 nm have been investigated. The diffraction efficiencies of the gratings ablated with various parameters were
measured by He-Ne laser at a wavelength of 632.8 nm. The 1-order diffraction efficiencies of the gratings improve from
1.7% to 2.3% with the increase of ablation speed from 20μm/s to 200μm/s, and decrease from 1.9% to 1.3% with the
increase of pulse energy from 70nJ to 110nJ, respectively. The experimental and theoretical results show that the
diffraction efficiencies of gratings can be improved by increase of ablation speed and grating constant or decrease of pulse energy.
<sub></sub>The morphology and mechanism of lithium niobate (LiNbO<sub>3</sub>) crystal ablated by a femtosecond laser pulse have been investigated. The ablated spots have been examined by means of scanning electron microscopy (SEM) and atom force microscopy (AFM). Sub-diffraction limit spots in LiNbO<sub>3</sub> crystal ablated by femtosecond laser have been obtained. The diameter of ablation spot is 400 nm and 800 nm when 170 nJ single pulse and 100 nJ, 17 pulses were used, respectively. There were about 200nm periodic ripples on the bottom surfaces of the spots at low laser energy. The result shows that the sub-diffraction limit structures may be formed by the multi-photon excitation, and the femtosecond laser ablation is an innovative tool for manufacturing LiNbO<sub>3</sub>-based optical devices.
The morphology of structural changes in MgAl<sub>2</sub>O<sub>4 </sub>transparent ceramic (MATC) crystal ablated by femtosecond laser pulse has been investigated. Experiments were performed using a commercial Ti:sapphire laser system with 800 nm/50 fs and the repetition ratio of 1 kHz. The damage threshold and ablation area for MgAl<sub>2</sub>O<sub>4</sub> transparent ceramic are investigated. The result shows that the darkening threshold and the damage threshold of MATC are measured to be 6.0×10<sup>12</sup> W/cm<sup>2</sup> and 2.4×1013 W/cm<sup>2</sup>, respectively. The ablated spots are examined by means of a charge coupled device (CCD) camera, scanning electron microscopy (SEM) and atom force microscopy (AFM). The infrared transmission ratio (IR transmission ratio) properties of MATC crystal ablated by femtosecond laser pulse are measured by micro-IR spectra. The result shows that the damaged area will increase linearly with the increasing energy of single pulse while will be approximate satisfied with Boltzmann distribution with the increasing number of multiple pulses. The spot ablated at optimized energy pulses (near the damage threshold energy) can improve the IR transmission ratio of MATC crystal from 82% to 86%. Furthermore, the periodic ripples about 100 nm in width and 200 nm in neighbouring ripples distance are formed on the bottom surfaces of ablation holes.