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Several approaches exist to induce the internal modifications in fused silica by femtosecond laser irradiation depending on the dose: direct writing of refractive index change (type I modification), birefringence control by nanogratings for geometric phase elements and polarisation sensitive imaging (type II modification) and new phenomena arising from double pulse utilisation. In this presentation, we focus on two recent topics: enhancement of nanograting formation using a double pulse processing and fabrication of high-efficient diffractive optical elements (DOE) by I type modification in fused silica.
Most of the studies show that the orientation of the LIPSS is perpendicular to the first pulse polarisation. However, the intra-volume modifications with the induced nanogratings have the depth dimension where the double-pulse fabrication can provide more sophisticated morphology depending on the temporal delay and energy relation between two pulses. The nanogratings induced using the double-pulse irradiation with perpendicular polarisations demonstrates the grid-like structure at ~ 10 ps temporal delay, while the 45 degrees tilted gratings appear without delay between pulses. Variation of the nanograting period was observed in the case of parallel polarisation. Those new phenomena can be widely used for writing the two-dimensional diffraction gratings or the information coding applications and requires more deep investigations.
Most of the effects are observed at focusing with high numerical aperture objectives, which working range is limited by spherical aberration below 1 mm depth. However, that makes possible to restrict the radial size of modification below the diffraction limit and extend the longitudinal length of modification up to 50 µm during the single scan. Therefore such approach is capable of recording more compact volume diffractive optical elements with the total diffraction efficiency > 90%. By varying the refractive index within ~1x10-4 to 1x10-3, it is possible to get up to π/24 phase retardance resolution attractive to design the phase change optical elements for the low-loss Top-hat beam shaping and multi-beam splitters.
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